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Correlatingspecificconductivitywithtotalhardnessinlimestoneanddolomitekarstwaters

ArticleinEarthSurfaceProcessesandLandforms·February2006

DOI:10.1002/esp.1232

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WiesławaEwaKrawczykUniversityofSilesiainKatowice

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Specific conductivity in karst waters 221

Copyright © 2005 John Wiley & Sons, Ltd. Earth Surf. Process. Landforms 31, 221–234 (2006)

Earth Surface Processes and LandformsEarth Surf. Process. Landforms 31, 221–234 (2006)Published online 20 December 2005 in Wiley InterScience (www.interscience.wiley.com). DOI: 10.1002/esp.1232

Correlating Specific Conductivity with TotalHardness in Limestone and Dolomite Karst WatersWieslawa Ewa Krawczyk1* and Derek C. Ford2

1 Faculty of Earth Sciences, University of Silesia, 41-200 Sosnowiec, ul. B´dzi7ska 60, Poland2 School of Geography and Geology, McMaster University, Hamilton, ON L8S 4K1, Canada

AbstractUnder field conditions modern digital conductivity meters give standardized, rapid andreproducible measurements. Here we investigate the accuracy of their estimates of the com-position of karst waters, as total hardness (TH, as mg/L CaCO3) for limestone and dolomite.These are the fundamental measures of process in carbonate karst geomorphology. PHREEQCtheoretical curves for the dissolution of pure calcite/aragonite and dolomite in water at25 °C are compared with water analyses from karst studies worldwide. Other principal ionsencountered are sulphates, nitrates and chlorides (the ‘SNC’ group). From carbonate karsts,2309 spring, well and stream samples were divided into uncontaminated (SNC <<<<< 10%),moderately contaminated (10 <<<<< SNC <<<<< 20%), and contaminated (SNC >>>>> 20%) classes. Wherespecific conductivity (SpC) is less than 600 µµµµµS/cm, a clear statistical distinction can be drawnbetween waters having little contamination and substantially contaminated waters withSNC >>>>> 20%. As sometimes claimed in manufacturers’ literature, in ‘clean’ limestone watersTH is close to 1/2SpC, with a standard error of 2–3 mg/L. The slope of the best-fit line for1949 samples covering all SNC classes where SpC <<<<< 600 µµµµµS/cm is 1·86, very close to the 1·88obtained for clean limestone waters; however, the value of the intercept is ten times higher.The regression line for clean limestone waters where SpC >>>>> 600 µµµµµS/cm helps to distinguishpolluted waters from clean waters with possible endogenic sources of CO2. In the range250 <<<<< SpC <<<<< 600 µµµµµS/cm, dolomite waters can be readily distinguished from limestone waters.Copyright © 2005 John Wiley & Sons, Ltd.

Keywords: karst; hydrochemistry; specific electrical conductivity; total hardness

Received 6 October 2004;Revised 1 March 2005;Accepted 2 March 2005

*Correspondence to:W. E. Krawczyk, Faculty ofEarth Sciences, University ofSilesia, 41-200 Sosnowiec, ul.B´dzi7ska 60, Poland.E-mail: wkraw@us.edu.pl

Introduction

Today there are many conductivity meters designed to measure the electrical conductivity of water or other liquidsunder field conditions. A majority are digital and standardized to report specific conductivity (SpC) at 25 °C. Measure-ments are simple, rapid, and reproducible to ±1 or 2 µS/cm. The instruments are small, light in weight, robust andinexpensive. Their use in the field gives immediate results, avoiding the problems of aeration and temperature changethat often arise when water samples are collected for later analysis in the laboratory.

