International Geology Review, Vol. 44, 2002, p. 191–221.Copyright © 2002 by V. H. Winston & Son, Inc. All rights reserved.

Perry et al..fm Page 191 Wednesday, July 3, 2002 2:31 PM

The Hydrogeochemistry of the Karst Aquifer Systemof the Northern Yucatan Peninsula, Mexico

EUGENE PERRY,Department of Geology and Environmental Geosciences, Northern Illinois University, DeKalb, Illinois 60115

GUADALUPE VELAZQUEZ-OLIMAN, AND LUIS MARIN

Instituto de Geofisica, Universidad Nacional Autonoma de Mexico, Ciudad Universitaria, CP 04510, Mexico, D.F.

Abstract

Based on groundwater geochemistry, stratigraphy, and surficial and tectonic characteristics, thenorthern Yucatan Peninsula, Mexico, a possible analog for ancient carbonate platforms, is dividedinto six hydrogeochemical/physiographic regions: (1) Chicxulub Sedimentary Basin, a Tertiary basinwithin the Chicxulub impact crater; (2) Cenote Ring, a semicircular region of sinkholes; (3) Pock-marked Terrain, a region of mature karst; (4) Ticul fault zone; (5) Holbox Fracture Zone–Xel-HaZone; and (6) Evaporite Region. Regional characteristics result from tectonics, rock type, and pat-terns of sedimentation, erosion, and rainfall. The Cenote Ring, characterized by high groundwaterflow, outlines the Chicxulub Basin. Most groundwater approaches saturation in calcite and dolomitebut is undersaturated in gypsum. Important groundwater parameters are: SO4/Cl ratios related toseawater mixing and sulfate dissolution; Sr correlation with SO4 and saturation of Lake Chichan-canab water with celestite, indicating celestite as a major source of Sr; high Sr in deep water ofcenotes, indicating deep circulation and contact of groundwater with evaporite; and correlation ofCa, Mg, and SO4, probably related to gypsum dissolution and dedolomitization. Based on geochem-istry we propose: (1) a fault between Lake Chichancanab and Cenote Azul; (2) deep seaward move-ment of groundwater near Cenote Azul; and (3) contribution of evaporite dissolution to karstdevelopment in the Pockmarked Terrain. Chemical erosion by mixing-zone dissolution is importantin formation of Estuario Celestun and other estuaries, but is perhaps inhibited at Lake Bacalarwhere groundwater dissolves gypsum, is high in Ca, low in CO3, and does not become undersatu-rated in calcite when mixed with seawater.

Introduction

THIS STUDY MAKES USE of groundwater geochemis-try, including natural geochemical tracers, to evalu-ate groundwater movement and rock/waterinteraction in the karstic carbonate-sulfate platformthat constitutes the northern Yucatan Peninsula,Mexico. Some of the study results may be broadlyapplicable in coastal karst regions and in betterunderstanding early diagenetic processes in ancientcarbonate platforms. Other results, particularlythose emphasizing Sr chemistry and isotopic compo-sition, may significantly augment borehole data indetermining the nature of the Chicxulub ImpactBasin.

The upper part of the Yucatan aquifer, developedin nearly horizontal Tertiary limestone and dolos-tone and probably evaporite, has both cavernous(fracture) and intergranular (porous medium) per-meability. Exceptionally permeable zones are devel-

oped along faults, perhaps generated by EoceneCaribbean plate movements (Rosencrantz, 1990) onthe east and by crustal relaxation and/or basin load-ing following K/T impact of a large bolide in thenorthwest (Fig. 1, Table 1).

Over almost the entire northern region, a freshwater lens is underlain by a marine saline intrusion,except near the coast there are neither extensiveaquitards (Table 1) nor permanent streams. Ground-water in the Yucatan aquifer system receives ionsfrom two major sources, dissolution of minerals andmixing with the seawater intrusion. Aspects ofregional hydrogeology are presented in papers byBack and Hanshaw (1970), Perry et al. (1989,1995), Stoessell et al. (1989, 1993), Marin et al.(1990), Moore et al. (1992), Marcella and Stoessell(1994), Reeve and Perry (1994), Stoessell (1995),Steinich and Marin (1997), and Pope et al. (2001).

Here we report geochemical data related to flowpatterns and mixing relations within the karst aqui-

1910020-6814/02/583/191-31 $10.00

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fer system of the northern Peninsula. The objectivesof this study have been to: (1) delineate the contri-bution of the saline intrusion to groundwatergeochemistry; (2) elaborate the flow characteristicsof highly permeable zones known as the CenoteRing (aligned sinkholes) and the Ticul fault thatdominate groundwater movement of a significantpart of the region; and (3) better understand the con-tribution of the minerals calcite and dolomite andthe evaporite minerals gypsum/anhydrite and celes-tite to groundwater chemistry. In pursuing theseobjectives, we have found the ions Cl, SO4, and Sr ingroundwater to be particularly useful. Ca, Na, andMg provide additional information.

In much Yucatan groundwater, especially thatnear the north coast, Cl is a conservative or nearlyconservative tracer derived from the saline intru-sion. Sulfate comes from the saline intrusion and/orfrom one or more evaporite sources, making the SO4/

Cl ratio of groundwater a useful tracer. Throughoutthe northern Peninsula, Sr/Cl in groundwater isinvariably higher than the seawater value and indi-cates dissolution of celestite (from evaporite) and/oraragonite. Variations in Sr/SO4 and 87Sr/86Sr offerfurther means to distinguish sources of Sr in ground-water. We shall report 87Sr/86Sr results elsewhere.

Yucatan Hydrogeology

Although no recent comprehensive water budgetestimate for the Peninsula has been made, precipi-tation data are available (Chavez-Guillen, 1986,1988). Precipitation varies across Yucatan from 500mm annually at Progreso (on the northwest coast,Fig. 1) to 1500 mm/yr near Xcan, with an average forthe state of Yucatan of 1025 mm (corresponding to avolume of precipitation of 40,000 million cubicmeters for the entire state). Annual precipitation in

FIG. 1. Map of study area showing terrains, faults, groundwater flow directions, and place names.

KARST AQUIFER SYSTEM 193

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neighboring Quintana Roo varies from 900 to 1500mm/yr (Chavez-Guillen, 1986).

Hanshaw and Back (1980) proposed a water bud-get for the Peninsula in which they estimated thatannual loss by evapotranspiration is about 900 mm,and that subtracting that amount from rainfall leavesabout 150 mm⋅yr–1 as the mean annual recharge

over an area of approximately 65,500 km2. Theyestimated further that human consumption was 350 ×106m3⋅yr–1 of groundwater. Because much of thenon-agricultural human consumption is returneddirectly to the aquifer, this would leave more than~9,500 × 106m3⋅yr–1 as the total discharge alongabout 1,100 km of coastline. Their result is based on

TABLE 1. Important Features Affecting or Affected by Hydrogeology

Feature1 Discussion

Faults

Cenote Ring (CR) Semicircular system of faults. Developed in Tertiary rocks overlying the 65 Ma K/T Chicxulub Impact Crater. Separates Chicxulub Sedimentary Basin (inside) from Pockmarked Terrain and other features with long exposure history (outside). The CR is a zone of high permeability with a groundwater divide near well UNAM2 (Figs. 1 and 2).

Holbox Fracture Zone (HFZ)

Outlined by cenotes (sinkholes). Roughly parallels faulted east coast. Probably related to Eocene tectonic events in the Caribbean. Manifested by elongated cenotes, often connected by broad “swales,” some of which are 100 km long. Water movement is northward.

Ticul fault Thrust up to south. Tertiary evaporite– and gypsum-anhydrite–bearing K/T impact breccia closer to surface in upthrust block. Surficial evidence: conspicuous ridge. Geochemical evidence: transport of water of distinctive composition from E-W groundwater divide in vicinity of Lake Chichancanab WNW to west arm of another structural feature, the CR.

Other physiographic/stratigraphic features

Result from interaction of stratigraphy, structure, and tectonics since Cretaceous.

Chicxulub Sedimentary Basin

Basin of subsidence during much of Paleogene. Lower permeability, fewer cenotes than areas outside CR. Groundwater chemistry dominated by mixing with saline intrusion.

Evaporite Region Lower permeability than northern Peninsula. Transient streams. Groundwater of variable quality, but in all cases characterized by high SO4 and relatively low Cl. L. Chichancanab,

on western border, is approximately saturated with respect to gypsum and celestite.

North Coast Dune ridge almost continuous except where crossed by CR, HFZ, and Ria Lagartos (another permeable zone of relatively high discharge). Groundwater beneath dune and inland from dune is confined by a thin, impermeable caliche layer.

Northern East Coast Fault-bounded coast. Mixing-zone dissolution causes rapid erosion along fractures to produce embayments. Steeper water table gradients than on north coast, partly because of greater recharge. Extensive development of caves along fractures.

Pockmarked Terrain Underlying Tertiary evaporite and K/T gypsum-anhydrite–bearing breccia may have been exposed to karstification, especially during Paleogene uplift and erosion. High permeability evidenced by many cenotes.

Typical ground surface Pervasive caliche layer up to about 3 m thick with little soil cover. Particularly well developed in Chicxulub Sedimentary Basin.A. Impedes infiltration—where continuous, infiltration occurs mostly through subsidence cracks. Caliche is, in many places, underlain by a thin, porous, uncemented layer of carbonate (sahcab in Maya) capable of retaining some moisture within the vadose zone.B. Along the North Coast, CaCO3-saturated groundwater comes to the surface and evapo-

rates, causing precipitation of carbonate cement that fills cracks in the caliche layer. This layer is highly impermeable and forms a narrow coastal aquitard (Perry et al, 1989).

1 Shown in Figure 1.

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an estimate of recharge by Lesser (1976), whichgeneralizes widely varying rainfall patterns. Arevised water budget estimate that recognizes theregional variability in precipitation and incorporatessignificant changes in urban activity and commer-cial agricultural practices is warranted, but isbeyond the scope of this paper.

Precipitation falling on the northern YucatanPeninsula rapidly penetrates a surface calcretelayer through fractures and moves through a highlypermeable vadose zone to the water table, which isnowhere more than 20 m below the land surface ormore than 4 m above mean sea level (MSL) in north-western Yucatan. Water table depth from the landsurface increases in and south of the Sierrita deTicul, reaching measured depths beneath the sur-face of 32 m at Santa Elena (Fig. 1) and 87 m in theSierrita de Ticul near Oxkutzcab (Fig. 1).

Because of exceptionally high permeability, thewater table in northwestern Yucatan is directlyrelated to sea level, and it is less than 3 metersabove MSL 100 km inland (Marin et al., 1990). Thehydraulic gradient for the northern region is 7–10mm/km (Marin et al., 1987; Marin, 1990). In 1997,J. Zhang and E. C. Perry measured water level ele-vations along a first order survey line of the InstitutoNacional de Estadistica, Geografia, e Informatica(INEGI) and found that the water level of LakeEsmeralda, Q. R. (Fig. 1) is 4 m above MSL. This isan approximate maximum water level elevation forthe entire northern Peninsula.

As a consequence of rapid infiltration, there areno surface streams more than a few hundred meterslong in the northern Peninsula (Perry et al., 1989,1995). Our work shows that important groundwaterdivides exist within the Cenote Ring near Tekit (Fig.1), as reported by Steinich et al. (1996) and nearLake Chichancanab. The near flatness and low ele-vation of the water table implies that the residencetime of groundwater in the northern part of theYucatan aquifer is short, as was proposed by Backand Lesser (1981). Stable isotope variation ingroundwater resulting from tropical storm events(Perry et al., 1999, 2001) is consistent with a shortresidence time. In the next section of this paper wenote that a lag of several months occurs between therainy season and peak discharge of groundwater tothe northwest coast, a phenomenon related torecharge. Data are insufficient at present to quantifythe residence time(s) of groundwater in the aquiferof the northern Yucatan Peninsula.

Although its open character makes the north-western Yucatan aquifer sensitive to contamination(Pacheco et al., 2001), the natural characteristics ofthis aquifer have, at present, been only locallyaltered by human activities. Nevertheless, the500,000 people of the city of Merida have signifi-cantly impacted the system locally (Marin et al.,2001), and intensive modern commercial agricul-ture may soon have an even greater regional impact(Miller, 1990).

Important geochemical information comes from aseries of 100 to 700 m holes drilled by the Univer-sidad Nacional Autónoma de México (UNAM)(Urrutia-Fucugauchi et al., 1996; Rebolledo-Vieyraet al., 2000). These include UNAM2, UNAM5,UNAM6, and UNAM7 (Fig. 1). Water analyses pre-sented here demonstrate that seawater intrusion ispresent at UNAM2 (within the Cenote Ring) andUNAM5. This intrusion is present over much of thestudy area beneath a fresh-water lens ranging from15 m thick to values calculated (from the Ghyben-Herzberg relation) to be 100 m (Steinich and Marin,1997).

Hydrogeochemical/Physiographic Regions

As a result of this study, it is possible to identifysix distinct hydrogeochemical/physiographicregions, all developed in carbonate rocks of theYucatan Peninsula (Fig. 1, Table 1). Distinctivecharacteristics of these regions result from tectonics(faulting, uplift), patterns of sedimentation and ero-sion, rainfall patterns, and rock type (Perry et al.,1995; Pope et al., 1996; McClain, 1997). Criteria fordefining these regions are based primarily onresearch presented here and on work of Velazquez-Oliman (1995), Tulaczyk et al. (1993), Perry et al.(1995), and McClain (1997).

Chicxulub Sedimentary Basin

The most conspicuous and best-studied of thehydrogeologic regions of northwest Yucatan is theChicxulub Sedimentary Basin, a Tertiary basin ofsedimentation that occupies much of the terminalCretaceous Chicxulub bolide impact crater (Hilde-brand et al., 1991, 1995; Pope et al. 1991, 1996,2001; Sharpton et al., 1993; Schuraytz et al., 1994;Morgan et al., 1997; Pope, 1997). This basinreceived up to 200 m of marine carbonate sedimentsduring the late Eocene and the Oligocene epochs atthe same time that much of the northeastern Penin-sula was exposed to erosion and karst formation

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(Lopez Ramos, 1973; Perry et al., 1995, Pope et al.,1996; McClain, 1997). An important hydrogeologicconsequence of this differential weathering is that,as demonstrated by cenote abundance (Hildebrandet al., 1995) and supported by resistivity measure-ments (Tulaczyk and Perry, unpubl.), cavern perme-ability is less developed within the Chicxulub Basinthan elsewhere on the Peninsula.

