Perchlorate Occurrence in the Texas SouthernHigh Plains Aquifer System

by W. Andrew Jackson, Srinivasa Kumar Anandam, Todd Anderson, Tom Lehman, Ken Rainwater,Srinath Rajagopalan, Moira Ridley, and Richard Tock

AbstractIn the spring of 2002, the Texas Commission on Environmental Quality determined that perchlorate (ClO4

�) was presentin ground water from the McMillan and Paul Davis well fields that supply potable water for the city of Midland. Researchersbegan a large-scale sampling program to determine the source(s) and distribution of perchlorate in the area’s ground water.This document summarizes the findings of a large-scale investigation in nine counties carried out from July to December2002. This program included public water system (PWS) wells and private wells in Andrews, Borden, Dawson, Ector,Gaines, Glasscock, Howard, Martin, and Midland counties, which occupy a total area of 23,960 km2. Water samples weretested for perchlorate and a suite of common ions. From a total of 254 wells sampled in the nine counties, 179 wells (70%)had detectable perchlorate concentrations (>0.5 ppb) and 88 wells (35%) had perchlorate concentrations equal to or above4 ppb. The highest perchlorate concentration found at a private well was 58.8 ppb in Dawson County, while the highestconcentration detected for a well in PWS was 45.6 ppb in city of Midland, Midland County. Perchlorate positively correlated(a < 0.0001) with Cl�, F�, Br�, SO4

2�, Mg2+, and K+ but not with NO2�, NO3

�, Na

+, or Ca2+. Research to date has identi-fied the most likely sources to be (1) a natural mineralogical impurity; (2) agricultural fertilizers containing perchlorate; (3)in situ generation of perchlorate by electrochemical reactions; or (4) some combination of the three. This study suggests thatthere may be significant sources other than the traditional industrial processing of perchlorate, and the distribution of per-chlorate in ground water is likely more widespread than previously suspected.

IntroductionPerchlorate (ClO4

�) has recently gained national atten-tion due to its increasingly widespread occurrence and in-dications it may pose a health risk to particular populationsat low ppb (lg/L) concentrations. Perchlorate contamina-tion in surface water and ground water in the United Stateshas been reported to range from ppb to ppm (mg/L) levels,affecting the drinking water of 15 million people (Logan2001). Perchlorate salts are widely used by the aerospace,defense, and chemical industries in solid rocket propel-lants, matches, signal flares, fireworks, explosives, andautomobile air bag inflators (Motzer 2001). Some naturallyoccurring fertilizers mined from Chilean caliche also con-tain perchlorate (Susarla et al. 1999, 2000; Urbansky et al.2000, 2001; Motzer 2001). Approximately 150 perchloratemanufacturers and users have been identified, distributedamong 44 states. California, Nevada, and Utah are the threemost affected states, while Arizona, Texas, New York,Maryland, and Arkansas have also experienced significantperchlorate releases. In addition, the Colorado River, which

provides drinking water and irrigation water for millionsof people in the Western United States, currently has lowlevels of perchlorate from Lake Mead to Mexico (Logan1998).

The major health concern for perchlorate exposure isthe interference with thyroid hormone function by compet-ing with iodide ion (Urbansky 1998). The carcinogenic,developmental, reproductive, and immunotoxic effects ofperchlorate are also of concern (Nerenberg et al. 2002).Perchlorate is not currently regulated in the Safe DrinkingWater Act, but state advisories vary from 1 to 18 ppb.Eventually, a federal drinking water maximum contaminantlevel as low as 1 to 6 ppb may be adopted (Na et al. 2002;Logan and Lapoint 2002).

To date, the most studied source of perchlorate con-tamination appears to be the historic disposal of wastewater from high-pressure washout of residual rocket pro-pellant, necessitated by the limited shelf life of ammoniumperchlorate, which required regular replacement and dis-posal of old stocks. These activities have occurred over thepast four decades.

Perchlorate is a strong oxidizing agent, as the most oxi-dized form of chlorine that exists in water. Perchlorate isPublished in 2005 by the National Ground Water Association.

Ground Water Monitoring & Remediation 25, no. 1/ Winter 2005/pages 137–149 137

chemically very stable and has low reactivity even inhighly reducing environments (Logan 1998). Inorganicperchlorate salts are generally extremely soluble. The highwater solubility and poor adsorption of perchlorate to typi-cal soil and organic carbon make perchlorate highly mobilein the environment (Logan 2001).

In the spring of 2002, representatives of the Texas Com-mission on Environmental Quality (TCEQ), as a part ofthe Unregulated Contaminant Monitoring Regulation deter-mined that perchlorate was present in some locations inthe McMillan (Midland County) and Paul Davis (Andrewsand Martin counties) well fields that supply potable waterfor the city of Midland, Texas. These wells are located inthe High Plains Aquifer System, locally referred to as theOgallala Aquifer. Given the importance of this aquifer sys-tem to the region as well as the inability to find a likelysource of the perchlorate, the Texas Tech University WaterResources Center (TTUWRC) was contracted by the TCEQto initiate a preliminary study across nine counties nearMidland. The objectives of the study were (1) to determinethe extent of the perchlorate contamination in the HighPlains Aquifer System and (2) to identify the probablesource(s) of contamination.