The purpose of this study is to evaluate how adequately simple field SpC measurements may be used to estimatesome of the most important parameters in dissolution, which is the dominant process in karst landform evolution. Incarbonate (limestone and dolomite) karsts a key parameter is the total hardness (TH) of the water, i.e. the sum of Ca2+

and Mg2+, which is conventionally expressed as mg/L of dissolved CaCO3. It is also desirable to be able to estimatewhether (and how far) the SpC values are affected by the presence of foreign ions such as K+, Na+, Cl−, NO3

−, SO42−,

derived from natural or anthropogenic sources.We have found only a few published equations showing the relationships between SpC and total hardness (TH),

each for a particular geographic region. For karst springs and domestic wells in Pennsylvania, White (1988, Table 5·7)gives SpC = 1·70 TH + 50 when SpC is expressed in µS/cm and TH as mg/L CaCO3. For tropical karst groundwatersin Malaysia, Crowther (1989) obtained SpC = 1·56 TH − 39 (n = 217, R2 = 0·98). In karst springs and rivers in the WesternTatra Mountains the relationship was SpC = 1·87 TH + 16·5, n = 211, R2 = 0·98 (Krawczyk and Opolka-Gadek, 1992).

222 W. E. Krawczyk and D. C. Ford

Copyright © 2005 John Wiley & Sons, Ltd. Earth Surf. Process. Landforms 31, 221–234 (2006)

Waters circulating in Proterozoic marbles in the ÍnieΩnik Klodzki (Sudety Mountains) and discharging as springsyielded SpC = 2·0 TH + 16·4, n = 94, R2 = 0·98 (Krawczyk, 1993).

Field data used in the study described below are from our own work or published collections such as thosereported above. Most of the waters were analysed for all ions listed in Table I and ion balance errors were <5%. In afew instances, only Ca2+, Mg2+ and HCO3

− were measured. The selection is worldwide in extent in order that karstspecialists everywhere may have confidence in the findings.

Specific electrical conductivityThe specific electrical conductivity (SpC) of any water is proportional to the concentration of ions within it. Thespecific conductivities for the quantitatively important cations and anions likely to be found in natural waters are givenin Table I. Conductivity is expressed in µS/cm (micro-Siemens per centimetre). The SI standard unit is mS/m butµS/cm prevails in the literature because of its more appropriate scale. The obsolete µmho/cm is also encounteredoccasionally in modern studies.

Pure water itself dissociates in natural conditions, (H2O → H+ + OH−), but the effect is tiny and may be neglectedhere. Atmospheric CO2 dissolves in falling rain

H2O + CO2 ⇔ H2CO03 ⇔ H+ + HCO−

3 (1)

in concentrations that are inversely proportional to the ambient temperatures. The HCO3− may further dissociate into

H+ and CO32− ions, but this effect only becomes quantitatively significant at pH values of 8·5 or higher. The load of

hydrogen and bicarbonate (HCO3−) ions in most rains will yield SpC values around 10 or so µS/cm. Over the sea and

in coastal areas dissolved sea salt (NaCl) particles usually increase conductivity to ~50–60 µS/cm, e.g. in Svalbard(Krawczyk et al., 2002). SpC values in runoff from melting glaciers and snowfields are mostly below 110 µS/cm(e.g. 54–105 µS/cm in the Werenskioldbreen river during ablation periods (Krawczyk, 1994)). Similar or slightlylower SpC values are found in water flowing on the silicate rocks because of their low solubility. The range of SpCmeasured in uncontaminated waters in carbonate karsts may be much greater, ~150–1000 µS/cm, the particular valuesdepending on a multiplicity of factors. Conductivity meters therefore can readily discriminate freshly fallen rain fromwaters long resident on or in soluble rocks, or detect the passage of water flowing from silicate and siliciclastic rocksonto karst. The aim of this paper is to show how SpC measurements can further differentiate quantities of dissolvedsolids within carbonate karst areas, and thus help to suggest the natural evolutionary paths of the waters conveyingthem. In densely populated, farmed or industrialized, regions many waters are now polluted. The conductivity metercan help to detect them; apart from the practical utility of such detection, this avoids distortion that will result fromtheir incorporation into models of natural development.