Cenote Ring

Bounding the Chicxulub Basin is the CenoteRing, a circular alignment of cenotes (karst sink-holes). This ring is an important hydrogeologic fea-ture because it is the surface manifestation of a zoneof high permeability, developed in Tertiary carbon-ate rocks. It is a major conduit for groundwater,which it carries from the southern interior northwardto the Gulf of Mexico coast at Bocas de Dzilam andEstuario Celestun (Fig. 1) (Perry et al., 1995). TheRing has a center approximately coincident with thecenter of the Chicxulub Impact Crater (Pope et al.,1993, 1996; Hildebrand et al., 1995; Perry et al.,1995). Perry et al. (1995), Hildebrand et al. (1995),and Pope et al. (1996) have suggested involvementof reactivated K/T ring faults in Cenote Ring forma-tion. Other hypotheses include “post-impact subsid-ence induced by slumping and viscous relaxation inthe rim” (Pope et al., 1991, p. 105), and “slumpingin the rim of the buried crater, differential thick-nesses in the rocks overlying the crater, or solutioncollapse within porous impact deposits” (Pope et al.,1993, p. 93).

The area circumscribed by the Ring is character-ized by a considerably lower density of cenotes thanthe area outside the Ring. Buffler et al. (1995)attributed cenote density to permeability variationcorrelated with a facies change (which they consid-ered to have produced the Ring). As stated above,Tulaczyk and Perry (unpubl.) suggested duration ofsubaerial exposure and weathering as a principalreason both for difference in permeability and cen-ote density inside and outside the Ring. This is con-sistent with the evolution of surface featuresreported by Pope et al. (1996). While sedimentationoccurred in the basin outlined by the Ring, erosionand karst weathering were taking place outside theRing.

The places where the Cenote Ring discharges tothe Gulf of Mexico (Bocas de Dzilam and EstuarioCelestun) are broad, shallow water bodies withopenings to the Gulf through the coastal dune sys-tem. They contain water with salinity intermediate

between fresh Yucatan groundwater and seawater.We note, as do Pope et al. (2001), that there is a sea-sonal variation in coastal zone recharge. Dilution ofsaline water in Estuario Celestun by groundwaterapparently lags several months behind regionalrecharge.

Perry and Velazquez-Oliman (1996) have evalu-ated mixing relations between Yucatan groundwaterand seawater, and they proposed that the broadopenings at Bocas and Estuario are maintained, inpart, by dissolution, in a groundwater mixing zone,of some of the aragonite and high-magnesium calcitesand and silt that is carried by the east-to-west–moving longshore current. The proposed process issimilar to the mixing-zone dissolution observed onthe east coast of the Peninsula by Back et al. (1979,1986) and by Stoessell et al. (1989); the process dif-fers in that on the north and northwest coast thematerial dissolved is unconsolidated sediment,whereas in east coast localities such as Xel-Ha (Fig.1) it is consolidated rock that is dissolving. Notably,there is no opening to the sea at Lake Bacalar (Fig.1), despite evidence of high groundwater flow. Alikely reason for this is discussed in the section onHydrogeologic Aspects of the Evaporite Region.

Pockmarked Terrain

The Pockmarked Terrain is a broad region innorth central Yucatan (Fig. 1) in which cenotesoccur in exceptional abundance in relatively maturekarst. According to Pope et al. (1996), its surfacefeatures are the oldest and most elevated of thenorthernmost Peninsula. Perry et al. (1996) havepresented evidence, amplified here, that this karstdevelopment is, in part, a process resulting from dis-solution of CaSO4 of Tertiary evaporite and/or K/Tanhydrite/gypsum–bearing impact breccia. Thewestern boundary of the Pockmarked Terrain coin-cides with the Cenote Ring and marks the boundarybetween the area outside underlain by evaporitebeds and terminal Cretaceous evaporite-bearingimpact breccia (Ward et al., 1995) and the areainside in which evaporitic horizons are absent or toodeep to interact with groundwater.

The stratigraphic information necessary to evalu-ate this hypothesis must be extrapolated from else-where, primarily from core of holes 2, 5, 6, and 7 ofthe UNAM drilling project (Urrutia-Fucugauchi etal., 1996) taken near the southern margin of theChicxulub Impact Crater (Figs. 1 and 2). This corecontains the following information about evaporiteminerals (primarily gypsum/anhydrite but also

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minor halite and celestite) occurring at depths shal-low enough to have interacted with groundwater dur-ing the Cenozoic Era (summarized from Urrutia-Fucugauchi et al., 1996; Rebolledo-Vieyra et al.,2000; Ward, Mihai Lefticariu, and Perry, unpubl.).UNAM 2 (inside the crater and 559 m deep)encountered no impact breccia and only a minoramount of evaporite. Below 172 m, core fromUNAM5 (near Sta. Elena) contains limestone/dolo-mite collapse breccia and evaporite horizons.Impact breccia containing some evaporite is presentfrom 332 m to the bottom of the hole at 504 m. Incore from UNAM6 (near Peto) evaporite occurs at256 m and evaporite-bearing breccia at depthsbelow 283 m. Impact breccia with a cap of evapor-ite-bearing carbonate occurs in core from UNAM7near Tekax at depths below 207 m.

Reinterpretation of core and logs of PEMEXexploratory wells Y4 and 5A (Figs. 1, 2; Ward et al.,

1995) also confirms the presence of evaporite-bear-ing breccia within the Pockmarked Terrain. Exten-sive weathering, the presence of evaporite mineralsat depth, and proximity to the coast have probablyinfluenced permeability development in north cen-tral Yucatan. W. C. Ward (pers. commun., November2001) suggests that possible block faulting andfracture density are factors that warrant fieldinvestigation.

Ticul faultThe surface expression of the Ticul fault is an

escarpment trending WNW for about 100 km (Fig.1). Our water analyses confirm that the Ticul faultzone transports SO4-rich water from the vicinity ofLake Chichancanab–Lake Esmeralda and dis-charges it into the western Cenote Ring nearKopoma (Figs. 1 and 2) (Velazquez-Oliman, 1995;Perry and Velazquez-Oliman, 1996).

FIG. 2. Study area. Contours are of equivalent ratio 100 × (SO4)/(Cl) (given as numbers where data are too sparse tocontour). Dark dots are cenotes (taken from Hildebrand et al., 1995). Circles are PEMEX wells.

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Holbox Fracture Zone–Xel-Ha Zone

The Holbox Fracture Zone–Xel-Ha Zone is aregional feature within the karst plain of northeast-ern Yucatan (Fig. 1). It consists of >100 km longchains of elongated solution depressions locallyknown as sabanas. Alignment of sabanas (South-worth, 1985; Tulaczyk et al., 1993) closely followsoffshore tectonic structures related to plate bound-aries presumably inactive since the Late Eocene.The hydrogeologic relation of the Holbox FractureZone to the Pockmarked Terrain on the west and theEvaporite Region to the southeast remains to beinvestigated. Work by Back et al. (1979, 1986),Stoessell et al. (1989), and Coke et al. (1991) showsthat a saline intrusion is important along the eastcoast, whereas Tulaczyk et al. (1993) found no evi-dence for such an intrusion more than a few kilome-ters south of the eastern north coast. Our work inthis area has been limited to determination of SO4/Cl ratios in a number of water samples (see Table 3).These data (plotted on Fig. 2) are consistent withwidespread contact of groundwater with a salineintrusion.

Evaporite Region

Few geologic or hydrogeologic data exist for thearea east and south of Lake Chichancanab (near thesouthern part of the Yucatan–Quintana Roo border,Fig. 1), which we identify here as the EvaporiteRegion. The region is characterized by high andvariable sulfate/chloride ratios. Our work indicatesthat some sulfate-rich groundwater from here movessouthward and westward and markedly influencesgroundwater chemistry to the west and north. There-fore, we include here the results of a reconnaissancestudy of the area by Perry, Jiren Zhang, andVelazquez-Oliman made in March and June, 1996,and reported in Perry et al. (1996) and Velazquez-Oliman and Perry (1996). K. Pope (pers. commun.,August 2000) reports that the region containingwater with high sulfate/chloride ratios extends intoBelize and perhaps Guatemala.

Lake Chichancanab, which is located on theapproximate boundary between the highly perme-able rocks of northwest Yucatan and the less perme-able eastern region, is a useful marker for thenorthwestern limit of the Evaporite Region. Surfaceelevation of Lake Esmeralda (the sister lake of LakeChichancanab, Fig. 1) was 4 m above MSL whenmeasured by Perry and Zhang in 1997. The water-table elevation decreases both east and west of thislake, making this a groundwater divide. South and

east of Lake Chichancanab, topography changesdramatically; swamps are common, and ephemeralstreams are present.

The water table from Lake Chichancanab eastand south appears to be less directly controlled bysea level than in northwest Yucatan, indicating thatintergranular permeability is relatively low. Evi-dence for this lower permeability includes: (1)swampy terrain at elevations well above sea level;(2) presence of water with variable sulfate concen-tration that ranges to values much higher than else-where in the study area (which could be sustainedfor a long time only if flow is limited); and (3) rela-tively slow recovery of the lake level after stormevents.

Surface rocks of the Evaporite Region have beenmapped as Eocene and Miocene–Pliocene. Bothunits reportedly contain evaporites, and (as notedabove) evaporite is present above the K/T breccia inUNAM core. The eastern shore of Lake Chichan-canab is an extensive low ridge of somewhat brecci-ated rock that we suggest may mark a fault that hasbrought to the surface sulfate-bearing evaporite con-sidered to be of Eocene age by Lopez Ramos (1973).Alternatively, the sulfate-bearing rock could be K/Timpact breccia.

Sampling and Analysis

The areas sampled in most detail for this studyare the Chicxulub Basin, the Cenote Ring, and theTicul fault zone in the western and northwestern partof the Yucatan Peninsula. Data from those areashave been supplemented by reconnaissance studiesin other parts of the Peninsula. Except for samplesspecifically noted in Table 2, all fresh groundwatersamples collected for this study were taken fromactively pumping municipal wells operated by theJunta de Agua Potable y Alcantarillado del Estadode Yucatán (JAPAY) or the equivalent agency inQuintana Roo.

Complete analyses are reported for all samplestaken in the first phase of the study (June 1993through March 1995). For some samples collectedbetween October 1995 and June 1996, we reportonly anion data (Cl, SO4, NO3, and alkalinity) (Table3). Temperature, pH, conductivity, dissolved oxy-gen, and redox potential were measured in the fieldusing of a Datasonde 3 connected to a flow-throughdevice. Measurements were made after the systemhad come to equilibrium, as indicated by stablereadings of temperature, pH, and conductivity. As

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TAB

LE 2

. Maj

or Io

ns a

nd F

ield

Par

amet

ers

Lat(

°N)/L

ong(

°W)

Sam

ple

nam

eC

olle

ct d

ate

Ca

(meq

) M

g(m

eq)

Sr(m

eq)

K(m

eq)

Na

(meq

) SO

4(m

eq)

Cl

(meq

) N

O3

(meq

) H

CO

3(m

eq)

Ion

Bal

(%)

100

×SO

4/C

lT ºC

pH

Wel

ls o

r cl

osed

cen

otes

20.6

4/89

.68

Aba

laA

ug 9

4 6.

94

4.65

0.

049

0.12

6.

65

3.75

7.

62

N.D

.7.

16

–0.3

2 49

.23

27.4

5 6.

76

20.6

4/89

.68

Aba

laJu

n-93

5.

81

3.71

0.

029

0.43

4.

67

2.55

5.

11

N.D

.6.

31

2.34

49

.95

27.7

4 7.

11

20.6

4/89

.68

Aba

laM

ar 9

67.

03

4.38

0.

053

0.11

6.

74

4.18

8.

05

0.07

3 6.

43

–1.1

3 51

.96

28.1

0 6.

74

20.6

4/89

.68

Aba

laO

ct 9

56.

87

4.47

0.

049

0.21

6.

72

3.83

7.

59

0.09

4 6.

38

1.19

50

.49

27.5

9 7.

19

18.7

4/88

.45

Bac

alar

Pue

blo

Mar

96

5.84

1.

58

0.03

6 0.

13

2.05

0.

25

2.23

0.

007

6.66

2.

64

11.0

4 26

.00

6.85

19

.88/

89.2

2

Bec

anch

en1

Mar

96

4.02

2.

92

0.06

5 0.

47

17.7

6 4.

02

10.8

3 0.

774

8.31

2.

66

37.1

0 29

.20

6.89

21

.20/

88.7

9B

uctz

otz

Apr

94

3.90

2.

87

0.01

0 0.

14

2.82

0.

39

2.31

N

.D.

6.56

2.

58

16.6

5 27

.03

6.85

20

.20/

89.3

3C

anek

N.D

.5.

04

2.79

0.

021

0.10

4.

28

1.71

4.

32

0.08

1 6.

31

–0.7

1 39

.56

28.5

0 6.

87

19.9

0/88

.95

Cat

mis

Mar

96

15.1

2 7.

79

0.12

5 0.

41

10.5

1 19

.68

10.8

2 0.

213

5.02

–2

.55

181.

93

28.4

0 6.

73

20.8

5/90

.28

Cel

estú

n M

ar 9

57.

53

5.99

0.

057

0.31

7 16

.33

5.45

18

.53

N.D

.6.

58

–0.5

5 29

.42

26.6

0 6.

96

20.8

5/90

.28

Cel

estú

n M

ar 9

68.

85

6.39

0.

063

0.32

16

.18

6.48

19

.38

0.09

8 6.

3 –0

.71

33.4

1 26

.60

6.62

20

.85/

90.2

8C

eles

tún

Apr

94

9.00

7.

03

0.06

0 0.