Approach

SamplingInitially, the source of the perchlorate was presumed

to be localized and industrial in nature. Therefore, more

wells were to be sampled in Midland, Martin, andAndrews counties, as well as in six adjacent counties(Figure 1). TCEQ records were used to identify all publicwater system (PWS) wells in the nine counties. PWS ex-amples included cities, water utilities, mobile home parks,rural convenience stores, and industrial water producers.An appropriate subset of the identified wells was selectedto provide best possible geographical coverage. The TCEQhad already collected and analyzed ~180 water samplesfrom private and PWS wells for perchlorate. Based on theTCEQ PWS data on file, there were 348 supply wells in 70PWS in the target counties that could easily be sampled.Considering both the locations previously sampled by theTCEQ and the locations of the 348 available wells, 250wells were targeted for sampling and analysis in this study.Where possible, when a selected well could not be sam-pled, another well in the same area was used to replace theoriginal well. In addition to the PWS wells, a limited num-ber of private wells were sampled to improve the geo-graphical distribution of well locations.

Sampling was conducted using flow-through cells to en-sure representative samples. Total dissolved solids (TDS),pH, conductivity, temperature, dissolved oxygen, and oxi-dation-reduction potential were recorded at each well. Ingeneral, after well stabilization, four 250-mL containerswere filled. One container was shipped to the GroundWater Analysis Laboratory (GAL) at the Institute of Envi-ronmental and Human Health for perchlorate analysis, onewas shipped to the TTUWRC Environmental Science Lab-oratory for anion analysis, one was preserved and shipped

Figure 1. Distribution and type of wells sampled.

W.A. Jackson et al./ Ground Water Monitoring & Remediation 25, no. 1: 137–149138

to the Geosciences Laboratory for cation analysis, and onewas archived. Occasionally, the archived container wasshipped to the GAL for a duplicate perchlorate analysis. Asmall number (10%) of archived samples were shipped toExygen Research� (State College, Pennsylvania) for liquidchromatography-mass spectrometry/mass spectrometry(LC-MS/MS) analysis to further confirm detections of per-chlorate. All samples were analyzed for perchlorate, anions(Cl�, F�, Br�, NO3

�, SO42�, PO4

3�, HCO3� as alkalinity),

cations (Na+, Mg2+, K+, Ca2+), and a subset of metals (Sr,Fe, Zn, Cd, Cu, Pb).

AnalysesAnalysis of perchlorate followed EPA method 314.0

(Hautman and Munch 1999), with a Dionex DX-320(Dionex Corp., Sunnyvale, California) ion chromatograph(IC) equipped with a GP50 gradient pump, a CD20 con-ductivity detector, and an AS40 automated sampler. Pea-kNet� chromatography software was used to control thesystem. Ion separation was made with a Dionex IonPacAS16 (250 3 4.0 mm) analytical column. Conditions forthe system were flow rate of 1.0 mL/min, eluent solution of100 mM sodium hydroxide, and injection volume of 1000lL. Ion detection was by suppressed conductivity in theexternal water mode. A standard curve was constructedfrom constant volume injections of calibration standardsof 4, 25, 50, 100, and 200 ppb. Computer-generated peakareas were used to measure sample concentrations in anexternal standard mode. Using the aforementioned analyti-cal method, the method detection limit for perchlorate was0.51 ppb.

Anions (other than perchlorate) in water samples weredetermined following EPA Method 300.0 by using a Dio-nex DX-500 IC, an IonPac AS14A column (4 3 250 mm),8 mM Na2CO3/1 mM NaHCO3 eluent, and an Anion AtlasElectrolytic Suppressor. Major cations and the other metalswere analyzed on an ICP-AES (Direct Reading Echelle,Leeman Labs Inc., Hudson, NH, USA). The peak readingtime and the background reading time were optimized foreach element before analysis. A 100-ppm commercial

standard (SPEX-Certiprep) was diluted to create the fol-lowing calibration standards: solution blank, 0.01, 0.05,0.1, 2, 4, and 10 ppm. At least three different check stand-ards were run every 10 samples to ensure that calibrationwas being maintained. Calibration was repeated daily toconfirm that calibration held true throughout the analysis.

Results

Occurrence of PerchlorateThe sampling teams were able to collect samples from

217 PWS wells and 37 private wells. The PWS total wasless than the original 250-well target as many wells werenot available due to mechanical or operational limitations.Borden County had no PWS wells located within thecounty. From the total of 217 PWS wells sampled in thenine counties, 159 wells (73%) had detectable perchlorateconcentrations (>0.5 ppb) and 76 wells (35%) had perchlo-rate concentrations that were equal to or above 4 ppb(Table 1). In Gaines, Andrews, and Dawson counties, over90% of the PWS wells had detectable perchlorate, while<40% of the wells in Glasscock and Howard counties haddetectable perchlorate. The percentage of wells with per-chlorate above 4 ppb was smaller than the detectable per-centage, but two counties, Dawson and Gaines, still hadover 50% of the tested PWS wells with perchlorate >4 ppb.When considered separately, 20 (54%) of the 37 privatewells had detectable perchlorate concentrations (>0.5 ppb)and 12 wells (32%) had perchlorate concentrations equal toor above 4 ppb. The highest perchlorate concentrationfound at a private well was 58.8 ppb in Dawson County,while the highest concentration detected for a PWS was45.6 ppb in Midland County.

Results of the confirmation sampling by LC-MS/MSwere generally in close agreement with the reported valuesfrom the GAL. In all cases, perchlorate identified by theGAL was confirmed as perchlorate by LC-MS/MS analy-sis. Samples analyzed by LC-MS/MS were on averagewithin 6.4 ± 12.3% of samples analyzed by Method 314.