Theoretical Considerations

The theoretical bicarbonate dissolution curvesThe waters in the majority of limestone karstlands studied worldwide have been found to contain chiefly calcium Ca2+

and bicarbonate HCO3− ions, plus lesser quantities of magnesium (Mg2+) ions. It is accepted that the dissolution of

Table I. Specific electrical conductivity values for 1 meq/L of anions and cations. Datafrom the work of Eith et al. (2001)

Cations ΛΛΛΛΛ+++++ (µµµµµS/cm per meq/L) Anions ΛΛΛΛΛ−−−−− (µµµµµS/cm per meq/L)

H+ 350 OH− 198Ca2+ 60 HCO3

− 45Mg2+ 53 SO4

2− 80Na+ 50 Cl− 76K+ 74 NO3

− 71NH4

+ 73 NO2− 72

Milliequivalents per litre (meq/L) = (millimoles of solute ion × charge of the ion)/litre.

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calcite or aragonite by water containing carbon dioxide (the carbonation reaction) thus is the predominant process. Thereaction is

CaCO3 + CO2 + H2O ⇔ Ca2+ + 2HCO3− (2)

Dissolution of 50·05 mg of CaCO3 in one litre of water yields 1 milliequivalent (meq/L) of Ca2+ and 1 meq/L ofHCO3

−. This is defined as the ‘total hardness’ (TH) of the solution, equal to one milliequivalent or 50·05 mg/L CaCO3.Taking the values for the specific conductivity of one meq/L of calcium and of bicarbonate from Table I, thetheoretical SpC of this solution will be 105 µS/cm (1 × 60 + 1 × 45). This is a very approximate estimate, however,because the conductivity meter measures the ‘activity’ of the ions, ai, rather than their concentration, ci. Activityis always somewhat less than concentration due to inter-ionic effects resulting mostly from electrical attractionsand repulsions, and the pairing of ions (Stumm and Morgan, 1996). The relationship is governed by the activitycoefficient, γi, where γi = ai/ci. In this example the SpC value is reduced to 92·9 µS/cm. Activity coefficients canbe calculated with the extended form of the Debye–Hückel equation or with Davies’ equation. The former considersthe charge of the ion, zi; two constants, A = 0·5085 and B = 0·3281 for water at 25 °C; the ionic strength of thesolution, I, and ion size, ri (ri = 6 Å for Ca2+ and 4 Å for HCO3

−). The extended Debye–Hückel equation is

log

γ ii

i

Az I

Br I=

−+

2

1(3)

Davies’ equation is derived in part empirically (Krauskopf and Bird, 1995):

log

.γ = −+

−

Az

I

II2

10 2 (4)

Computer programs for speciation modelling use Davies’ equation and multiple iterations. Activities of Ca2+ andHCO3

− obtained with the popular PHREEQC v.2 (Parkhurst and Appelo, 1999) are slightly lower, partly because theadditional ion pair CaHCO3

+ is included.The higher the Ca2+ and HCO3

− concentrations, the higher is the ionic strength and the lower are the activitycoefficients and consequent calcium and bicarbonate activities. Data for the theoretical curves relating SpC to dis-solved calcium carbonate (calcite or aragonite), with Ca2+ concentrations expressed as total hardness in mg/L CaCO3

at 25 °C, are given in Table II for the Debye–Hückel (D–H), Davies’ and PHREEQC computations. For the ranges ofSpC typical in uncontaminated karst water the differences are <5%. We may assume that SpC = f(TH) in karst waterswill plot above the theoretical lines because SpC measures ionic activity but total hardness measures Ca2+ and Mg2+

concentrations.The best fit expression for the PHREEQC theoretical calcite dissolution curve is

SpC = 1·79 TH − 0·0009 TH2 + 4 × 10−7 TH3 + 3·43 (5)

For practical purposes, little error is introduced if this curve is divided into two around a breakpoint at SpC ≈ 600 µS/cm,enabling the use of simple linear equations, which in this case are

Table II. Theoretical specific conductivity values using the Debye–Hückel (D–H), Davies’ (D) equations and modelling usingPHREEQC v.2 for the dissolution of pure calcite (or aragonite) and ideal (stoichiometric) dolomite by the bicarbonate reaction

TH [as mg/LCalcite SpC [µµµµµS/cm] Dolomite SpC [µµµµµS/cm]