36

15.9

2 6.

24

17.4

4 N

.D.

6.56

3.

43

35.7

7 26

.37

6.75

20

.85/

90.2

8C

eles

tún

Dec

93

9.03

6.

34

0.06

0 0.

31

15.5

5 6.

41

18.6

5 N

.D.

6.41

–0

.30

34.3

8 26

.45

6.65

20

.96/

88.6

0C

enot

illo

Mar

95

6.19

2.

41

0.02

0.

10

5.15

0.

59

6.70

N

.D.

7.52

–3

.27

8.81

25

.50

6.76

20

.74/

89.8

3C

hoch

ola

Oct

95

5.42

4.

13

0.03

2 0.

16

9.12

3.

06

10.4

4 0.

083

6.57

–3

.30

29.3

2 27

.60

6.72

20

.74/

89.8

3C

hoch

ola

Aug

94

7.27

5.

06

0.03

8 0.

17

9.17

2.

76

10.9

1 N

.D.

6.83

2.

85

25.3

5 28

.05

6.84

20

.03/

89.1

7C

olok

e A

kal

Mar

96

4.56

1.

72

0.01

3 0.

15

4.81

0.

87

4.71

0.

097

5.61

–0

.17

18.5

6 29

.50

6.83

20

.86/

90.2

1

Cru

z, C

eles

tun1

Dec

93

4.36

1.

97

0.02

7 0.

13

4.37

1.

72

5.53

N

.D.

4.72

–4

.90

31.1

7 27

.68

6.89

21

.27/

88.9

3D

zila

m d

e G

onza

lez

Apr

94

5.89

2.

11

0.01

8 0.

09

4.62

0.

44

5.19

N

.D.

6.23

3.

57

8.42

27

.27

6.80

20

.75/

89.2

8H

omun

Jun

93

5.46

3.

42

0.01

3 0.

07

2.57

0.

35

3.11

N

.D.

7.44

2.

88

11.1

5 28

.85

7.56

20

.86/

90.4

0

Hot

el G

utie

rrez

1M

ar 9

57.

91

8.73

0.

061

1.19

32

.53

7.66

33

.94

N.D

.8.

02

0.80

22

.56

25.9

0 6.

99

20.7

3/89

.17

Huh

iJu

n 93

5.

28

3.73

0.

018

0.07

2.

99

0.38

3.

36

N.D

.7.

36

4.25

11

.44

27.3

1 7.

07

20.6

8/90

.46

I

sla

Are

na1

Mar

95

9.49

8.

79

0.06

0.

67

35.7

9 7.

31

38.2

6 N

.D.

8.08

1.

06

19.1

1 27

.50

6.8#

20.9

4/89

.04

Izam

alM

ar 9

54.

46

2.13

0.

01

0.04

2.

15

0.24

2.

51

N.D

.5.

96

0.50

9.

49

27.3

0 6.

92

20.7

9/89

.03

Kan

tuni

lA

pr 9

4 4.

72

3.63

0.

018

0.11

4.

11

0.46

4.

63

N.D

.7.

87

–1.4

0 9.

90

27.4

6 6.

62

20.9

2/89

.98

Kin

chil

Dec

93

6.14

3.

36

0.01

9 0.

15

6.87

1.

30

9.42

N

.D.

6.49

–1

.99

13.8

1 27

.17

6.87

20

.62/

90.1

6K

ocho

lD

ec 9

3 7.

32

4.40

0.

046

0.23

11

.15

5.26

11

.95

N.D

.6.

13

–0.4

1 43

.99

27.9

0 6.

81

20.6

2/90

.16

Koc

hol

Aug

94

7.76

4.

58

0.04

9 0.

25

11.3

0 4.

93

11.6

6 N

.D.

6.10

2.

70

42.2

5 28

.23

6.83

20

.66/

89.9

0K

opom

aSe

p 93

8.

42

6.46

0.

059

0.34

15

.45

7.31

17

.25

N.D

.6.

29

–0.2

1 42

.34

27.6

6 6.

88

20.6

6/89

.90

Kop

oma

Jun

93

7.75

6.

09

0.05

2 0.

36

14.3

7 6.

72

14.6

0 N

.D.

6.42

1.

57

46.0

1 27

.73

6.77

20

.56/

89.4

6M

ahzu

cil

Oct

95

5.53

3.

66

0.04

3 0.

17

4.46

2.

20

5.11

0.

127

6.49

–0

.22

43.1

0 27

.49

6.88

20

.48/

89.3

5M

ama

Jun

93

6.90

3.

89

0.05

3 0.

12

4.85

3.

20

5.23

N

.D.

6.54

2.

71

61.2

2 26

.98

7.05

KARST AQUIFER SYSTEM 199

Perry et al..fm Page 199 Wednesday, July 3, 2002 2:31 PM

20.3

8/89

.39

Man

iJu

n 93

6.

66

4.27

0.

047

0.14

5.

64

4.82

5.

76

N.D

.6.

60

–1.2

4 83

.67

27.3

8 7.

01

20.5

8/90

.03

Max

canu

Aug

94

5.76

3.

58

0.02

5 0.

14

5.87

2.

46

5.70

N

.D.

7.23

–0

.03

43.1

3 28

.60

6.81

20

.58/

90.0

3M

axca

nuD

ec 9

3 5.

16

3.07

0.

021

0.11

4.

44

1.98

4.

46

N.D

.7.

31

–3.5

9 44

.44

27.9

6 6.

59

20.6

2/89

.61

Muc

uych

eO

ct 9

55.

12

2.11

0.

009

0.27

1.

78

0.79

1.

87

0.15

8 6.

31

0.71

42

.08

27.6

6 7.

05

20.5

5/89

.86

Opi

chen

Dec

93

9.20

5.

35

0.05

8 0.

22

9.15

6.

16

10.1

9 N

.D.

6.93

1.

48

60.5

2 27

.81

6.85

20

.55/

89.8

6O

pich

enA

ug 9

4 7.

82

6.32

0.

063

0.29

13

.05

7.74

13

.11

N.D

.6.

47

0.40

58

.99

28.4

9 6.

71

20.5

5/89

.86

Opi

chen

Jun

93

8.03

5.

91

0.06

0 0.

28

12.0

8 7.

85

12.0

8 N

.D.

6.51

–0

.16

64.9

7 27

.96

6.92

20

.30/

89.4

1O

xkut

zcab

Sep

93

4.61

2.

89

0.02

2 0.

11

6.70

2.

45

5.30

N

.D.

6.10

1.

77

46.1

6 28

.19

6.99

20

.11/

88.9

3P

eto

Sep

93

6.31

4.

30

0.12

2 0.

62

8.87

5.

02

7.37

N

.D.

7.13

1.

78

68.0

1 27

.55

6.83

20

.11/

88.9

3P

eto

Mar

95

6.18

4.

28

0.12

0.

61

9.22

4.

48

7.17

N

.D.

7.49

3.

23

62.4

2 26

.20

6.99

20

.87/

88.6

3Q

uint

ana

Roo

Mar

95

5.11

2.

32

0.02

0.

07

4.45

0.

52

5.60

N

.D.

6.95

–4

.41

9.37

26

.60

7.07

20

.49/

89.5

9Sa

calu

mJu

n 93

7.

15

5.28

0.

047

0.20

11

.04

6.06

10

.95

N.D

.6.

36

0.76

55

.38

27.6

0 6.

95

20.4

9/89

.59

Saca

lum

Aug

94

8.23

5.

40

0.05

4 0.

19

10.1

0 5.

85

11.8

0 N

.D.

6.54

–0

.48

49.5

6 27

.64

6.79

20

.88/

89.8

9Sa

mah

ilM

ar 9

65.

45

2.90

0.

016

0.17

6.

65

0.90

7.

79

0.09

0 6.

7 –0

.92

11.5

0 27

.20

6.68

20

.83/

89.9

8Sa

n A

nton

io T

edzi

dzD

ec 9

3 6.

46

5.01

0.

032

0.23

10

.85

3.08

12

.44

N.D

.6.

90

0.33

24

.78

27.6

3 6.

53

20.1

8/89

.66

Sayi

lD

ec 9

3 5.

97

2.83

0.

010

0.14

2.

04

0.59

2.

17

N.D

.8.

03

0.97

27

.20

28.0

1 6.

57

21.1

2/89

.73

Sier

ra P

apac

alA

ug 9

4 5.

31

3.36

0.

022

0.20

9.

44

1.25

10

.98

N.D

.6.

59

–1.3

2 11

.43

28.2

1 6.

85

20.4

6/89

.67

San

Jose

Tip

ceh

Dec

93

9.28

6.

18

0.06

3 0.

23

12.5

6 8.

30

14.5

6 N

.D.

6.11

–1

.16

57.0

3 27

.28

7.08

20

.60/

89.0

1So

tuta

Jun

93

5.77

2.

75

0.01

5 0.

32

3.04

0.

52

3.52

N

.D.

6.82

4.

58

14.6

7 27

.28

7.18

20

.33/

89.6

4St

a. E

lena

Dec

93

8.28

5.

15

0.06

8 0.

18

7.75

7.

99

8.68

N

.D.

6.43

–3

.76

92.0

2 28

.05

6.85

20

.33/

89.6

4St

a. E

lena

Mar

95

7.49

4.

77

0.06

0.

16

7.82

7.

00

7.66

N

.D.

6.35

–1

.70

91.3

9 27

.80

6.89

20

.60/

90.1

1Sa

nto

Dom

ingo

Dec

93

4.70

1.

36

0.01

5 0.

12

2.27

1.

09

2.45

N

.D.

4.80

0.

74

44.6

4 27

.80

6.80

20

.73/

89.4

8Te

coh

Aug

94

4.37

3.

31

0.01

4 0.

09

4.01

0.

47

4.46

N

.D.

7.06

–0

.87

10.6

1 28

.13

6.75

20

.73/

89.4

8Te

coh

Jun

93

5.13

3.

48

0.01

4 0.

09

3.91

0.

48

4.63

N

.D.

7.10

1.

69

10.4

1 27

.92

7.11

20

.19/

89.2

7Te

kax

Sep

93

5.08

3.

28

0.02

2 0.

14

6.82

2.

45

6.99

N

.D.

6.36

–1

.46

35.0

4 28

.58

6.92

20

.53/

89.3

3Te

kit

Sep

93

4.95

3.

03

0.03

8 0.

06

3.14

0.

90

3.57

N

.D.

6.49

1.

14

25.2

3 27

.65

6.97

20

.65/

89.4

6Te

lcha

quil

loJu

n 93

4.

60

3.25

0.

009

0.33

2.

90

0.55

3.

42

N.D

.6.

95

0.80

16

.20

27.4

4 7.

01

21.1

4/88

.94

Tem

axA

pr 9

4 4.

41

1.58

0.

011

0.02

1.

61

0.17

1.

24

N.D

.5.

67

3.73

13

.42

27.7

3 6.

85

20.6

9/89

.65

Tem

ozon

Sur

Mar

96

4.16

3.

35

0.01

8 0.

30

5.49

1.

19

6.53

0.

129

5.8

–1.2

6 18

.25

28.1

0 6.

92

20.4

0/89

.53

Ticu

lD

ec 9

3 6.

71

3.28

0.

035

0.08

5.

42

3.91

6.

99

N.D

.5.

52

–2.8

2 55

.98

28.4

5 6.

76

20.4

0/89

.53

Ticu

lJu

n 93

5.

41

3.92

0.

034

0.13

7.

32

3.62

7.

05

N.D

.5.

97

0.53

51

.42

28.6

1 7.

01

20.9

0/88

.75

Tunk

asA

pr 9

4 6.

34

2.67

0.

022

0.10

4.

56

0.43

5.

25

N.D

.7.

00

3.85

8.

14

26.7

3 6.

78

20.0

7/89

.04

Tzu

caca

bM

ar 9

511

.11

6.02

0.

09

0.24

12

.74

12.7

5 13

.26

N.D

.5.

64

–2.3

4 96

.09

27.5

0 7.

01

20.0

7/89

.04

Tzu

caca

bSe

p 93

11

.27

5.75

0.

088

0.23

9.

91

11.3

6 10

.37

N.D

.5.

54

–0.0

4 10

9.64

27

.75

6.74

20

.87/

89.7

3U

man

Mar

96

5.50

1.

14

0.01

3 0.

07

3.66

0.

46

4.03

0.

065

6

–0.8

0 11

.34

28.7

0 6.

70

20.3

7/89

.78

U

xmal

1D

ec 9

3 9.

05

5.78

0.

074

0.24

10

.77

9.10

10

.12

N.D

.7.

39

–1.3

3 89

.84

28.9

6 6.

76

20.8

3/89

.19

Xoc

chel

Mar

95

5.27

3.

00

0.02

0.

04

4.21

0.

57

5.59

N

.D.

7.48

–4

.24

10.1

8 27

.30

6.76

(tab

le c

onti

nues

)

200 PERRY ET AL.

Perry et al..fm Page 200 Wednesday, July 3, 2002 2:31 PM

TAB

LE 2

. Con

tinue

d

Lat(

°N)/L

ong(

°W)

Sam

ple

nam

eC

olle

ct d

ate

Ca

(meq

) M

g(m

eq)

Sr(m

eq)

K(m

eq)

Na

(meq

) SO

4(m

eq)

Cl

(meq

) N

O3

(meq

) H

CO

3(m

eq)

Ion

Bal

(%)

100

×SO

4/C

lT ºC

pH

Ope

n w

ater

18.6

5/88

.41

Azu

l Cen

ote

Mar

96

22.4

96.

990.

190

0.06

1.64

25.7

51.

250.

003

4.61

–0.3

920

56.3

227

.40

7.35

20.8

8/90

.36

Bal

deoc

era

Mar

95

23.9

683

.95

0.20

8.89

406.

3647

.26

489.

70N

.D.

3.48

–1.6

19.

6526

.20

6.86

N.D

.C

anal

Mar

95

7.79

55.

602

0.05

60.

377

20.8

35.

4324

.86

N.D

.6.

11–2

.46

21.8

524

.90

7.51

20.8

3/90

.38

Cel

estu

n, D

umac

2M

ar 9

518

.33

76.8

70.