Table 1Frequency of Perchlorate Detections Above 0.5 and 4 ppb

Percent withPerchlorate

Percent withPerchlorate

Percent withPerchlorate

County PWS Wells >0.5 >4 Private Wells >0.5 >4 Total >0.5 >4

Andrews 17 100 29 6 83 33 23 96 30Borden 0 0 0 7 0 0 7 0 0Dawson 12 92 67 4 100 75 16 94 69Ector 46 43 4.3 3 33 0 49 42 4Gaines 38 100 65 1 100 100 39 100 66Glasscock 4 25 0 4 25 0 8 25 0Howard 8 37 13 5 80 60 13 54 32Martin 22 77 27 4 100 75 26 81 34Midland 70 74 41 3 0 0 73 71 39Total 217 73 35 37 54 32 254 70 35

W.A. Jackson et al./ Ground Water Monitoring & Remediation 25, no. 1: 137–149 139

Estimate of Total Perchlorate MassThe 1997 saturated thickness maps of the High Plains

Aquifer in these nine counties were obtained from theUSGS (Fisher et al. 1998). USGS used water table eleva-tion and aquifer base data from 1085 wells to determinethe saturated thickness of the entire Ogallala Aquifer. Thissaturated thickness was represented in the form of rangesof 0 to 15 m, 15 to 30 m, and 30 to 60 m. Average satu-rated thicknesses of 7.6, 23, and 45 m for each intervalwere used for calculation purposes. A concentration con-tour plot for the entire study area was interpolated basedon the concentration of perchlorate in individual wells.The entire study area comprising nine counties wasdivided into square cells of equal area (50.3 km2). Totalmass of perchlorate for each county, M, was determined by

M ¼X

i

CiTiAin (1)

where Ci = average concentration in cell i, Ti = average sat-urated thickness, Ai = cell area, and n = effective porosity(0.30). Total estimated mass of perchlorate in the nine-county area is ~740,000 kg (Table 2). However, this esti-mate is most probably conservative as it only considers theOgallala Aquifer. Several of the sampled wells were inadjacent formations, but the saturated thicknesses of thoseformations were not known.

DistributionA more detailed analysis of the distribution of perchlo-

rate reveals few major trends and somewhat perplexingpatterns (Figure 2). For the most part, the sampling effortsadequately represented each county spatially. Two immedi-ate conclusions can be drawn: (1) there is no continuousplume-like pattern discernable and (2) there are large varia-tions in perchlorate concentrations on a very small scale.The first conclusion indicates that it is unlikely that onesingle-point source is responsible for the presence of per-chlorate. A widespread phenomenon appears to be morelikely, such as subsurface mineralogy or infiltration fromdiffuse surface sources. A more detailed analysis of the dis-tribution shows that wells within very close proximity candiffer widely in their perchlorate concentrations, indicating

that (1) the perchlorate is confined to some specific zoneswithin the saturated thickness; (2) the source is localizedand thus dispersion has not occurred to an appreciableextent; or (3) the source is intermittent, producing waves ofconcentrations that due to the short sampling time periodhave not been detected. Some apparent trends illustrated bythe distribution map are simply artifacts of well locations(well clusters), which largely follow the paleochannels thatcharacterize the most productive parts of the OgallalaAquifer. Ground water in the High Plains Aquifer Systemis generally considered to have a regional average velocityof 0.3 m/d. In actuality, many of the PWS wells are locatedin paleochannels in which the actual velocity may be sub-stantially >0.3 m/d.

Temporal Stability of PerchlorateIn order to determine the stability of perchlorate con-

centrations in wells with reported detections, 26 wellswere sampled twice. Fifteen wells were previously sam-pled during April 23, 2002 to May 1, 2002 by TCEQ foranalyses in their lab, and then sampled again during Sep-tember 11, 2002 to September 18, 2002 by TTUWRC re-searchers for analysis in this project. Eleven wells weresampled on September 10, 2002 by the TTUWRC teamand then sampled again on November 12, 2002 by TCEQstaff and analyzed by the TTU labs. The concentration ofperchlorate in each well is remarkably stable (Table 3).

GeochemistryThe water sample analyses included field measure-

ments along with laboratory analyses of anion, cation, andtrace metal concentrations. The field data comprised meas-urements such as water temperature (19 to 23�C, witha few outliers up to 26.2�C), pH values (all near neutral,6.52 to 7.66), and specific conductivity (0.687 to 3.22 mS/cm, with some outliers to 7 mS/cm). The water samplescontained fairly high levels of TDS, which varied from0.33 to 4.56 g/L. A number of parameters were signifi-cantly correlated to perchlorate (P < 0.001) including thewell parameters TDS and conductivity; total anions, Cl�,SO4

�2, F�, and Br�; and cations, K+ and Mg2+ (Table 4).Other solutes are correlated at lower significance levels.Also of interest are two major cations (Na+ and Ca2+),which are not correlated well (a > 0.01). All of these corre-lations are positive, indicating a general trend of increasingperchlorate concentration with increase in any of the afore-mentioned anions or cations. Three possible interpretationsinclude (1) ground water concentration due to evaporationexceeding discharge; (2) infiltration of surface water withdissolution of specific minerals; or (3) upwelling of brinewater from lower formations or improper disposal of brinewater from surface impoundments. A detailed discussion ofthese interpretations is beyond the scope of this paper, buta few general comments can be made.