CaCO3] D–H D PHREEQC D–H D PHREEQC

100·1 180·0 179·0 177·0 174·7 173·5 171·7200·2 341·1 337·8 331·3 331·8 327·8 322·1300·3 492·2 486·1 473·5 479·8 472·0 461·1400·4 636·6 626·8 607·0 621·5 609·0 591·9500·5 775·7 761·8 734·0 758·4 740·5 716·6600·6 910·5 892·1 855·5 891·3 867·6 836·6700·7 1041·7 1018·4 972·5 1020·9 990·9 951·2

224 W. E. Krawczyk and D. C. Ford

Copyright © 2005 John Wiley & Sons, Ltd. Earth Surf. Process. Landforms 31, 221–234 (2006)

SpC = 1·51 TH + 17·1 (6)

where SpC < 600 µS/cm, and

SpC = 1·19 TH + 134·1 (7)

where SpC > 600 µS/cm.The dissolution of dolomite is given by

CaMg(CO3)2 + 2H2O + 2CO2 ⇔ Ca2+ + Mg2+ + 4HCO3− (8)

Dissolution of 92·13 mg/L of dolomite creates 1 meq/L of Ca2+, 1 meq/L of Mg2+ and 2 meq/L of HCO3−. The total

hardness of this solution has a value of 100·1 mg/L CaCO3 (=2 meq/L). A first estimate of the SpC is 204 µS/cm but,allowing for activities and the ion size of 8 Å for Mg2+, it is reduced to 171·7–174·7 µS/cm (Table II). The theoreticalrelationship between SpC and total hardness in dolomite plots below the calcite dissolution curve because the SpC ofthe magnesium ion is 53 µS/cm per meq/L, which is 7 µS/cm per meq/L less than Ca2+. The best fit polynomialequation for the PHREEQC curve is

SpC = 1·74 TH − 0·0008 TH2 + 4 × 10−7 TH3 + 3·03 (9)

For SpC < 600 µS/cm the linear equation is

SpC = 1·47 TH + 18·3 (10)

and for SpC > 600 µS/cm

SpC = 1·18 TH + 125·8 (11)

Contributions of the other principal ions that may be encountered in karst watersSulphate (SO4

2−) is the ion not produced in the bicarbonate reaction that is most commonly encountered in the world’skarst waters. Where it is detected, it chiefly contributes to balancing the Ca2+ and Mg2+ cations in solutions. It canderive from two principal sources—the dissolution of gypsum (CaSO4·2H2O) or the oxidation of H2S to producesulphuric acid (H2SO4), a strong acid that rapidly dissolves carbonate minerals. H2S can derive from exhalative,organic decompositional or local inorganic (e.g. pyrite) weathering sources.

Gypsum is common as a minor component in many limestones and dolomites, especially the latter. It may bedisseminated within the carbonate crystalline mass, fill vugs (small voids) in it, or occur as separate interbeds. Broadlyspeaking, it is ten times as soluble as calcite (Ford and Williams, 1989, p. 42) and so may furnish significant quantitiesof the common ion, Ca2+, and the foreign ion, SO4

−, to waters even where its total volume within the carbonate rock issmall. For example, it is present at a few percent or more in low stage discharges from many regional limestonesprings (Worthington and Ford, 1995). Its presence will serve to boost the SpC value above that which correlates witha given total hardness in purely bicarbonate solutions. It may reduce rates of carbonate rock dissolution by thecommon ion effect (Ford and Williams 1989, p. 72).

Exhalative H2S is associated with some volcanic de-gassing and with the bacterial reduction of sulphate rocks insedimentary basins. It is partly or largely responsible for excavation of some of the world’s most spectacular limestonecaves, such as Carlsbad Caverns and Lechuguilla Cave, New Mexico (see e.g. Hill, 1995), but nevertheless must beconsidered a highly localized process that will not contribute solute ions to most karst waters. Similarly, H2S from thedecay of vegetal matter in anoxic waters appears to be quantitatively significant only in some saltwater coastal karstsituations (Whitaker and Smart, 1994) and perhaps rare deep ponds or cave traps inland, and thus will also be highlylocalized.