157.

1235

1.52

42.5

438

0.16

N.D

.4.

093.

0911

.19

26.1

#27.

0#20

.85/

90.3

6C

eles

tun,

Est

ero

Poz

oM

ar 9

58.

956

9.22

40.

070

0.74

536

.52

7.86

37.2

4N

.D.

6.84

3.32

21.1

026

.10

7.04

19.8

8/88

.77

Chi

chan

cana

bM

ar 9

630

.35

16.5

40.

342

0.29

8.67

51.1

46.

610.

319

2.26

–3.5

577

4.04

26.5

07.

9319

.88/

88.7

7C

hich

anca

nab

Mar

95

33.9

719

.77

0.39

0.10

9.15

54.6

98.

26N

.D.

1.56

–0.8

966

2.47

25.7

08.

4520

.55/

90.4

3E

l Rem

ate

Mar

95

8.24

5.43

0.05

0.34

20.2

56.

2620

.62

N.D

.5.

972.

1830

.36

26.5

07.

0519

.14/

88.1

7N

ohbe

c L

ake

Mar

96

2.47

0.79

0.01

10.

141.

351.

081.

130.

000

2.33

2.32

95.9

523

.10

8.42

18.6

6/88

.41

Bac

alar

lago

onJu

n 96

N.D

.N

.D.

N.D

.N

.D.

N.D

.22

.21

2.93

0.02

02.

70N

.D.

757.

2030

.72

7.78

20.2

7/87

.49

Car

was

hJu

n 96

N.D

.N

.D.

N.D

.N

.D.

N.D

.1.

5315

.68

0.03

56.

72N

.D.

9.73

27.0

76.

9020

.83/

90.3

8C

eles

tun,

Dum

acJu

n 96

N.D

.N

.D.

N.D

.N

.D.

N.D

.38

.79

395.

310.

170

3.84

N.D

.9.

8129

.90

7.80

18.6

5/88

.41

Cen

Azu

l 20m

Jun

96N

.D.

N.D

.N

.D.

N.D

.N

.D.

24.6

11.

210.

020

4.59

N.D

.20

33.3

429

.28

7.03

18.6

5/88

.41

Cen

Azu

l 62m

Jun

96N

.D.

N.D

.N

.D.

N.D

.N

.D.

25.7

31.

220.

021

4.54

N.D

.21

08.5

429

.06

7.44

18.6

5/88

.41

Cen

Azu

l, Su

per

Jun

96N

.D.

N.D

.N

.D.

N.D

.N

.D.

24.8

21.

200.

020

4.03

N.D

.20

61.9

331

.41

7.73

20.4

9/87

.73

Cob

a L

ago

Jun

96N

.D.

N.D

.N

.D.

N.D

.N

.D.

0.65

7.93

0.00

23.

03N

.D.

8.22

30.0

28.

4520

.49/

87.7

3C

oba

Rai

nJu

n 96

N.D

.N

.D.

N.D

.N

.D.

N.D

.0.

020.

030.

002

0.00

N.D

.63

.77

27.9

07.

3219

.76/

88.7

1E

smer

alda

Mar

96

N.D

.N

.D.

N.D

.N

.D.

N.D

.28

.95

4.02

0.00

05.

46N

.D.

720.

2625

.50

7.69

19.4

7/88

.10

Oco

m, L

.M

ar 9

6N

.D.

N.D

.N

.D.

N.D

.N

.D.

4.83

4.96

0.00

485.

28N

.D.

97.3

824

.86

7.75

Dee

p sa

mpl

esN

.D./N

.D.

UN

AM

2, 3

00 m

Mar

95

48.8

710

2.69

0.52

10.0

653

1.51

62.6

763

3.29

N.D

.4.

51–0

.49

9.90

27.2

06.

78N

.D./N

.D.

UN

AM

2, 3

00 m

A

ug 9

446

.80

97.1

70.

515

10.0

052

6.3

69.1

961

2.46

N.D

.4.

62–0

.40

11.3

028

.60

6.52

N.D

./N.D

.U

NA

M2,

350

m

Aug

94

48.6

210

7.7

0.54

310

.93

559.

369

.05

606.

73N

.D.

4.36

3.34

11.3

830

.00

6.8#

N.D

./N.D

.U

NA

M5,

300

mM

ar 9

575

.52

99.4

30.

648.

3053

1.61

72.1

662

7.06

N.D

.2.

810.

9511

.51

32.0

06.

74N

.D./N

.D.

UN

AM

5 30

0 m

A

ug 9

475

.63

98.1

70.

678

8.00

475.

586

.91

569.

47N

.D.

3.11

–0.1

115

.26

31.4

76.

54N

.D./N

.D.

UN

AM

5 40

0 m

Aug

94

86.2

710

1.6

0.73

27.

9747

3.4

86.3

057

7.49

N.D

.2.

900.

2514

.94

30.0

06.

8#

1 Sam

ple

take

n fr

om a

wel

l tha

t is

not a

ctiv

ely

pum

ping

.2 V

alue

s w

ith

the

suff

ix “

#” a

re e

stim

ated

.

KARST AQUIFER SYSTEM 201

Perry et al..fm Page 201 Wednesday, July 3, 2002 2:31 PM

TAB

LE 3

. Par

tial A

naly

ses

and

Fiel

d Pa

ram

eter

s of

Wel

ls

Lat(°

N)/L

ong(

°W)

Sam

ple

nam

eC

olle

ct d

ate

SO4

(meq

)C

l(m

eq)

NO

3(m

eq)

HC

O3

(meq

)10

0 ×

SO4/

Cl

T ºCpH

20.8

5/90

.28

Cel

estu

n A

PJu

n 96

6.07

1

8.67

0

.047

6.

49

32.4

9 26

.30

6.76

20.6

5/87

.94

Che

max

Jun

960.

73

7.91

0

.069

7.

31

9.19

26

.51

6.83

19.4

9/88

.59

Chu

nhuh

ub C

entr

o A

g1M

ar 9

618

.93

2.10

0.

00

4.82

90

1.43

26

.50

6.67

19.5

8/88

.59

Chu

nhuh

ub P

uebl

oM

ar 9

67.

29

4.18

0.

19

6.98

17

4.40

26

.60

6.93

20.4

9/87

.73

Cob

a ag

ua p

ot.

Jun

960.

58

5.92

0.0

46

6.98

9.

80

25.5

0 6.

85

21.2

1/87

.72

Col

. Yuc

atan

Jun

960.

63

4.99

0

.132

6.

72

12.5

6 25

.91

6.74

20.8

4/88

.53

Dzi

tas

Mar

96

0.92

2.

99

0.6

452

4.36

30

.77

26.9

0 6.

79

19.9

0/88

.94

Dzi

uche

Mar

96

7.79

2.

27

0.05

5.

62

343

.50

26.3

0 6.

75

21.4

1/87

.89

Dzo

not C

arre

tero

Jun

961.

94

19.5

7 0

.078

6.

77

9.91

26

.03

6.99

19.4

0/88

.62

El R

amon

alM

ar 9

613

.24

1.21

0.

01

5.16

10

94.2

1 26

.40

6.76

19.5

5/88

.04

Feli

pe C

ar P

uert

Mar

96

10.0

6 6.

90

0.04

6.

23

145.

80

25.8

2 6.

43

21.0

7/87

.49

Kan

tuni

lkin

Jun

960.

20

1.34

0

.028

7.

36

14.9

4 25

.40

6.86

20.6

6/89

.90

Kop

oma

Oct

95

4.65

11

.42

0.10

6.

01

40.7

2 27

.66

6.75

21.3

9/88

.15

Loch

eJu

n 96

1.02

12

.72

0.05

3 _

8.00

26

.09

6.88

20.4

8/89

.35

Mam

aO

ct 9

6_

5.33

0.

08

6.58

53

.53

26.8

7 6.

86

20.0

3/88

.28

Mel

chor

Oca

mpo

Jun

960.

33

1.62

0.

253

5.10

20

.27

27.1

7 6.

99

19.2

4/88

.55

Nue

vo I

srae

lM

ar 9

625

.06

6.05

0.

10

N.D

.41

4.21

26

.00

6.82

20.5

5/89

.86

Opi

chen

(Cen

ote)

Oct

95

1.34

2.

33

0.30

7.

31

57.5

7 28

.79

7.16

20.5

5/89

.86

Opi

chen

Oct

95

6.86

13

.75

0.15

N

.D.

49.8

8 N

.D.

N.D

.

20.5

5/89

.86

Opi

chen

Mar

96

6.09

12

.67

0.12

N

.D.

48.0

7 N

.D.

N.D

.

20.3

0/89

.41

Oxk

utzc

abO

ct 9

51.

95

4.19

0.

0661

N

.D.

46.5

0 28

.14

6.97

20.3

0/89

.41

Oxk

utzc

abM

ar 9

61.

68

4.13

0.

07

5.92

40

.68

28.6

0 6.

97

21.3

0/88

.27

Pana

baJu

n 96

0.71

10

.64

0.06

0 7.

95

6.63

26

.80

7.01

20.1

1/88

.93

Peto

Oct

95

4.06

7.

07

0.48

N

.D.

57.4

4 27

.35

6.78

20.1

1/88

.93

Peto

Mar

96

3.82

7.

16

0.50

7.

21

53.3

5 27

.70

6.82

(tab

le c

onti

nues

)

202 PERRY ET AL.

Perry et al..fm Page 202 Wednesday, July 3, 2002 2:31 PM

TAB

LE 3

. Con

tinue

d

Lat(

°N)/L

ong(

°W)

Sam

ple

nam

eC

olle

ct d

ate

SO4

(meq

)C

l(m

eq)

NO

3(m

eq)

HC

O3

(meq

)10

0 ×

SO4/

Cl

T ºCpH

20.0

4/88

.54

Sabo

nJu

n 96

1.28

4.

14

0.06

3 6.

23

30.8

5 26

.80

6.93

20.4

9/89

.59

Saca

lum

Oct

95

5.79

12

.02

0.11

29

6.19

48

.17

27.4

1 7.

14

21.2

4/87

.43

San

Ang

elJu

n 96

0.10

0.

49

0.01

3 5.

03

21.3

0 27

.08

7.27

20.3

5/88

.22

San

Ped

roJu

n 96

0.21

1.

63

0.47

4 5.

15

13.0

3 27

.51

6.82

19.9

5/88

.88

Sta.

Ros

aM

ar 9

63.

65

2.60

0.

10

5.92

14

0.38

27

.20

6.77

20.1

9/89

.27

Teka

xM

ar 9

62.

01

7.41

0.

06

6.38

27

.13

28.3

0 6.

83

20.5

2/89

.34

Teki

tO

ct 9

51.

00

3.58

0.

0548

6.

46

27.9

3 27

.26

6.95

20.6

5/89

.47

Telc

haqu

illo

Oct

95

0.67

4.

06

0.18

55

6.72

16

.50

27.4

3 6.

86

20.2

4/88

.26

Tepi

chJu

n 96

0.30

3.

37

0.05

6 5.

84

8.82

26

.50

6.97

20.4

0/89

.53

Ticu

lO

ct 9

63.

48

6.57

0.

1113

N

.D.

52.9

0 28

.51

6.85

20.4

0/89

.53

Ticu

lM

ar 9

62.

79

6.06

0.

10

5.52

46

.04

28.9

0 6.

91

20.1

5/89

.22

Ticu

mM

ar 9

61.

33

4.57

0.

1613

5.

84

29.1

0 28

.30

6.96

20.1

9/88

.36

Tiho

suco

Jun

960.

35

4.02

0.

047

6.00

8.

75

26.4

3 6.

93

20.5

4/88

.27

Tixc

acal

cupu

lJu

n 96

0.43

2.

83

0.25

6 4.

65

15.0

0 N

.D.

N.D

.

21.1

4/88

.16

Tizi

min

Jun

960.

51

6.35

0.

240

6.54

8.

04

27.0

0 6.

84

20.0

7/89

.04

Tzuc

acab

Mar

96

11.1

2 10

.94

0.05

5.

46

101.

65

28.3

0 6.

83

20.6

8/88

.21

Vall

adol

id S

an P

edM

ar 9

60.

40

4.51

0.

0871

7.

18

8.76

27

.10

6.60

20.6

8/88

.21

Vall

adol

id P

PM

ar 9

60.

37

4.01

0.

1032

6.

92

9.36

27

.70

6.70

20.8

5/88

.16

Xca

n (C

enot

e)Ju

n 96

0.47

4.

22

0.08

6 6.

10

11.1

3 25

.50

6.83

20.8

0/87

.70

Xca

n2Ju

n 96

0.92

8.

56

0.10

7 6.

52

10.7

2 25

.74

6.87

20.8

3/89

.19

Xoc

chel

Jun

960.

57

5.68

0.

055

7.25

10

.04

27.4

4 6.

83

20.1

9/89

.38

Xoh

uaya

nM

ar 9

62.

00

4.37

0.

1516

5.

97

45.7

7 30

.40

6.90

20.8

2/89

.74

Xte

pen

Mar

96

0.90

9.

24

0.07

6.

59

9.74

27

.70

6.82

21.4

2/88

.02

Xyo

hact

unJu

n 96

1.55

15

.94

0.05

3 8.

28

9.72

26

.30

6.77

20.7

5/89

.72

Yaxc

opoi

lM

ar 9

60.

92

9.37

0.

07

6.85

9.

82

28.8

0 6.

84

20.9

4/87

.86

Yoco

tzon

otJu

n 96

0.40

1.

90

0.14

5 5.

21

21.2

1 25

.69

7.65

1 Sam

ple

take

n fr

om a

wel

l tha

t is

not a

ctiv

ely

pum

ping

.

KARST AQUIFER SYSTEM 203

Perry et al..fm Page 203 Wednesday, July 3, 2002 2:31 PM

noted in Table 2, pH and temperature of four sam-ples are estimated. Two of these are deep samplescollected in a stainless steel bottle that requiredhours to retrieve, making direct measurementimpossible.