Intrusion of brine water is unlikely to be a cause as sol-ute ratios are very similar to those typically related to theHigh Plains Aquifer System. Following the work of Woodand Jones (1990), solute ratios were calculated for Cl�/Br�, Na+/K+, and Cl�/SO4

2�. These ratios were used byWood and Jones (1990) to identify possible sources of the

Table 2Estimated Mass of Perchlorate in Study Area

Ground Water

County Area (km2) Mass (kg)

Andrews 3842 84,000Borden 2296 1600Dawson 2304 180,000Ector 2294 11,000Gaines 3844 190,000Glasscock 2317 50,000Howard 2330 110,000Martin 2340 88,000Midland 2356 34,000Total 23,923 740,000

W.A. Jackson et al./ Ground Water Monitoring & Remediation 25, no. 1: 137–149140

solutes present in the saline lakes of the Southern HighPlains of Texas. The average Cl�/Br�, Na+/K+, and Cl�/SO4

2� ratios of the High Plains Aquifer have been reportedas 140, 8, and <1, respectively, whereas the Deep-BasinBrine Aquifer (Paleozoic in age) has ratios of 290 (Cl�/Br�), 170 (Na+/K+), and 190 (Cl�/SO4

2�). In the presentstudy, these ratios were 186 (Cl�/Br�), 6 (Na+/K+), and <1(Cl�/SO4

2�), which match closely the values reported forthe High Plains Aquifer (Table 4).

Aquifer StratigraphyThe High Plains Aquifer and Edwards-Trinity (Plateau)

Aquifer are the two principal aquifer systems presentwithin the study area. The Cenozoic Pecos Alluvium Aqui-fer and the Dockum Aquifer are also present but areexploited to a limited degree in this region (e.g., Ashworthand Hopkins 1995).

The High Plains Aquifer (or Ogallala Aquifer) consistsof sandstone and conglomerate of the Late Tertiary(Miocene-Pliocene) Ogallala Formation overlain by a surfi-cial veneer of permeable Pleistocene and Holocene eoliandeposits (Geologic Atlas of Texas 1976; Holliday 1989).The Ogallala Formation is typically <30 m thick in thestudy area but exceeds 60 m in thickness locally (Cronin1961, 1969; Gustavson 1990; Gustavson et al. 1991; Seni1980). Over most of the study area, Cretaceous strata lie

beneath the Ogallala Formation and vary in thicknessowing to erosional relief prior to deposition of the OgallalaFormation (Brand 1953; Fallin 1988, 1989; Nativ andGutierrez 1988). Two intervals within the Cretaceous sec-tion are generally saturated with fresh to moderately salineground water; one in the basal sandstone and conglomerateof the Antlers Sandstone and the other in jointed limestoneof the Comanche Peak Limestone and Edwards Limestone(Knowles et al. 1984; Ashworth et al. 1991; Peckham andAshworth 1993). These strata appear to be in hydrauliccontinuity with the Ogallala Formation and are usuallyincluded with the Ogallala within the High Plains Aquifer,although the nature of cross-formational flow betweenthese units is not well established.

The Edwards-Trinity (Plateau) Aquifer extends north-ward from the Edwards Plateau into the southernmostthree counties in the study area (Anaya 2001; Ashworthet al. 1991; Walker 1979). The aquifer includes all Creta-ceous strata from the base of the Trinity Group (AntlersSandstone) to the top of the Edwards Group. The AntlersSandstone consists of 15 to 60 m of weakly cemented sand-stone and conglomerate (e.g., Walker 1979). The EdwardsLimestone consists of thickly bedded cherty dolomiticlimestone and is typically <60 m thick in the study area.The limestone beds have interconnected solution cavities,fractures, and bedding plane joints and comprise an

Figure 2. Perchlorate occurrence in the Southern High Plains of Texas. ND, nondetect.

W.A. Jackson et al./ Ground Water Monitoring & Remediation 25, no. 1: 137–149 141

important aquifer farther south on the Edwards Plateau.Although the Edwards Limestone and the Antlers Sand-stone are generally thought to comprise a single aquifer, inthe study area the zone of saturation is below the lime-stone, and fresh ground water is apparently confined to theAntlers Sandstone (Walker 1979).

The Cenozoic Pecos Alluvium Aquifer extends north-ward from the Pecos River Valley into the southwesternpart of the study area (Jones 2001; Ashworth and Flores1991). This aquifer is primarily used as a source of irriga-tion water to the west and south. Few water wells exploitthis aquifer within the study area. The Triassic DockumGroup is present beneath the entire study area and sounderlies the High Plains, Edwards-Trinity, and CenozoicPecos Alluvium aquifers. The Dockum Group is typically<150 m thick where it outcrops in the study area but ap-proaches 600 m in thickness in the subsurface. Sandstoneand conglomerate beds within the Dockum Group are per-meable water-bearing units (Dutton and Simpkins 1986;Bradley and Kalaswad 2001). Ground water in the DockumAquifer is under confined or partially confined conditionsand is generally of poor quality, except near the outcropbelt where recharge occurs, such as by downward leakagefrom the Cenozoic Pecos Alluvium Aquifer (Dutton andSimpkins 1986; Bradley and Kalaswad 2001). As a result,municipal use of the Dockum Aquifer is found primarily to

the west and east of the study area. Elsewhere, the poorwater quality, deep pumping depths, and low yields discour-age its use (Bradley and Kalaswad 2001). Few wells withinthe study area use ground water from the Dockum Aquifer.