Pyrite (FeS2) is a common minor constituent in shale partings that are found in most limestones and dolomites.Galena (PbS) and sphalerite (ZnS) may also be present, though normally in much smaller amounts. At the surface andin oxygen-rich groundwater these sulphides are readily oxidized to form sulphuric acid. The full summary reaction forpyrite and calcite may be written

4FeS2 + 15O2 + 8H2O + 16CaCO3 → 16Ca2+ + 8SO42− + 16HCO3

− + 2Fe2O3 (12)

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Copyright © 2005 John Wiley & Sons, Ltd. Earth Surf. Process. Landforms 31, 221–234 (2006)

Equation (12) shows a theoretical extreme case where carbonic acid (H2CO3) is entirely absent from the calcitedissolution process, i.e. all H+ is from oxidation of pyrite, creating a solution that contains equal equivalent con-centrations of bicarbonate and sulphate anions plus calcium as the only cation. Such a solution with 2 meq/L Ca2+,1 meq/L HCO3

− and 1 meq/L SO42− would yield an SpC value of 197·7 µS/cm.

We must emphasize that this is a hypothetical case that is unlikely to be encountered in real instances because boththe quantity and the dispersion of pyrite (or other sulphide minerals) will be insufficient in oxidizing environments topromote H2SO4 production at rates far exceeding H2CO3 production and dissociation. The practical effect in mostinstances will be to stack the H2SO4-sourced ions on top of the calcite (or, where appropriate, the aragonite ordolomite) curves, increasing the specific conductivity readings and the total hardness values estimated from them.

The other ions commonly found in karst waters are sodium (Na+), potassium (K+), chloride (Cl−) and nitrate (NO3−).

All of these are usually present in very low quantities, however. Sodium and potassium can be derived from dissolu-tion of silicate minerals; dissolved silica itself is found in quite a wide range of concentrations, indicating that a littledissolution of silicate minerals takes place. The sodium and chloride can be also of marine origin; a few mg/L to tensof mg/L are measured in most rains close to sea coasts, and some can be transported far inland as airborne particles.Nitrate is produced by natural processes in soils, the oxidation of organic compounds containing nitrogen yieldingnitrate ions in the final stage of nitrification. Comparatively high concentrations of sodium, potassium, chloride,sulphate and nitrate may often be indicators of agricultural or industrial pollution. In addition, phosphate (PO4

3−) ionsare found in groundwater that has drained fields treated with phosphate fertilizers or where phosphate detergents leakfrom containment. The presence of one or more of these ionic species must be postulated where measured SpC valuesdeviate far from the linear relationships cited above and illustrated below.

Selected Field Studies

Limestone karstWe illustrate the relationships between specific conductivity and total hardness with the sequence of case studiesgiven below. These have been ordered to display the effects of increasing quantities of data and/or geologicaland geomorphological complexity. Figure 1(a) and (b) shows examples from geographically small regions withinwhich the geologic and other conditions are comparatively uniform. The Durmitor karst of Montenegro (Figure 1(a))is a good representative example of a clean (non-polluted) Jurassic and Cretaceous limestone terrain in the DinaricKarst. The water samples were from karst springs, lakes, streams and the Tara River, analysed by the first author usingfield laboratory methods as outlined by Krawczyk (1996). There is a quite a wide range of conductivity, from 50 to550 µS/cm. There is little evidence of agricultural or other surface sources of pollution; the highest nitrate concentra-tion, recorded on one occasion in one spring, was only 10 mg/L. Some long-range atmospheric transport of sulphatesand chlorides into the region is possible.

In Figure 1(a), line A is the best fit to the 36 water samples where foreign ions (sulphate plus nitrate and chloride(SNC)) summed to less than 10% of total anion equivalents (i.e. HCO3

− > 90%). The correlation coefficient for thisdata set is R2 = 0·99. The slope value in the equation (Figure 1(a)) is only ~0·4 greater than in the theoretical case forpure calcite dissolution (equation (6)). The data from Durmitor thus gives us a good instance of the accuracy withwhich SpC may predict total hardness over a wide range of values in a clean, rather homogeneous, limestone karst.