In five cases, unconventional valving systems atthe well site made it impossible to measure or col-lect water directly from a well. In these cases(Abala, Homun, Mama, San Jose Tipceh, and Uxmal[Fig. 1]), samples were taken from leaks around thepump bearing (Mama, Homun, and San JoseTipceh), a bucket (Abala, June 1993), or a storagetank containing unchlorinated water (Uxmal). Thelatter samples may have anomalously high pH val-ues (reflecting evaporation and CO2 loss). As dis-cussed in a subsequent section, saturation indices ofcarbonate phases in some or all of these five sam-ples may be unreliable; however, there is no evi-dence that relations between non-carbonate ionswere significantly modified.

All water samples collected between 1993 andMarch 1995 were passed through a 0.2 micron filterand collected in polyethylene bottles that had previ-ously been washed in 5 or 10% nitric acid. Thesebottles, after washing and rinsing, were stored full ofdeionized water that was decanted in the field.Finally, the bottles were rinsed with sample waterbefore filling. Three separate bottles of each filteredsample (for alkalinity, anions, and cations) wererefrigerated in an ice cooler during transportation.Alkalinity determinations were normally donewithin 24 hours (by Gran titration, using a Hachpipette and 1.6 N H2SO4). Because of the need totransport samples by air, cation samples wereuntreated until they arrived in DeKalb, Illinois; thenfilled bottles used for cation analysis were acidifiedwith concentrated nitric acid to pH 2. These acidi-fied samples were left in their original full bottles fora minimum of two weeks before analysis to allow anyprecipitated carbonate to redissolve. Chloride andsulfate were determined by ion exchange chroma-tography using a Dionex anion suppressor and aconductivity cell. Cation analyses (Na, K, Ca, Mg,and Sr) were made in a Beckman V DC plasma spec-trometer. For all ions except potassium, analyticalprecision is approximately ± 5% or better. Somepotassium analyses may have errors of ± 10%. Allcomplete analyses reported here (Table 2) have ionbalances equal to or better than 5%.

Two procedural modifications were made forsamples collected between October 1995 and June1996: (1) some samples were collected by injecting

the sample through a 0.2 micron filter into 7 or 10ml “Vacutainers,” and (2) samples for NO3

– analysiswere collected in bottles rinsed with dilute HClinstead of HNO3. Vacutainers are commerciallyavailable evacuated test tubes that are fitted withrubber septa; they are widely used for taking bloodsamples; they are light and easily transported; bestof all, they are readily available in a number of phar-maceutical supply houses in Yucatan. Duplicateanalysis of samples taken in Vacutainers and inwashed polyethylene bottles gave essentially identi-cal results. Anion analyses from the most recentsampling cycle were performed with a Dionex 500ion exchange chromatograph fitted with an AS4A-SC exchange column.

Discussion

Sources of ions

We discuss here geochemical characteristics ofgroundwater and probable controls on groundwatergeochemistry. Natural processes that affect northernYucatan groundwater include: (1) mixing with sea-water; (2) dissolution/precipitation of CaCO3 andCaMg(CO3)2; (3) dissolution or precipitation ofphases other than CaCO3 and CaMg(CO3)2 (mostimportantly CaSO4/CaSO4⋅2H2O, SrSO4, and per-haps NaCl); (4) evapotranspiration/precipitation(essentially, loss or gain of nearly pure water); and(5) (locally) redox reactions, particularly sulfatereduction. These processes can be modified byhuman activity such as: (1) pumping water for agri-cultural, municipal, or industrial applications; (2)adding organic and inorganic contaminants, eitherat the surface or by injection; and (3) altering theland surface through construction or through substi-tution of one type of vegetation for another. Table 4summarizes sources and sinks for ions in groundand surface waters of the Yucatan Peninsula.

Seawater mixing and redox reactions

To a first approximation, Cl is a conservativetracer of the contribution of the saline intrusion ofthe Yucatan Peninsula to groundwater ion content ofthe freshwater lens over an extensive area (Stoessellet al., 1989; Moore et al., 1992; Perry et al., 1995).Our data (Tables 2 and 3) show that values close tothe seawater ratio of SO4/Cl are found in groundwa-ter of the freshwater lens throughout much of thenorthern Peninsula. For easy visualization, the sul-fate/chloride ratio is expressed as 100 × (SO4/Cl), inequivalents, which, for average seawater is 10.3.

204 PERRY ET AL.

Perry et al..fm Page 204 Wednesday, July 3, 2002 2:31 PM

TABLE 4. Chemical Sources and Sinks

Ion Source/sink Evidence

Cl in shallow groundwater and surface water

Source 1: Seawater (from saline intrusion)

A. Saline intrusion is present under much of the northern Peninsula. B. The Yucatan aquifer is highly permeable. C. 100 × SO4/Cl in groundwater approxi-mates the seawater value (10.3) over a wide area inland from coast, especially inside Ring of Cenotes. D. Na/Cl ≈ 1.17 (seawater value) in many shallow groundwater samples.

Source 2: Halite in evaporite

Southern and eastern groundwater samples in which Na/Cl ≈ 1. These same samples have 100 × SO4/Cl >> 10.3.

SO4 in shallow groundwater and surface water

Source 1: Seawater (from saline intrusion)

A. Saline intrusion is present under much of the northern Peninsula. B. The Yucatan aquifer is highly permeable. C. 100 × SO4/Cl is close to seawater value (10.3) over a wide area inland from coast, especially inside Ring of Cen-otes.

Source 2: Gypsum and/or anhydrite in evaporite

A. Abundant gypsum and anhydrite occur in Chicxulub impact breccia and overlying Tertiary evaporite. B. 100 × SO4/Cl variable and occasionally >> 10.3 in S and SE of study area. C. Presence of gypsum-saturated water in Lake Chichancanab and near-saturation of water in L. Esmeralda.

Sink: Redox reactions such as 2CH2O+SO4

– 2 = H2S + 2HCO3

–

Documented in water of deep cenotes by Socki (1984), Socki et al. (1984), and Stoessell et al. (1993). Not common in groundwater over much of the northern Peninsula as attested by 100 × SO4/Cl ≥ 10.3. (Values of 100 × SO4/Cl < 10.3 near the coast where water table is close to the land surface suggest redox reactions with water-saturated plant remains.)

H2S, particu-larly in deep cenotes

Source: Redox reactions(as above)

As for sulfate sink above. H2S present locally below saline interface. Sulfur isotope determinations in Cenote Xcolak by Socki (1984) show strong 34S enrichment in SO4 of bottom water.

Sr in shallow and deep groundwater and surface water

Source 1: Seawater (from saline intrusion)

Because Cl, SO4, and other ions in the fresh water lens and saline intrusion come from seawater, seawater must also be a source for Sr. It is a minor source as shown by Sr/Cl ratios that are variable but, in all cases, greater than the sea-water ratio.

Source 2: Celestite in evaporite.

In virtually all water with a high content of SO4, there is a strong correlation (R2>0.9) between Sr and SO4. Water in Lake Chichancanab and deep, saline water sampled within the breccia layer of UNAM5 are each approximately sat-urated with respect to celestite.

Source 3: Aragonite Aragonite typically contains 7000–9400 ppm Sr. Even groundwater that has a low concentration of SO4 has a higher Sr/Cl ratio than seawater. A logical source is aragonite (or other carbonate mineral).

Na Sources: Saline intrusion and halite

As for Cl.

Mg Source 1: Saline intrusion

Because Cl, SO4, and other ions in the fresh water lens and saline intrusion come from seawater, seawater must also be a source for Mg.

Source 2: CaMg(CO3)2, and “dedolomitization”

A. Most groundwater is near saturation with respect to dolomite. B. Mg is cor-related with SO4 in high-sulfate waters suggesting the reaction: Ca++ + CaMg(CO3)2 = Mg++ + 2CaCO3.

Ca Source 1: Saline intrusion

Because Cl, SO4, and other ions in the fresh water lens and saline intrusion come from seawater, seawater must also be a source for Ca.

Source 2: Minerals CaCO3, CaMg(CO3)2, and CaSO4 (gypsum and/or anhydrite)

A. Most groundwater is near saturation with respect to calcite and dolomite. B. Ca is correlated with SO4 in high-sulfate waters and is highest in waters with high sulfate.

Sink Dedolomitization in groundwater that has dissolved gypsum/anhydrite (refer to Source 2 of Mg).

KARST AQUIFER SYSTEM 205

Perry et al..fm Page 205 Wednesday, July 3, 2002 2:31 PM

Groundwater ratios, plotted in Figure 2, indicatethat mixing with the saline intrusion is the mostimportant source of ions in fresh groundwater over awide area. Of 129 fresh groundwater samples evalu-ated, mostly from municipal wells, 100×(SO4/Cl) isless than the seawater value by 10% or more in only11 cases, and in only four cases is this ratio lowerthan the seawater ratio by 20% or more. Twenty-nine samples, by far the largest sample group, fall inthe range between 9 and 14, close to the seawatervalue. Ratios higher than seawater are, of course, tobe expected where gypsum/anhydrite dissolution isinvolved (Fig. 2).

Ratios of (SO4)/Cl lower than the seawater valuecan be attributed to sulfate reduction. One impor-tant but restricted class of samples in which redoxreactions are important is comprised of deep cenotesthat extend through the saline interface (Socki et al.,1984; Stoessell et al., 1993). Tropical vegetationsinks through the water column of these sinkholesand decays in the presence of sulfate-rich salinewater. The result is depletion of oxygen in the lower,unmixed saline layer, followed by sulfate reductionand production of H2S and HS-. This is a distinctlylocal phenomenon that probably does not accountfor low ratios observed in the 11 water samples inTables 2 and 3. Those samples are from near thecoast, exactly where seawater mixing would beexpected to be most important. Precisely becausethey are from near the coast, they occur in shallowwells where the fresh water lens is near the surface,in close contact with plant roots that can supplyorganic matter for sulfate reduction within the freshwater lens itself. Thus, we conclude that the SO4/Clratio is a valuable indicator of seawater mixing butthat it must be evaluated with some caution.

Saturation indices and mineral dissolution

Saturation indices (SIs) of appropriate mineralsare useful in evaluating the extent to which waterchemistry is controlled by equilibrium with solidphases. SIs [log(Q/K)] for Yucatan groundwater andsurface water (Table 5) were calculated using thecomputer program PHREEQC (Parkhurst, 1995).There is no general agreement as to the appropriateequilibrium constant for dolomite; we use the valueof K = 10–17.15, which is the PHREEQC defaultvalue.

Surface waters, except for the coastal springBaleocera, are supersaturated with respect to bothcalcite and dolomite (Table 5). In contrast, 61 of 73groundwater samples, both shallow and deep, have

calcite SIs within ±0.24 of saturation. Of the eightshallow groundwater samples undersaturated bymore than 0.20, all except one were sampled duringrelatively cool weather in October through March.Because the rainy season on the Peninsula typicallybegins in June and ends in September, some or all ofthese samples could contain a component of mete-oric precipitation that had not yet equilibrated withthe aquifer. Of the eight groundwater samples super-saturated by more than 0.2, six were collected in thehot month of June, and five are from localities previ-ously mentioned where it is known that sampleevaporation took place. The median SI of calcite forall 83 groundwater and surface samples is 0.05, andthe mean value for all 68 shallow groundwater anal-yses is 0.00.

For all groundwater samples except the two withlowest Mg concentration (1.1 and 1.4 meq/l, Table2), an excellent correlation exists between dolomitesaturation and calcite saturation:

SIdolomite = 2.01SIcalcite (R2 = 0.91).

The most obvious reason that dolomite and cal-cite saturation should be related by a factor ofexactly two is through the pH measurement, whichaffects the calculated (CO3

–2)/(HCO3–) of the sam-

ple. There is a squared dependence on carbonateion in the solubility product constant of dolomite.Covariation of SIdolomite with SIcalcite suggests thatmuch of the scatter in SI data may be an artifact ofsampling that arises through evaporation duringsample collection and/or through pH measurementerror that we were unable to eliminate. Thus, weconclude that most groundwater from the northernYucatan Peninsula is precisely at equilibrium bothwith low magnesium calcite and with dolomite.

Most Peninsular groundwater and surface wateris strongly undersaturated with respect to gypsumand celestite (SrSO4). The two exceptions where sat-uration or near-saturation is known to occur aredeep groundwater within the saline intrusion andsurface water from Lake Chichancanab.

Groundwater within the saline intrusion, sam-pled at 300 m and at 400 m in well UNAM5 (Fig. 1),is precisely saturated with respect to both gypsumand celestite (Table 5). Ejecta from the ChicxulubImpact is present in this well at all depths below 332m, and this breccia, which contains abundantgypsum and anhydrite (Urrutia-Fucugauchi et al.,1996; Rebolledo-Vieyra et al., 2000; Ward, Mihai

206 PERRY ET AL.

Perry et al..fm Page 206 Wednesday, July 3, 2002 2:31 PM

TAB

LE 5

. Sat

urat

ion

Indi

ces

Lat (

o N)/

Long

(o W)

Sam

ple

nam

eC

olle

ct d

ate

Gyp

sum

Anh

ydri

teC

eles

tite

Cal

cite

Ara

goni

teD

olom

ite

Wel

ls o

r cl

osed

cen

otes

20.6

4/89

.68

Aba

laA

ug 9

4–1

.30

–1.5

1–1

.40

–0.0

4–0

.18

–0.1

0

20.6

4/89

.68

Aba

laJu

n 93

–1

.48

–1.6

9–1

.73

0.22

0.08

0.40

20.6

4/89

.68

Aba

laM

ar 9

6–1

.25

–1.4

5–1

.31

–0.1

0–0

.24

–0.2

3

20.6

4/89

.68

Aba

laO

ct 9

5–1

.29

–1.5

0–1

.38

0.34

0.20

0.64

18.7

4/88

.45

Bac

alar

Pue

blo

Mar

96

–2.3

9–2

.60

–2.5

50.

02–0

.12

–0.3

7

19.8

8/89

.22

Bec

anch

enM

ar 9

6–1

.51

–1.7

1–1

.24

–0.0

8–0

.22

–0.1

3

21.2

0/88

.79

Buc

tzot

zA

pr 9

4 –2

.37

–2.5

8–2

.90

–0.1

4–0

.28

–0.2

6

20.2

0/89

.33

Can

ekN

.D.