There is no obvious correlation of perchlorate pres-ence/absence or perchlorate concentration values with anysubsurface geologic feature or geographically with anynatural surface feature (e.g., proximity to saline lake ba-sins, proximity to surface drainages, and proximity toground water recharge areas or discharge areas). Perchlo-rate was detected in wells producing water from the HighPlains Aquifer in seven of the nine counties covered in thestudy area. Samples from the High Plains Aquifer in Bor-den County were not found to contain perchlorate, and theHigh Plains Aquifer was not sampled in Glasscock Countydue to its limited distribution in this county. Where per-chlorate was detected in the High Plains Aquifer, it wasfound in wells producing water exclusively from the Ogal-lala Formation, as well as in waters produced in part orentirely from Cretaceous (Edwards-Trinity) strata.

Perchlorate was also detected in wells producing waterfrom the Edwards-Trinity (Plateau) Aquifer in Ector,Howard, and Glasscock counties. However, the boundarybetween the High Plains and Edwards-Trinity (Plateau)aquifers is not precisely defined, and it is possible thatsome or many additional wells (e.g., in the Midland and

Table 3Temporal Variation in Perchlorate Concentrations

Initial Sampling Second Sampling

Water Source Date Perchlorate (ppb) Date Perchlorate (ppb)

G1650001A May 1, 2002 1.1 September 11, 2002 2.4G1650001AH May 1, 2002 4.0 September 11, 2002 2.8G1650001AO May, 1, 2002 1.5 September 11, 2002 2.3G1650001AR April 23, 2002 9.0 September 18, 2002 11G1650001AU April 29, 2002 9.1 September 18, 2002 13G1650001AV April 29, 2002 15 September 18, 2002 20G1650001BA April 23, 2002 20 September 18, 2002 47G1650001BG April 23, 2002 13 September 18, 2002 14G1650001BH April 23, 2002 19 September 18, 2002 20G1650001BM April 29, 2002 6.6 September 18, 2002 6.0G1650001D May 1, 2002 3.4 September 11, 2002 4.6G1650001L May 1, 2002 11 September 11, 2002 22G1650001O May 1, 2002 9.9 September 11, 2002 17G1650001S May 1, 2002 7.1 September 11, 2002 11G1650001U May 1, 2002 4.5 September 11, 2002 15G0020001D September 10, 2002 7.8 November 12, 2002 7.3G0020001H September 10, 2002 2.8 November 12, 2002 4.0G0020001J September 10, 2002 3.4 November 12, 2002 3.7G0020001K September 10, 2002 3.1 November 12, 2002 3.6G0020001L September 10, 2002 3.0 November 12, 2002 2.4G0020001M September 10, 2002 2.6 November 12, 2002 2.3G0020001N September 10, 2002 2.9 November 12, 2002 2.8G0020001P September 10, 2002 2.4 November 12, 2002 NDa

G0020001Q September 10, 2002 2.5 November 12, 2002 2.7G0020001R September 10, 2002 3.0 November 12, 2002 2.3G0020001T September 10, 2002 2.3 November 12, 2002 3.2

aND, Nondetect.

W.A. Jackson et al./ Ground Water Monitoring & Remediation 25, no. 1: 137–149142

Odessa areas) included here with the High Plains Aquifermay actually be producing water from the Edwards-Trinity(Plateau) Aquifer (see Ashworth and Hopkins 1995).

Perchlorate was detected in one well producing waterfrom the Dockum Aquifer in Howard County. Samplesfrom two wells producing water from the Dockum Aquiferin Borden County and one well in Andrews County did notcontain detectable perchlorate. No wells from the Ceno-zoic Pecos Alluvium Aquifer were sampled in this study.

Discussion

Possible SourcesThere are many possible sources that could be respon-

sible for the perchlorate present in the affected groundwater. These possibilities can be separated into five maincategories, including (1) industrial wastes; (2) militaryactivities; (3) agricultural activities; (4) indirect chemicalformation; and (5) natural geologic deposits. Backgroundliterature and available data are discussed for each hypo-thetical source.

Industrial WastesNumerous industries are known to historically or cur-

rently use, produce, or process perchlorate salts andperchloric acid. By far the most common use of perchlo-rate is as a solid oxidant in items such as rockets, matches,fireworks, energetics, and flares. It is also a waste productin industries processing mined nitrate, one example ofwhich is the Apache Powder Site (Arizona). Industriesinvolved in the manufacture or packaging of such productsare responsible for some very large-scale releases that ledto contamination of significant ground water resources inCalifornia, Nevada, and Arizona, as well as surface waterresources, most notably Lake Mead and the ColoradoRiver (Gullick et al. 2001; Urbansky 2002). In most ofthese cases, ground water contamination by these activitiesis expressed as traditional finite plumes that begin at theperchlorate source and migrate downgradient with theground water flow. Perchlorate can also be used as a gas-drying agent or a chemical reagent as a perchlorate saltor as perchloric acid. No widespread industrial productionor packaging facilities are known to be located in thenine-county region, although it can be very difficult toidentify all such industries. It is possible that some smallmanufacturing or processing facilities may have beenlocated in the region, but it is also highly unlikely that theycould be responsible for the presence of perchlorate inground water over such a large area.

The petroleum exploration industry has been generallyknown to use some perchlorate-containing products in thestudy area. Perchlorate is known to be an ingredient ina few of the many types of explosives (e.g., Magnagel)used in seismic surveys. The research team was not able togather specific historical information of the brands of ex-plosives or the timing of their applications in seismicexploration on the Southern High Plains and PermianBasin as such records are not publicly available. Perchloricacid has been used in well-fracturing practice but amountsand locations are not precisely known.