In Figure 1(b) the Durmitor data set with SNC < 10% has been expanded by adding other SNC < 10% samples fromthe Austrian and Italian Alps, from Poland and from limestone springs in Pennsylvania. For SpC below 600 µS/cm(n = 164) data fit the linear trend

SpC = 1·88 TH + 3·1 R2 = 0·96 (13)

which is very similar to the best fit for Durmitor alone. There are comparatively few limestone waters whereSpC > 600 µS/cm but SNC < 10%. Figure 1(b) includes 23 published analyses from karst springs in Argens andDordogne, France (Martin, 1991), and travertine depositing springs in Huanglong (Liu et al., 1995) and Baishuitai,China (Liu et al., 2003). The linear fit is

SpC = 1·43 TH + 129·3 R2 = 0·98 (14)

Most limestone waters with SpC higher than 600 µS/cm will contain significant concentrations of sulphate, nitrate orchloride. The exceptionally high proportion of Ca2+ and HCO3

− in the Argens, Dordogne, Baishuitai and Huanglong

226 W. E. Krawczyk and D. C. Ford

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Figure 1. (a) Correlation of SpC and total hardness (expressed as mg/L CaCO3) for limestone waters with SNC < 10% in theDurmitor region, Montenegro. The theoretical relationships (dashed lines) are for ion activities (lower) and ion concentrations (upper)resulting from the dissolution of calcite or aragonite, calculated with PHREEQC. (b) Correlation of SpC and total hardness (expressedas mg/L CaCO3) for a larger sample of clean limestone waters (n = 187) with SNC < 10%; waters with SpC < 600 µS/cm (opensquares) and SpC > 600 µS/cm (dark squares). The theoretical relationships (dashed lines) are for ion activities (lower) and ionconcentrations (upper) resulting from the dissolution of calcite or aragonite, calculated with PHREEQC.

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samples are believed to result from the addition of significant amounts of juvenile (deep) carbon dioxide (Martin,1991; Liu et al., 2003).

In many European limestone regions that we have sampled, waters impacted by human activity are more numerousthan those that reflect solely natural processes: SNC is often greater than 20%, and in many cases reaches 40%.Figure 2(a) shows examples from particular karsts known to be contaminated by agricultural activity, inadequatesewage disposal, airborne industrial pollution etc. A sample suite from Poland includes springs and spring-fed rivers inJurassic limestone in Ojców National Park (Hibszer et al., 2001; Hibszer and Krawczyk, 2003), drip waters contami-nated by natural nitrates in Zegar Cave (Krawczyk, unpublished) and springs in Roztocze (Íwieca, 1998). A secondsuite is from the well known Moravian Karst (Permo-Triassic limestones) in the Czech Republic. It includes springs,surface streams and cave pools (Krawczyk, unpublished). It is seen that the large majority of the samples plot abovethe PHREEQC line for dissolution of pure calcite, but the correlation coefficient for a linear best fit for waters withSpC < 600 µS/cm remains high (R2 = 0·89).

In Figure 2(b) this data set of contaminated limestone waters from Middle Europe is expanded to n = 484 byincorporating similar findings (SNC > 20%) from Canalobre Cave and Sierra Blanca in Spain (Andreu et al., 1999;Andreo and Carrasco, 1999), the Cartaro Grande region in Italy (courtesy of R. Drysdale), Herault in France (Marjoletand Salado, 1976), Izmir (Tarcan, 2001), Van (Ozler et al., 2001) and Koycegiz (Bayari et al., 1995) in Turkey,Damascus, Syria (Kattan, 1997), Guizhou Province, China (Han and Liu, 2004), Illinois, USA (Panno et al., 2001),Birongo, Venezuela (Urbani, 1995), and the Pinar del Rio region of Cuba (Krawczyk, unpublished).