–1.6

7–1

.88

–1.9

9–0

.05

–0.1

9–0

.18

19.9

0/88

.95

Cat

mis

Mar

96

–0.4

7–0

.68

–0.4

9–0

.04

–0.1

8–0

.21

20.8

5/90

.28

Cel

estú

n M

ar 9

5–1

.20

–1.4

1–1

.26

0.08

–0.0

60.

22

20.8

5/90

.28

Cel

estú

n M

ar 9

6–1

.07

–1.2

9–1

.17

–0.2

2–0

.36

–0.4

2

20.8

5/90

.28

Cel

estú

n A

pr 9

4 –1

.09

–1.3

0–1

.21

–0.0

6–0

.20

–0.0

9

20.8

5/90

.28

Cel

estú

n D

ec 9

3 –1

.07

–1.2

8–1

.19

–0.1

7–0

.31

–0.3

5

20.9

6/88

.60

Cen

otil

loM

ar 9

5–2

.06

–2.2

7–2

.50

–0.0

4–0

.18

–0.3

4

20.7

4/89

.83

Cho

chol

aO

ct 9

5–1

.47

–1.6

8–1

.65

–0.2

2–0

.36

–0.3

9

20.7

4/89

.83

Cho

chol

aA

ug 9

4 –1

.43

–1.6

4–1

.66

0.05

–0.0

90.

11

20.0

3/89

.17

Col

oke

Aka

lM

ar 9

6–1

.96

–2.1

6–2

.45

–0.1

4–0

.28

–0.5

3

20.8

6/90

.21

Cru

z, C

eles

tun

Dec

93

–1.6

9–1

.90

–1.8

5–0

.21

–0.3

5–0

.61

21.2

7/88

.93

Dzi

lam

de

Gon

zale

zA

pr 9

4 –2

.18

–2.3

9–2

.64

–0.0

6–0

.20

–0.4

0

20.7

5/89

.28

Hom

unJu

n 93

–2

.32

–2.5

3–2

.89

0.76

0.62

1.50

20.8

6/90

.40

Hot

el G

utie

rrez

Mar

95

–1.1

4–1

.36

–1.2

00.

140.

000.

47

20.7

3/89

.17

Huh

iJu

n 93

–2

.30

–2.5

2–2

.72

0.23

0.09

0.48

20.6

8/90

.46

Isla

Are

naM

ar 9

5–1

.11

–1.3

2–1

.25

0.05

–0.0

90.

22

20.9

4/89

.04

Izam

alM

ar 9

5–2

.50

–2.7

1–3

.10

–0.0

4–0

.18

–0.2

5

20.7

9/89

.03

Kan

tuni

lA

pr 9

4 –2

.27

–2.4

8–2

.64

–0.2

4–0

.38

–0.4

3

20.9

2/89

.98

Kin

chil

Dec

93

–1.7

6–1

.97

–2.2

10.

01–0

.13

–0.0

9

20.6

2/90

.16

Koc

hol

Dec

93

–1.1

7–1

.38

–1.3

1–0

.06

–0.2

0–0

.18

20.6

2/90

.16

Koc

hol

Aug

94

–1.1

8–1

.38

–1.3

2–0

.01

–0.1

5–0

.09

KARST AQUIFER SYSTEM 207

Perry et al..fm Page 207 Wednesday, July 3, 2002 2:31 PM

20.6

6/89

.90

Kop

oma

Sep

93

–1.0

4–1

.25

–1.1

40.

03–0

.11

0.11

20.6

6/89

.90

Kop

oma

Jun

93

–1.0

9–1

.30

–1.2

1–0

.09

–0.2

3–0

.12

20.5

6/89

.46

Mah

zuci

lO

ct 9

5–1

.56

–1.7

7–1

.61

–0.0

2–0

.16

–0.0

6

20.4

8/89

.35

Mam

aJu

n 93

–1

.34

–1.5

5–1

.40

0.22

0.08

0.35

20.3

8/89

.39

Man

iJu

n 93

–1

.20

–1.4

1–1

.29

0.15

0.01

0.27

20.5

8/90

.03

Max

canu

Aug

94

–1.5

1–1

.71

–1.8

2–0

.02

–0.1

6–0

.08

20.5

8/90

.03

Max

canu

Dec

93

–1.6

1–1

.82

–1.9

5–0

.27

–0.4

1–0

.61

20.6

2/89

.61

Muc

uych

eO

ct 9

5–1

.95

–2.1

6–2

.65

0.16

0.02

0.10

20.5

5/89

.86

Opi

chen

Dec

93

–1.0

3–1

.24

–1.1

80.

12–0

.02

0.16

20.5

5/89

.86

Opi

chen

Aug

94

–1.0

3–1

.24

–1.0

7–0

.14

–0.2

8–0

.20

20.5

5/89

.86

Opi

chen

Jun

93

–1.0

1–1

.21

–1.0

80.

08–0

.06

0.19

20.3

0/89

.41

Oxk

utzc

abSe

p 93

–1

.57

–1.7

8–1

.84

–0.0

0–0

.14

–0.0

4

20.1

1/88

.93

Peto

Sep

93

–1.2

2–1

.43

–0.8

8–0

.03

–0.1

7–0

.07

20.1

1/88

.93

Peto

Mar

95

–1.2

7–1

.49

–0.9

30.

13–0

.01

0.24

20.8

7/88

.63

Qui

ntan

a R

ooM

ar 9

5–2

.16

–2.3

8–2

.52

0.19

0.05

0.18

20.4

9/89

.59

Saca

lum

Jun

93

–1.1

3–1

.34

–1.2

60.

07–0

.07

0.17

20.4

9/89

.59

Saca

lum

Aug

94

–1.1

0–1

.31

–1.2

2–0

.02

–0.1

6–0

.06

20.8

8/89

.89

Sam

ahil

Mar

96

–1.9

4–2

.15

–2.4

2–0

.21

–0.3

5–0

.53

20.8

3/89

.98

San

Ant

onio

Ted

zidz

Dec

93

–1.4

4–1

.65

–1.6

9–0

.33

–0.4

7–0

.60

20.1

8/89

.66

Sayi

lD

ec 9

3 –2

.05

–2.2

6–2

.77

–0.1

6–0

.30

–0.4

8

21.1

2/89

.73

Sier

ra P

apac

alA

ug 9

4 –1

.84

–2.0

5–2

.17

–0.0

6–0

.20

–0.1

5

20.4

6/89

.67

San

Jose

Tip

ceh

Dec

93

–0.9

4–1

.15

–1.0

50.

260.

120.

49

20.6

0/89

.01

Sotu

taJu

n 93

–2

.12

–2.3

3–2

.65

0.35

0.21

0.54

20.3

3/89

.64

Sta.

Ele

naD

ec 9

3 –0

.96

–1.1

7–0

.99

0.03

–0.1

10.

02

20.3

3/89

.64

Sta.

Ele

naM

ar 9

5–1

.04

–1.2

4–1

.07

0.04

–0.1

00.

03

20.6

0/90

.11

Sant

o D

omin

goD

ec 9

3 –1

.82

–2.0

2–2

.26

–0.2

3–0

.37

–0.8

4

20.7

3/89

.48

Teco

hA

ug 9

4 –2

.28

–2.4

8–2

.72

–0.1

7–0

.31

–0.2

9

20.7

3/89

.48

Teco

h Ju

n 93

–2

.22

–2.4

3–2

.73

0.25

0.11

0.50

20.1

9/89

.27

Teka

xSe

p 93

–1

.55

–1.7

6–1

.86

–0.0

2–0

.16

–0.0

6

(tab

le c

onti

nues

)

208 PERRY ET AL.

Perry et al..fm Page 208 Wednesday, July 3, 2002 2:31 PM

Lat (

o N)/

Long

(o W)

Sam

ple

nam

eC

olle

ct d

ate

Gyp

sum

Anh

ydri

teC

eles

tite

Cal

cite

Ara

goni

teD

olom

ite

20.5

3/89

.33

Teki

tSe

p 93

–1

.94

–2.1

5–2

.00

0.06

–0.0

80.

07

20.6

5/89

.46

Telc

haqu

illo

Jun

93

–2.1

8–2

.39

–2.8

40.

10–0

.04

0.21

21.1

4/88

.94

Tem

axA

pr 9

4 –2

.63

–2.8

4–3

.18

–0.1

2–0

.26

–0.5

1

20.6

9/89

.65

Tem

ozon

Sur

Mar

96

–1.9

1–2

.12

–2.2

2–0

.12

–0.2

6–0

.17

20.4

0/89

.53

Ticu

lD

ec 9

3 –1

.26

–1.4

6–1

.48

–0.1

4–0

.28

–0.4

2

20.4

0/89

.53

Ticu

lJu

n 93

–1

.39

–1.5

9–1

.53

0.05

–0.0

90.

13

20.9

0/88

.75

Tunk

asA

pr 9

4 –2

.18

–2.3

9–2

.59

–0.0

1–0

.15

–0.2

5

20.0

7/89

.04

Tzuc

acab

Mar

95

–0.7

2–0

.93

–0.7

50.

200.

060.

28

20.0

7/89

.04

Tzuc

acab

Sep

93

–0.7

4–0

.95

–0.7

9–0

.05

–0.1

9–0

.24

20.8

7/89

.73

Um

anM

ar 9

6–2

.15

–2.3

5–2

.72

–0.1

7–0

.31

–0.8

4

20.3

7/89

.78

Uxm

alD

ec 9

3 –0

.91

–1.1

1–0

.93

0.03

–0.1

10.

03

20.8

3/89

.19

Xoc

chel

Mar

95

–2.1

3–2

.34

–2.5

0–0

.08

–0.2

2–0

.24

Ope

n w

ater

18.6

5/88

.41

Azu

l Cen

ote

Mar

96

–0.2

2–0

.43

–0.2

30.

680.

540.

98

20.8

8/90

.36

Bal

deoc

era

Mar

95

–0.6

0–0

.80

–0.6

2–0

.24

–0.3

80.

26

N.D

./N.D

.C

anal

Mar

95

–1.2

0–1

.42

–1.2

90.

570.

431.

14

20.8

3/90

.38

Cel

estu

n, D

umac

Mar

95

–0.7

1–0

.92

–0.7

4–0

.12

–0.2

70.

55

20.8

5/90

.36

Cel

estu

n, E

ster

o P

ozo

Mar

95

–1.1

0–1

.31

–1.1

50.

170.

030.

50

19.8

8/88

.77

Chi

chan

cana

b M

ar 9

60.

03–0

.19

0.14

0.91

0.77

1.69

19.8

8/88

.77

Chi

chan

cana

b M

ar 9

50.

07–0

.14

0.19

1.23

1.09

2.35

20.5

5/90

.43

El R

emat

eM

ar 9

5–1

.11

–1.3

3–1

.28

0.16

0.02

0.28

19.1

4/88

.17

Noh

bec

Lak

eM

ar 9

6–1

.99

–2.2

1–2

.28

0.75

0.61

1.12

Dee

p sa

mpl

es

N.D

./N.D

.U

NA

M2,

300

mM

ar 9

5–0

.26

–0.4

5–0

.17

0.08

–0.0

60.

68

N.D

./N.D

.U

NA

M2,

300

m

Aug

94

–0.2

4–0

.42

–0.1

3–0

.17

–0.3

10.

19

N.D

./N.D

.U

NA

M2,

350

m

Aug

94

–0.2

5–0

.42

–0.1

30.

11–0

.03

0.79

N.D

./N.D

.U

NA

M5,

300

mM

ar 9

5–0

.05

–0.2

2–0

.04

0.06

–0.0

80.

50

N.D

./N.D

.U

NA

M5,

300

m

Aug

94

0.04

–0.1

30.

07–0

.10

–0.2

40.

15

N.D

./N.D

.U

NA

M5,

400

mA

ug 9

4 0.

09–0

.09

0.09

0.16

0.02

0.62

KARST AQUIFER SYSTEM 209

Perry et al..fm Page 209 Wednesday, July 3, 2002 2:31 PM

Lefticariu, and Perry, 2001, unpubl.), is an obviouspossible source for SO4 and Ca.1

Water from Lake Chichancanab is saturated withrespect to both gypsum and celestite (Table 5). LakeChichancanab and its sister lake, Esmeralda, arecharacterized by gypsum precipitated around theirshores. In the past, at somewhat higher lake levels,these two lakes were merged (Hodell et al., 1995).Hodell et al. (2001) reported that gypsum precipita-tion from Lake Chichancanab is a sensitive climaticindicator and that sediment core from this lake con-tains a 2,600-year climate record. The unique char-acter of the water of Lakes Chichancanab andEsmeralda implies that the groundwater rechargethat feeds them must come into direct contact withabundant gypsum/anhydrite, probably along thesteep eastern shore of the lakes, which may be afault, upthrust to the east.

The mineral source of SO4, Ca, and Sr for LakeChichancanab water may be Eocene evaporite(Lopez Ramos, 1973) and/or Chicxulub impactbreccia. Traces of celestite (SrSO4) have been iden-tified in core from UNAM6 by Mihai Lefticariu(unpubl.) but not yet in the gypsum that precipitatesalong the shores of Lake Chichancanab. Neverthe-less, celestite saturation in water of that lake and thecorrelation of Sr with sulfate in groundwater andsurface water of the Peninsula (discussed below)make it clear that celestite, present in evaporite orevaporitic breccia, is a major source of Sr in thesewaters. We shall show that sulfate and Sr are impor-tant natural tracers for groundwater movement, par-ticularly in the southeastern and the westernmostparts of the study area.

Correlation of Sr and sulfate and the sources of these ions

Sulfate-rich groundwater (with high ratios of 100 ×SO4/Cl) radiates south and west from the vicinity ofLake Chichancanab, which, to recapitulate, lies on agroundwater divide (Fig. 2). In this groundwater, anexcellent correlation exists between Sr and SO4,given by the equation (excluding Peto) (Fig. 3A):

(Sr) = 0.0063(SO4) + 0.015 r 2 = .90.