The distribution of historical/current petroleum-relatedactivities has been mapped by the Railroad Commission(RRC). A set of geographic information system (GIS) cov-erages that included all boreholes/wells in the nine countieswas obtained from the RRC (Railroad Commission 2003).Locations in this database are defined as any drillingactivity, whether test hole or well (abandoned or active forproduction, injection, or other purposes). These spatiallyreferenced data, represented as single pixels, were overlaidon the nine-county study region with the associated distri-bution of perchlorate (Figure 3). As can easily be seen,there has been a very large amount of petroleum-relateddrilling activity in the area, with all nine counties showingsignificant numbers of locations. No obvious relation canbe seen between well location and perchlorate detection.However, this distribution represents all known wells andtest holes. It is possible that a hypothetical link betweenspecific well activities and perchlorate in ground water maybe discernable. Specific characteristics might include age,active/inactive condition, casing type, production or injec-tion, perforation procedure, or others. Until the suspectedlinkages with the petroleum industry are better known, it is

Table 4Statistical Summary of Correlations between

Ground Water Species

Data Set Dependent Variable r2 P n

Field dataConductivity (mS/cm) 0.21 <0.0001 252Specific conductivity(mS/cm)

0.22 <0.0001 251

TDS (ppt) 0.22 <0.0001 249Alkalinity asCaCO3 (ppm)

0.017 0.0569 208

AnionsNO2

� (ppm) 0.0028 0.401 252NO3

� (ppm) 0.0091 0.1287 253F� (ppm) 0.13 <0.0001 246Cl� (ppm) 0.14 <0.0001 246Br� (ppm) 0.23 <0.0001 246SO4

2� (ppm) 0.10 <0.0001 246Total anions(ppm)

0.19 <0.0001 254

CationsNa+ (ppm) 0.02 0.0175 230K+ (ppm) 0.12 <0.0001 230Ca2+ (ppm) 0.0055 0.2616 230Mg2+ (ppm) 0.146 <0.0001 230

Transition metalsMn (ppm) 0.00006 0.9068 230Fe (ppm) 0.0029 0.4153 230Cu (ppm) 0.0006 0.7111 230Zn (ppm) 0.0085 0.1625 230Cd (ppm) 0.000003 0.9791 230Pb (ppm) 0.0069 0.2085 230

Ion ratiosCl�/Br- 0.74 <0.0001 246SO4

�2/Cl� 0.43 <0.0001 246Na+/K+ 0.048 0.0009 229

W.A. Jackson et al./ Ground Water Monitoring & Remediation 25, no. 1: 137–149 143

difficult to draw more definitive conclusions. This issue isbeing studied further by the research team.

Agricultural ActivitiesAgriculture is another important industry in the study

area. Perchlorate salts are not used as primary chemicalsfor any agricultural purpose. However, perchlorate canexist as an impurity or by-product in two common agricul-tural products, mined nitrate fertilizers and chlorate defo-liants. For many years, perchlorate was thought to onlyoccur in Chilean nitrate deposits in the Atacama Desert,the most documented occurrence of natural perchlorate(e.g., Ericksen 1983). Other known and suspected sourcesof perchlorate are discussed in Natural Sources. Whilethere is some controversy over the current concentration ofperchlorate in Chilean fertilizers, certainly it can be saidthat these fertilizers historically contained perchlorate invarying concentrations. A study by Urbansky et al. (2001)found some lots of NaNO3 fertilizer to contain over 0.1%of perchlorate. Further, an earlier study (Tollenaar andMartin 1972) reported 0.12% to 0.26% perchlorate in Chil-ean nitrate deposits. A more recent study, however, showedthat changes made by the fertilizer industry have signifi-cantly reduced the concentration of perchlorate in commer-cial Chilean nitrate fertilizer (Urbansky et al. 2001).

Widespread perchlorate in ground water in this study areawould more likely represent historical releases and notmore recent events.

Land-use data on the irrigated acreage in the ninecounties in 1994 were provided by the Texas Water Devel-opment Board from its Texas Natural Resource Informa-tion System in the form of GIS shapefiles (Texas WaterDevelopment Board 2003) and overlaid on the nine-countyregion with the perchlorate results (Figure 4). It should benoted that only the irrigated acreage is shown; dry-landfarming acreage is as yet unavailable in GIS coverageform. Most of the irrigated cultivation is found in Gaines,Glasscock, and Dawson counties, with substantiallysmaller acreages in Midland, Andrews, Martin, and Ectorcounties. There was virtually no irrigated acreage inHoward and Borden counties. It is interesting to note thatGaines and Dawson counties, with substantial irrigatedacreages, both have the two highest percentages of perchlo-rate detections and concentrations above 4 ppb. GlasscockCounty, with the second largest number of irrigated acres,has the second lowest percentage of detections and concen-trations above 4 ppb. However, most of the area under irri-gation in Glasscock County is not above the High PlainsAquifer System but rather in the Edwards-Trinity, compli-cating the interpretation. Also of interest is the fact that the

Figure 3. Distribution of oil and gas well locations overlaid on the distribution of wells tested for perchlorate. ND, nondetect.

W.A. Jackson et al./ Ground Water Monitoring & Remediation 25, no. 1: 137–149144

county with the least-irrigated acreage (Borden) had nowells with detectable perchlorate. Until more data on boththe historical irrigated acreage and dry-land acreage arecompiled, little can be concluded regarding the linkbetween agricultural activity and presence of perchlorate.In addition, well placement in relation to agricultural activ-ity and ground water flowpaths should be considered. Forinstance, Andrews County has little irrigated acreage, butmany of the wells in Andrews County are located near itsnorthern border and downgradient from Gaines County,which has substantial irrigated acreage. Typical flow veloc-ities in the Ogallala range from 0.3 to 3 m/d, generallyfrom the northwest to the southeast. More sophisticatedapplication of regional ground water flow modeling isplanned as the research progresses.

Using the mass estimates of perchlorate from Table 2,it is possible to estimate the required mass of fertilizer thatwould be required to account for the perchlorate foundin each county, assuming the fertilizer contained 0.1% per-chlorate and 16% N by mass. Using either irrigated acreageor total acreage in each county, the loading rate of N peracre can be compared to typical loading rates (Table 5).The recommended loading rate of N ranges from 56 to 112kg/ha. For some counties, such as Gaines, application ofChilean nitrate for 2 years could account for the perchlo-rate present, assuming no transformation or other losses.For other counties, such as Martin, it would require 45 to90 years of application, if only the current irrigated acreage

is considered. Of course, this calculation is greatly simpli-fied as it does not consider dry-land farms or past changesin cultivated areas. However, it certainly indicates that his-toric use of Chilean nitrate could account for the perchlo-rate present in the study area.

Chlorates are commonly used as defoliants and forother limited applications. Chlorates are produced by elec-trolytic processes, and this process is theorized to also pro-duce perchlorate in smaller amounts. No data are availableon the concentration of perchlorate in chlorates; however,information obtained from the Texas A&M AgriculturalStation (Lubbock) indicated that chlorate was used on<5% of the total acreage of the High Plains. Combiningthis information with the low areal application rate makesit highly unlikely that chlorates are the source of perchlo-rate in the Southern High Plains.

Indirect ProductionOne possible previously unreported source is the pro-

duction of perchlorate within saturated and unsaturatedsediments due to electrochemical reactions that convertchloride to perchlorate. One current source could be light-ning strikes on buried metallic objects such as wells, pipe-lines, or tanks. A second current source could be cathodicprotection systems for oil wells, water wells, pipelines,and underground storage tanks. It is also well known thatgalvanic cells can exist in water supply wells that includesteel well screen, steel well casing, steel discharge pipes,

Figure 4. Distribution of irrigated farmland overlaid on the distribution of wells tested for perchlorate. ND, nondetect.

W.A. Jackson et al./ Ground Water Monitoring & Remediation 25, no. 1: 137–149 145

and electrical submerged well pumps. A complete discus-sion of this hypothesis is beyond the scope of this paper.Certainly, given the number of buried metallic structures,there is an abundance of candidates that may have cathodicprotection and thus could participate in a perchlorate pro-duction reaction. Information that must be collected in-cludes type of protection used, anode placement, anodetype, applied currents, and description of the reactions inthe subsurface environment.

Military SitesMilitary sites are commonly found to be contaminated

by a variety of chemicals, occasionally including perchlo-rate. The study area in question does contain a number offormerly used defense sites (FUDS). These sites were usedfor bombing practice ranges, and all were reported closedat the end of World War II. In most cases only practicebombs were dropped, some of which may have had a spot-ting charge. In fewer instances, explosives were used dueto the presence of impact craters. To date, the researchteam has found no information on the use of perchlorateby the military before 1950. One practice bomb known tobe used was the M38A2, which contained ~1.3 kg of blackpowder (74% KNO3, 11% sulfur, 15% charcoal). It is cer-tainly possible that the nitrate used to make the black pow-der could have come from the Chilean deposits witha small percentage of perchlorate.

The distribution of these FUDS is indicated in Figure 5,which combines the nine-county region, FUDS locationand size, and the perchlorate observations. It should benoted that the actual shapes and orientations of the practiceranges are approximate. Seven of the nine counties containFUDS, no FUDS were found to exist in either Gaines orDawson counties, and only one very small site (0.13 ha)was located in Howard County. The known distribution ofthese sites and the apparent absence of these sites upgra-dient in two of the most affected counties make it unlikelythat these FUDS are responsible for the widespread per-chlorate distribution. It is still possible that multiple sour-ces are responsible for the distribution of perchlorate and, ifso, FUDS may be responsible for some of the detections.

Natural SourcesevaporatesUntil recently, the prevailing opinion was

that perchlorate was only found naturally in the AtacamaDesert. However, recently, perchlorate has been found insome evaporates and potash ores (Orris et al. 2003). Thiswork substantiates older studies in which other naturalsources of perchlorate were reported (e.g., Becking et al.1958). Minerals of prime interest include sylvinite (KCl)and hanksite (Na22K[SO4]9[CO3]2Cl). However, moreresearch is needed to assess the overall abundance and fre-quency of perchlorate in these minerals as well as otherperchlorate-bearing minerals. In addition, a better under-standing of the formation of perchlorate in natural con-ditions would also aid in determination of the likelihood ofperchlorate presence in geologic formations.