If the surface waters of Lake Chichancanab andCenote Azul (Fig. 1) are included, the equationbecomes:

(Sr) = 0.00664(SO4) + 0.0136 r2 = .98. (1)

This relation, as well as the lack of significantcorrelation of Sr with Cl and the uniform excess ofSr/Cl compared to seawater (Fig. 3B), indicates thatthe high Sr values in Peninsular groundwater arefrom dissolving evaporite.

Equation (1) fails for some water with very lowsulfate concentration (Fig. 3A), indicating one ormore minor additional sources of Sr that areswamped at high SO4 concentration. Calcite andaragonite are probable Sr contributors. Aragonite,an abundant component of marine carbonate sedi-ments, can selectively incorporate several timesmore strontium than calcite: typically 7000–9400ppm in aragonite produced inorganically or by cor-als, compared to 1000 ppm in calcite (Veizer, 1978,1983; Morse and McKenzie, 1990). Lime muds andmost molluscs contain 4500 ppm or less of Sr (Morseand McKenzie, 1990).

We use a factor, F(Sr) to explore the source of Srin the groundwaters of low TDS where water chemis-try is dominated by carbonate dissolution and sea-water mixing:

F(Sr) is an estimate (in ppm) of the Sr content ofthe putative carbonate that dissolved to provide theobserved Ca content of a given groundwater.2 Chlo-ride content of the water is used to correct values ofmCa and mSr for seawater contribution, and SO4 isused to correct for gypsum contribution. Twenty-twoof the samples for which complete analyses areavailable also have SO4/Cl ratios close to seawater.The chemistry of these waters presumably is notappreciably influenced by evaporite dissolution.These 22 samples have F(Sr) values ranging from

1Evaporite layers are present below 173 m in this well(Rebolledo-Vieyra et al., 2000), but groundwater associatedwith these could not be sampled because the well is cased to350 m.

2The numbers 0.00033, 0.038, and 0.103 in Equation (2) are,respectively, the chemical equivalent ratios of Sr, Ca, and SO4to Cl in seawater.

F Sr( )

87.6 2mSr0.00033mCl

–( ) 4× 105××

40 × 2mCa0.038mCl

–( ) 2mSO4

0.103mCl–

–-------------------------------------------------------------------------------------------------------------------.

=

(2)

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1640 (Telchaquillo) to 5350 ppm (Bacalar Pueblo),with an average value of 2625 ppm. This intermedi-ate range of Sr values suggests that both aragoniteand calcite are dissolving and releasing Sr to theaquifer, a conclusion consistent with petrographicobservations. This conclusion is reinforced by thefact that the highest value (Bacalar Pueblo) is from awell in Pleistocene rock likely to contain apprecia-ble aragonite, whereas the low value, at Telchaqui-llo, is in Eocene rock that, presumably, hasundergone diagenetic recrystallization for a muchlonger time.

Groundwater chemistry conclusively demon-strates that the main source(s) of high sulfate and

strontium in the freshwater lens of the Yucatan aqui-fer must be shallow, specifically that it must residein or above the freshwater lens. Evidence comesfrom the composition of saline groundwater in con-tact with evaporitic breccia at 400 m in wellUNAM5 (Tables 2 and 5). That water is approxi-mately saturated with respect to gypsum and has asulfate content of 86 meq/l, yet its value for100(SO4/Cl) is 15, only 1.4 times the seawater ratioand 140 times less than the ratio in Cenote Azul. Ifwater below the saline interface became saturatedwith respect to gypsum and subsequently mixedwith water of the freshwater lens, the resulting sul-fate/chloride ratio would be only slightly greater

FIG. 3. A. Strontium versus sulfate (meq/kg). Except for Peto (verified and unexplained) an excellent correlationexists for strontium versus sulfate (R2 = 0.9). The mixing line is shown. B. Strontium versus chloride (meq/kg) in ground-water. The strontium content of groundwater does not correlate with chloride content and is much higher than it would beif seawater mixing were the major source of strontium. The mixing line is shown.

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than the seawater value. This evidence is directlyopposed to the idea that a possible source of sulfatefor the high sulfate/chloride ratios observed in thewestern Cenote Ring could be gypsum of impactbreccia lying below the saline interface, as was sug-gested by Pope et al. (2001). Note that Sr is some-what more sensitive as a tracer of ions derived fromdissolution of evaporites than is sulfate. The Sr/Clratio (in equivalents) of the same UNAM5 water is1.27 × 10–3, approximately 4 times the ratio in sea-water; nevertheless, that is still 120 times less thanthe ratio in Cenote Azul.

At least one shallow source of sulfate and stron-tium is available: the water of Lake Chichancanab,which is saturated with respect to both gypsum andcelestite. Furthermore, the pattern of sulfate/chlo-ride ratios in Figure 2 indicates that a groundwatersource is located near this lake.

The constraint that the evaporite source must beshallow is significant because, although Tertiaryevaporite and evaporite-bearing K/T boundary brec-cia are ubiquitous outside the Cenote Ring, theyhave only been reported at depths greater than 170m. Therefore, a breccia source for groundwater sul-fate would require the existence of an unreportedstructural feature such as the fault we have postu-lated on the east bank of Lakes Esmeralda andChichancanab.

Rapid weathering combined with lush tropicalflora makes surface geological mapping difficult onthe Yucatan Peninsula, and the surface expressionof an evaporite horizon is problematic because rain-fall is sufficient to suppress outcrops of gypsum.Thus, direct confirmation of the source of groundwa-ter sulfate may require drilling to obtain rock cores.In support of participation of the K/T breccia, wenote that this breccia is likely to be accompanied bya clay-rich altered suevite layer (Urrutia-Fucugau-chi et al., 1996; Rebolledo-Vieyra et al., 2000), andthe presence of such a layer of low permeabilitycould explain the presence of swampy regions and ofsurface water east and south of Lake Chichancanab.

Evidence for a deep cavern system

The Pockmarked Terrain in the north central partof the Yucatan Peninsula, directly east of the Chic-xulub Impact Basin, contains the greatest density ofcenotes on the Peninsula (Hildebrand et al., 1995).These karst features must reflect specific weather-ing conditions and processes. Among these, changesin sea level, which has been dramatically lower inthe past (Fairbanks, 1989) and extensive Paleogene

weathering outside the Chicxulub Basin (Pope et al.,1996; McClain, 1997) are well known, and thedirect correlation of the groundwater table with sealevel beneath Yucatan was documented by Perry etal. (1995). We propose here that the presence of sul-fate-bearing impact breccia at shallow depth and thedistinctive qualitative differences that are associ-ated with weathering processes between sulfate andcarbonate minerals may also be key geomorphic fac-tors. Karst processes in sulfate rocks are signifi-cantly different from those occurring in carbonaterocks because gypsum and anhydrite are more solu-ble than calcite; and dissolution of sulfate mineralsis independent of pH. In consequence, dissolutionof sulfate in groundwater is less dependent on posi-tion of a rock unit with respect to the water table orthe saline interface and relatively more dependenton groundwater movement than is the case with car-bonate dissolution.

The Pockmarked Terrain was exposed to exten-sive Paleogene weathering, particularly during theOligocene (Pope et al., 1996; McClain, 1997). TheK/T boundary breccia occurs at a depth of less than200 m below the surface in PEMEX well Y4 (Fig. 1),near the Pockmarked Terrain (Ward et al., 1995).Thus, it is reasonable that in the Pockmarked Ter-rain, sulfate of the boundary breccia was exposed tomoving groundwater during times of low seastand.

Information about deep cavern permeability is tobe expected from deep cenotes. Three cenoteswithin the Pockmarked Terrain that have been stud-ied are deep enough to pass through the freshwaterlens and extend into the saline intrusion. These areXcolak (with a 120 m water column as measured bya line dropped from the surface), Ucil (105 m), andLabon (65 m) (Fig. 1). At the time of this study,Labon was heavily contaminated by a nearby cattlefeedlot and was not studied beyond confirmation ofa fresh-saline water interface. At a water depth of 50m, Xcolak has a sharp interface between well-mixedfresh water and anoxic water that becomes progres-sively more saline with depth. Abundant tropicalvegetation falling into Xcolak provides organic mat-ter for redox reactions that reduce sulfate of thesaline interface (Socki, 1984, Socki et al., 1984).Cenote Ucil, with a saline interface at 65 m, is par-tially protected by a cavern ceiling from entry oforganic debris, but it nevertheless contains abun-dant H2S in its saline bottom water (Socki, 1984;Gmitro, 1986).

From strontium data for water in Cenote Ucil, itis very probable that there is groundwater movement

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between the deep saline water in the cenote and anunderlying SO4-rich layer of evaporite and/orimpact breccia. Except for probable H2S loss (Sockiet al., 1984), there is only limited mixing betweenthe freshwater lens and the underlying saline intru-sion. Below the 65 m saline interface, Sr contentincreases regularly with depth to a value of 33.5ppm (0.76 meq/l) for the deepest sample accessible(105 m) (Fig. 4). As discussed previously, the onlyother groundwater in Yucatan known to have a com-parable strontium concentration is the saline waterin direct contact with the impact breccia in wellUNAM5 at 400 m. Seawater, which has a maximum

Sr concentration of only 7.6 ppm, could explain onlyabout 20% of observed Sr in deep Ucil water, basedon a comparison of the sodium concentration of thiswater and of seawater (Table 6, Fig. 4).

We have also shown in Equation 1 that through-out the Peninsula there is an excellent correlationbetween Sr concentration and sulfate concentration.Strontium, rather than sulfate, must be used as ameasure of the evaporite contribution to Ucil waterfor two reasons. First, as noted earlier, the sulfatecontent of deep water from the saline intrusion con-tains too much SO4 to be sensitive to sulfate addedby evaporite dissolution. Second, Socki (1984) and

TABLE 6. Strontium and Sodium Content of Water in Cenote Ucil

Depth(m)

Sr(ppm)

Na(ppm)

Pct. of seawater2 Sr from seawater2

(ppm)

201 1.3 170 <2 <0.1

901 21.5 5500 51 3.9

1001 30.6 7625 71 5.4

1051 33.5 8600 80 6.0

853 17.0 5976 55.5 4.2

1From Gmitro, 1986.2 Calculated from sodium content.3This study.

FIG. 4. Depth versus sodium and strontium content (ppm) for Cenote Ucil. Sodium is a measure of the seawatercontent (10,700 ppm in undiluted seawater). The cenote passes through a saline interface (shown by the sharp break inslope of Na versus depth). “Excess Sr” is the amount by which strontium in the water exceeds that contributed by sea-water.

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Socki et al. (1984) have demonstrated that redoxprocesses have caused major SO4 depletion beneaththe saline interface at Ucil.

We return to the question of the likelihood thatK/T boundary breccia and/or overlying evaporite isaccessible to groundwater that can be sampled fromthe surface. To do so, we consider Cenote Xcolak,deeper than Ucil and in many respects similar butfor which no Sr data are currently available. Reliefabove Xcolak is about 20 m., and the part of the cen-ote floor that can be directly measured is 120 m fromthe surface. Thus, the main floor of this cenote is atleast 20 + 120 = 140 m below surface topography. InPEMEX well Y4 (Fig. 1), near the Pockmarked Ter-rain, the boundary breccia occurs at a depth belowthe surface of less than 200 m (Ward et al., 1995)—that is, only about 60 m deeper than the directlymeasured (therefore minimum) floor depth of CenoteXcolak.3 Sea level 18,000 years ago was about 110m below present sea level (Fairbanks, 1989; Coke etal., 1991), and, because the water table of the north-ern Yucatan Peninsula is closely related to sea level,the water table at Xcolak at that time would havebeen approximately coincident with the main cenotefloor. Therefore, it is likely that during Pleistoceneand Paleogene times of low sea level (Kinsland etal., 2000), evaporite–K/T boundary breccia of thePockmarked Terrain was exposed to active ground-water circulation.

Undersaturation of groundwater with respect tocarbonate typically occurs near the water table, inresponse to high soil PCO2, and in the mixing zonebetween fresh and saline water. Almost all ground-water from northern Yucatan is close to saturationwith respect to calcite (Table 5 and previous discus-sion). In contrast, almost all groundwater is under-saturated with respect to gypsum/anhydrite. Thus,virtually any circulating Yucatan groundwater,including deep, saline groundwater, is capable ofdissolving gypsum or anhydrite.

A possible contributing factor to dissolution ofevaporite is that sulfate in fresh K/T boundary brec-cia has a large anhydrite component (Urrutia-Fucugauchi et al., 1996; Rebolledo-Vieyra et al.,2000); and, at shallow depth and 30°C, anhydrite isunstable with respect to gypsum (Macdonald, 1953).

Thus, entry of water into the breccia layer results inan initial conversion of anhydrite to gypsum with anexpansion of 28%. If, at shallow depth, the replace-ment was a mol-for-mol process (instead of occur-ring at constant volume), the result would befracturing and permeability enhancement of overly-ing rock.

It is worth noting that the Sr anomaly reported inUcil water is extremely diminished in passingthrough the saline interface, and thus would not bediscovered by studying shallow groundwater. Forexample, in contrast to Ucil bottom water, waterfrom a depth of 20 m in that cenote (within the fresh-water lens) has an Sr content of 1.3 ppm, only mar-ginally higher than the average value of 0.9 ppm forshallow Yucatan groundwater in contact with calciteand aragonite. Sulfate also fails as an indicatorbecause (as a result of redox processes) it is actuallydepleted, compared to seawater, in the highly reduc-ing bottom water of Ucil.

Covariation of calcium, magnesium, and sulfate ions as evidence of dedolomitization

As one test of the processes acting on theYucatan aquifer, we have examined how our samplesdiffer from two-component mixtures of seawater(analysis taken from Drever, 1997) and CaCO3-satu-rated groundwater. From the (presumably conserva-tive) chloride content of our water samples wecalculated a mixing ratio (R) (Plummer and Back,1980):

(3)

This factor is then applied to each ion in every sam-ple, for example, for calcium:

[“Excess” CaSample] =

(mCa)Sample – [(mCa)Buctzotz + RSample ×((mCa)Sea – (mCa)Buctzotz)]. (4)

In this equation groundwater from Buctzotz(Table 2) is considered typical of groundwater sam-ples affected only by seawater mixing and CaCO3dissolution. A hidden assumption of this equation isthat most carbonate dissolution occurs near thewater table and that there is minimal CO2 produc-tion in the deeper parts of the freshwater lens.Although the calculation is an imperfect approxima-tion, it has the effect of normalizing for seawater

3In PEMEX well 5A, which is closer to Xcolak (Fig. 1), brec-cia is reported at 400 m, but the minimum depth could bemuch shallower because the log for that well was constructedfrom only four samples in 1000 m of section (Ward, pers.commun., November 2001).

RSample

mCl( )Sample

mCl( )Buctzotz

–

mCl( )Sea

mCl( )Buctzotz

–----------------------------------------------------------------------.=

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mixing and for ubiquitous calcium carbonate disso-lution, and it is a useful aid in identifying ionsources such as gypsum dissolution (and, probably,a calcium sink: dedolomitization). As would beexpected if gypsum dissolution occurs, a direct cor-relation exists between “excess” sulfate, as deter-mined by Equation (3) and “excess” calcium (Fig.5A) for those samples whose (SO4/Cl) ratios aregreater than seawater.

A correlation also exists between “excess” sul-fate and “excess” Mg (Fig. 5A). Furthermore, thesum of the “excess” cations (Ca + Mg) versus“excess” sulfate has a slope of nearly 1, a relationthat cannot be explained by simple dissolution butis an expected consequence of “dedolomitization,” aprocess in which dolomite of the aquifer is replaced

by calcite, taking calcium ions from groundwaterand releasing magnesium ions to groundwater (Backet al., 1983). R. Stoessell (pers. commun., Septem-ber 5, 2000) pointed out that, for kinetic reasons,this process is unlikely to occur so long as waterremains saturated or supersaturated with respect todolomite. It is entirely possible, however, that thehighly seasonal recharge events of the Yucatan Pen-insula could produce an upper groundwater layerassociated with gypsum-bearing portions of theaquifer in which the Ca/Mg ratio favors replacementof dolomite by calcite. It was noted above that alleight of the groundwater samples in Table 5 that areundersaturated in carbonate were collected duringthe rainy season. Although the dedolomitizationhypothesis lacks direct evidence, it is supported by

FIG. 5. A. Relation between “excess” cations calcium and magnesium (as defined in text) and “excess” sulfate ingroundwater. B. Calcium, magnesium, and (Ca + Mg) versus sulfate in groundwater (meq/kg).

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the common occurrence of dedolomitization in high-sulfate groundwater of other aquifers containinggypsum (Back et al., 1983; Plummer et al., 1990).Except in the most sulfate-rich waters of theYucatan aquifer system, the relation between Ca,Mg, and (Ca + Mg) versus SO4 is not clear withoutapplying Equation 3 (Fig. 5B).

Degree to which chloride is conservativeTo this point, we have considered chloride to be

a conservative ion whose concentration is a measure

of the amount of seawater from the saline intrusionthat has mixed with groundwater of the freshwaterlens. Figure 6A shows that the Cl/Na ratio of mostfresh groundwater of the northwest Peninsula variesbetween the values of 0.95 and 1.2, and the stron-gest maximum in this ratio is ~1.15 (Fig. 6A). Thisis close to the expected range of 1.0 (for halite disso-lution) to 1.17 (for seawater dilution). An interestingpoint is that covariation of Cl/Na and SO4 is essen-tially random except that the three samples withhighest sulfate concentration have Cl/Na ratios

FIG. 6. A. Histogram of Cl/Na showing that most groundwater samples have ratios consistent with a combination ofseawater mixing and halite dissolution. B. Ratio Cl/Na versus SO4 for groundwater. Most samples fall between or closeto the range expected for seawater mixing and halite dissolution. There is no apparent correlation between Cl/Na andsulfate content, except perhaps that the Cl and Na in three highest sulfate samples appear to have come from halitedissolution.

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indistinguishable from the halite dissolution valueof 1 (Fig. 6B). A probable explanation is that highsulfate values occur in the Cenote Ring and theTicul fault where flow velocity is high. In thesezones two controls on (Cl/Na) are involved: (1) trans-portation of water that has been in contact withevaporite (Cl/Na = 1); and (2) increased mixing ofsaline water (Cl/Na = 1.17) from the saline intrusionas a consequence of high flow rate. Support for thesecond mechanism comes from the fact that: (1)despite its high salinity and its high sulfate concen-tration (4.9 times that of any shallow groundwaterfrom the northwest Yucatan Peninsula), the chlorideconcentration of Lake Chichancanab is only abouthalf that of groundwater found in several wells asso-ciated with the Ticul fault and the Cenote Ring; and(2) Lake Chichancanab is a likely source and/orproxy for the source of sulfate-rich groundwaterentering the Ticul fault. The question arises as towhy, if evaporite is involved, highly soluble halite isnot as important a contributor to groundwater ions asgypsum/anhydrite. We suggest that most accessiblehalite may already have been removed from the sys-tem and that the halite that remains is present asinclusions in other minerals (including fluid inclu-sions) or otherwise has restricted access to ground-water (Gonzalez-Partida et al., 2000).

Hydrogeologic aspects of the Evaporite RegionThe area that we have labeled the Evaporite

Region extends from southern Quintana Roo (from

somewhere between Tulum and Felipe CarrilloPuerto) south to the Belize border and westward toLake Esmeralda and gypsum-saturated Lake Chich-ancanab (Fig. 1). It is the only one of the regionsdefined here that could be defined exclusively onthe basis of groundwater geochemistry (Fig. 7). TheEvaporite Region is also distinctive geomorphicallyand is characterized by swamps, lakes, and ephem-eral streams that mark this as a region of less perme-able near-surface rocks than those to the north. Twoaspects of water analyses support the conclusion ofrestricted permeability: (1) sulfate concentrationsand sulfate/chloride ratios are both higher than inthe northern Peninsula (Fig. 1, Tables 2 and 3); and(2) ion concentrations vary widely over short dis-tances.4 The persistence of gypsum in contact withthe near-surface aquifer implies restricted flow, andthe steep gradients in ion concentration imply lim-ited circulation.

Located in southeastern Quintana Roo within theEvaporite Region and near the east coast (Fig. 1),Cenote Azul is a large (more than 100 m across),deep (>65 m) lake with unusual water geochemistryand hydrogeologic properties. Its form and depthimply that it is a karst sinkhole and is therefore partof an interconnected cavern system. It contains

4For example, for nearby wells in the Chunhuhub Agricul-tural Station and Chunhuhub Pueblo (Fig. 1), the values are,for Cl 2.1 versus 4.2 meq/kg; for SO4 18.9 versus 7.3 meq/kg;and for 100 × (SO4/Cl) 903 versus 174, respectively.

FIG. 7. SO4/Cl versus Cl showing regional differences in water chemistry.

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water with high sulfate and strontium content (25.75and 0.19 meq/kg, respectively) and low chloride andsodium content (1.25 and 1.64 meq/kg, respec-tively). The value for 100 × (SO4/Cl) is about 2000.Low chloride and low sodium values indicate that,despite its depth, this cenote is disconnected fromthe local coastal aquifer, which exchanges with sea-water. Water from the nearby public supply systemfor Bacalar is in apparent contact with seawater andhas the following chemistry: chloride = 2.23, sulfate =0.25 meq/kg; and 100 × (SO4/Cl) = 11 (Table 3).Oxygen and deuterium of water from Cenote Azul lieclose to the meteoric water line (Perry et al., 2001),indicating only minor evaporation. To maintainwater in this deep cenote, with ion chemistry unre-lated to local coastal groundwater chemistry andwith only minor evaporation, requires a largehydraulic gradient and high flow, suggestingrecharge from a remote area. The high SO4 and Srvalues (which fit Equation 1) indicate that evaporiteand/or boundary breccia is present in the flow path.Cenote Azul, Chunhuhub Agricultural Station, LakeEsmeralda, Lake Chichancanab, and Nuevo Israel(another site of high SO4/Cl) are in a direct line thatis a projection of the east bank of Lake Chichan-canab. That observation suggests the presence of afault through these localities.

Lake Bacalar, a long, narrow lake close to andeast of Cenote Azul (Fig. 1), has water chemistrysimilar to but less extreme than Cenote Azul (Table2). Its low chloride content (avg. 2.93 meq/l) andhigh ratio of SO4/Cl (100 × SO4/Cl = 760) indicatelack of mixing with seawater despite close proximityto the coast. This, in turn, implies a high flow rate of“fresh” groundwater. The question arises why, incontrast with other places of high groundwater flowto the coast such as Estero Celestun and Bocas deDzilam, Lake Bacalar does not maintain a directconnection to the ocean. A likely answer (Perry etal., 2001) is that, because Lake Bacalar is fed bywater that drains an area of gypsum-bearing rocks,these waters are supersaturated with respect to cal-cite (Table 5). A mixture of Lake Bacalar water withseawater remains supersaturated with respect to cal-cite. There is thus no tendency for chemical erosionas there is in other coastal discharge points.

Results and Conclusions

The Yucatan aquifer system provides a uniquecombination of simple mineralogy, a layer-cakestratigraphy, fault conduits, a saline intrusion, and

relatively little anthropogenic influence that makesthis a valuable natural laboratory for the study ofkarst groundwater processes. This study has con-firmed that the saline intrusion beneath the northernYucatan Peninsula is truly extensive, penetrating atleast 100 km inland. Within the Chicxulub Basinand behind a broad coastal region on the north andeast, the ion chemistry of the fresh groundwater lensis controlled by dissolution of calcium carbonateand by mixing with the underlying saline intrusion.

Several structural/tectonic features stronglyinfluence the hydrogeology of the northern Penin-sula. The most important of these in the northwest isthe Chicxulub Impact Basin, a basin of subsidenceduring much of the Tertiary Period. Hydrogeologiceffects are twofold: (1) sedimentary rocks within thebasin are significantly less permeable than those tothe northeast, primarily because the rocks withinthe basin have been subjected to considerably lesssubaerial weathering; and (2) a semicircular systemof faults, the Cenote Ring, which forms a hinge zonearound the edge of the basin, is a region of high per-meability that channels groundwater flow to thecoast. Two other fault systems, possibly associatedwith Eocene plate movement, the Ticul fault in thesouthern part of the study area and the Holbox Frac-ture System in the northeast, also channel ground-water flow. We postulate here an additional faultsystem extending from the vicinity of Lake Chichan-canab to Cenote Azul in Quintana Roo that may bechanneling groundwater southeast from the centralPeninsula to the coast. Virtually all groundwater onthe Peninsula is at or near saturation with respect todolomite and low magnesium calcite. In addition,calcium sulfate from near-surface evaporite and/orK/T evaporite-bearing breccia strongly influence(s)groundwater chemistry in the south and east of thestudy area and along the Ticul and western Ringfault systems. Dissolving evaporite minerals provideions, particularly SO4 and Sr, that are extremely use-ful natural tracers for monitoring groundwater move-ment. These ions, together with Cl and SO4/Clratios, permit a distinction between groundwater (inthe southeast) whose ion chemistry derives prima-rily from evaporite dissolution, and fresh groundwa-ter (in the north and northeast) whose ion chemistryis dominated by calcite dissolution and by mixingwith the saline intrusion. A zone of intermediateSO4/Cl ratios along the Ticul fault and northwestCenote Ring demonstrates groundwater movementaway from high sulfate regions.

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High Sr content of deep groundwater in CenoteUcil, within the Pockmarked Terrain in north-cen-tral Yucatan, implies the presence of a deep cavernsystem, developed in K/T boundary breccia, thatmay be an important part of the Peninsular aquifersystem during times of low seastand. In addition, itis possible that this ancient, deep karst cavern sys-tem established during one or more lower seastandsnot only exists, but that it is now actively discharg-ing groundwater to the sea offshore from CenoteAzul. Possibly the saline intrusion is deep or absentbeneath Cenote Azul because that cenote is in theEvaporite Region, a region characterized by highrecharge in which the shallow aquifer has lowerintergranular permeability than rocks on the north-ern Peninsula. If sulfate in the Evaporite Regioncomes predominantly from K/T impact breccia, low-ered permeability of the aquifer there may resultfrom the presence of a layer of suevite, altered toclay minerals. Results of this investigation suggestthat a more detailed study of strontium and sulfatecharacteristics of Peninsular groundwater will beuseful in evaluating the extent and character ofejecta deposits of the terminal Cretaceous Chicxu-lub bolide impact.

Groundwater reaching the Celestun Estuary andXel Ha (and other estuaries of the northern Penin-sula) mix with seawater to produce a mixed waterthat is undersaturated with respect to calcite; thisaggressive groundwater can contribute significantlyto coastal erosion by chemical means. Whereasthese and other zones of high groundwater dischargeon the coast of the Yucatan Peninsula remain opento the sea, it is notable that Lake Bacalar lacks adirect opening to the sea. This may be the directresult of the influence of calcium sulfate dissolutionon groundwater in the Lake Bacalar region. Thehigh-sulfate groundwater of the region does not forma water unsaturated with respect to calcite whenmixed with seawater.

Acknowledgments

Perry acknowledges financial support from NSFGrants EAR-9304840 and EAR-9706203 and theNorthern Illinois University (NIU) Graduate School.Field assistance and many useful discussions wereprovided by Slawomir Tulaczyk, Jana McClain,Jiren Zhang, Birgit Steinich, Alberto Trejo, Fran-cisco Velazquez, Jorge Vidal, Esther Perry, and fac-ulty of the Universidad Autonoma de Yucatan(particularly Ing. Ismael Sanchez and Ing. Jose

Gamboa). Velazquez-Oliman acknowledges a gradu-ate fellowship from the Consejo Nacional de Cienciay Tecnologia and from the Direccion General deAsuntos del Personal Academico (DGAPA). Marinacknowledges a grant from (DGAPA) of the Univer-sidad Nacional Autonoma de Mexico (IN107595).One or more drafts of this paper were reviewed by J.Herman, K. Pope, R. Stoessell, and W. Ward.Responsibility for any errors rests with us.

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