ConclusionsThis paper describes the preliminary investigation of

largest continuous occurrence of perchlorate in ground watercurrently known in the United States. The presence of per-chlorate was first noted in two well fields operated by thecity of Midland, Texas, during sampling by the TCEQ aspart of the Unregulated Contaminant Monitoring Regulation.The initial expectation by the TCEQ was that the perchloratewas due to some localized industrial source that would causea finite plume that could easily be delineated. After collec-tion and analyses of 254 well samples, it is apparent that theinitial expectation was far from reality as significant perchlo-rate concentrations were found in eight of the nine countiesand in multiple aquifers. This occurrence is unusual in thatcommon industrial sources are not apparently responsible.Based on currently available information, the most likelysources are the historical use of fertilizers with some per-chlorate content, natural evaporite minerals containing per-chlorate within the subsurface, application of perchloratecompounds in oil exploration, or electrochemical generation.

Based on these findings, the TCEQ has contracted theTTUWRC to broaden its regional investigation. Phase IIbegan in 2003 and encompasses 43 more Texas counties tothe south, east, and north of the original study area. Thesecond phase is scheduled for completion in 2004.

Table 5Estimated Fertilizer Loading Rates at 0.1% Perchlorate in KNO3

Area (km2) Mass (kg) Load (kg N/ha)

County Total Irrigated Perchlorate Fertilizer Total Irrigated

Andrews 3843 36.2 84,000 84,000,000 36.0 3817Borden 2299 3.4 1600 1,600,000 1.1 787Dawson 2304 134.6 180,000 180,000,000 124.7 2136Ector 2295 7.8 11,000 11,000,000 8.2 2405Gaines 3847 2109.9 190,000 190,000,000 79.8 146Glasscock 2319 204.5 50,000 50,000,000 35.4 401Howard 2331 5.4 110,000 110,000,000 74.1 31,738Martin 2343 28.5 88,000 88,000,000 61.8 5076Midland 2358 77.7 34,000 34,000,000 23.4 711Total 23,938 2608 740,000 740,000,000 50.5 463

W.A. Jackson et al./ Ground Water Monitoring & Remediation 25, no. 1: 137–149146

Editor’s Note: The use of brand names in peer-reviewedpapers is for identification purposes only and does not con-stitute endorsement by the authors, their employers, or theNational Ground Water Association.

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Biographical SketchesDr. Andrew Jackson is currently an Associate Professor of

Civil Engineering at Texas Tech University. He conducted hisgraduate work at Louisiana State University in EnvironmentalEngineering and his undergraduate work in Biology at RhodesCollege. Dr. Jackson’s general field of expertise is related to theremediation of hazardous wastes in the surface and subsurface.He serves as an associate editor of the Journal Water Air and SoilPollution and is on the editorial board of the Journal of Environ-mental Toxicology and Chemistry.

Srinivas Anandam received his M.S degree from Departmentof Civil Engineering at Texas Tech University, Lubbock, Texas. Hespecialization include hydrogeology and remediation. He receiveda B.S. degree from Anna University, India. He is currently lookingfor career in the field of groundwater modeling and remediation.

Dr. Todd Anderson is an Associate Professor of Environmen-tal Toxicology and Director of Groundwater Analysis Lab at TheInstitute of Environmental and Human Health, Texas Tech Univer-sity, Lubbock, Texas. Dr. Anderson got his M.S and Ph.D. in

W.A. Jackson et al./ Ground Water Monitoring & Remediation 25, no. 1: 137–149148

Environmental Toxicology from The University of Tennessee, Knox-ville, TN. His field of interest is in the fate and chemistry of con-taminants (particularly organic contaminants) in the environmentand in the use of plants for cleanup of contaminated sites (phyto-remediation), specifically, the role of rhizosphere microorganismsin enhancing microbial degradation of contaminants in soil.

Dr. Tom Lehman is a professor in Department of Geo-sciences, Texas Tech University, Lubbock, Texas. Dr. Lehman gothis Ph.D. from University of Texas, Austin, Texas. His researchinterests include paleontology, sedimentology, and Lithology.

Dr. Ken Rainwater is the Director of water Resources anda Professor in Civil Engineering in Texas Tech University, Lub-bock, Texas. Dr. Rainwater got his Ph.D. in Civil Engineeringfrom University of Texas, Austin, Texas. His field of expertise isgroundwater quality and quantity.

Srinath Rajagopalan is a doctoral candidate in Environmen-tal Engineering at the Texas Tech University (Department of CivilEngineering, Box 41022, Lubbock, TX 79415). He holds a B.E. inCivil Engineering from Madras University, India and a M.S.degree from Texas Tech University. His field of interest includes

source and fate of compounds in the environment, groundwaterflow and contaminant transport modeling. He is currently work-ing as a research assistant for Dr. Andrew Jackson and plans tograduate by May 2005.

Dr. Moira K. Ridley is an associate professor of geology inthe Department of Geosciences at Texas Tech University, Lub-bock, TX. Her specialty is in aqueous geochemistry, specificallyexperimental and modeling studies at mineral-solution interfaces.She received BSc. and MSc. degrees from the University of CapeTown, and a Ph.D. degree from the University of Nebraska –Lincoln.

Dr. Richard Wm. Tock currently holds an appointment as anAdjunct Professor of Chemical Engineering at Texas Tech Univer-sity, Lubbock, Texas. Before retiring from Texas Tech University in2004, Dr. Tock taught and conducted research in the areas ofpolymer science and environmental technology. He received hisPh.D. in chemical engineering from the University of Iowa, Iowacity, Iowa. For the past three years, Dr. Tock has served as aGovernor’s appointee to the Texas Commission EnvironmentalTechnology (TCET).

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