Once again, almost all of these waters plot above the theoretical curve for calcite dissolution. For 330 samples withSpC < 600 µS/cm the best fit is SpC = 1·65 TH + 86·3, with an acceptable R2 = 0·87. In comparison with the resultsof Figure 1(b), the intercept value is much increased in Figure 2(a) and (b), indicating that in these contaminatedwaters the mean ‘background’ SpC is around 100 µS/cm (the range is 86·6–136.3). The scatter of points whereSpC > 600 µS/cm renders the SpC = f (TH) relationship unacceptable (R2 = 0·40); the concept of stacking common orforeign ions derived from other dissolution systems noted above is well displayed in this area of the figure.

Dolomite karstThe literature contains comparatively few detailed studies of waters on dolomite terrains, perhaps because karstdevelopment is often more limited as a consequence of the lesser solubility of the mineral. Figure 3(a) presents84 SpC < 600 µS/cm samples of sinking streams, wells and springs on the Niagaran dolostones (Silurian) of Ontario,(Enyedy-Goldner, 1994; Ford, unpublished) and the Ordovician dolostones of Pennsylvania (Langmuir, 1971), streamsin the Apuan Alps (Drysdale, unpublished) and springs in the Sierra Blanca, Spain (Andreo and Carasco, 1999), plus130 analyses of SpC > 600 µS/cm waters from mostly dolomitic rocks in the Barkly Tableland, Australia (Drysdaleet al., 2002), and Carratraca Spa, Spain (Vallejo and Duran, 2001). In the majority of these samples SNC < 10%, butin some samples not all major ions were measured.

The best fit for the SpC < 600 µS/cm set is

SpC = 1·56 TH + 35·2 R2 = 0·89 (15)

Although there is a higher background, the fit is very close to the theoretical equation (10). For Ontario alone (n = 36)the correlation improves to R2 = 0·97 and for Pennsylvania (n = 25) to R2 = 0·94, pointing to the influence locallithological factors have on solubility. In a recently completed investigation, the second author was able to useEquation (15) and field SpC measurements over the spring thaw period to separate the sources of 47 springs along a1500 m frontage of the Niagara Escarpment, Ontario, into three distinct types of karst aquifer.

For the 130 water samples where SpC > 600 µS/cm

SpC = 1·25 TH + 207·3 R2 = 0·69 (16)

Both the slope and intercept values are higher than in the theoretical equation (11); the intercept value is 65% higher,indicating that these waters may be contaminated. However, the correlation coefficient is significantly better than inthe polluted limestone karst waters.

In a classic study in Pennsylvania, Langmuir (1971) compared spring and well water samples drawn from dolomiteswith others from nearby limestones. As a check on the validity of the presumed dolomite or limestone sources, it washypothesized that water samples predominantly dissolving dolomite would yield Ca2+/Mg2+ < 1·5, while all sampleswith Ca2+/Mg2+ ≥ 1·5 indicated predominance of limestone dissolution. For our analysis here, water samples withSpC > 600 µS/cm have been deleted from Langmuir’s data set because of their small number.

228 W. E. Krawczyk and D. C. Ford

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Figure 2. (a) Correlation of SpC with total hardness (expressed as mg/L CaCO3) for polluted limestone karst water (SNC > 20%)from sites in central Europe; waters with SpC < 600 µS/cm (open diamonds) and SpC > 600 µS/cm (dark diamonds). The theoreticalrelationships (dashed lines) are for ion activities (lower) and ion concentrations (upper) resulting from the dissolution of calcite oraragonite, calculated with PHREEQC. (b) Correlation of SpC with total hardness (expressed as mg/L CaCO3) for 484 pollutedlimestone karst waters from Europe, Asia and the Americas; waters with SpC < 600 µS/cm (open diamonds) and SpC > 600 µS/cm(dark diamonds). The theoretical relationships (dashed lines) are for ion activities (lower) and ion concentrations (upper) resultingfrom the dissolution of calcite or aragonite, calculated with PHREEQC.

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Results from the Langmuir data are shown in Figure 3(b). The limestone and dolomite waters are quite clearlydifferentiated from each other. The difference in the slope values in the two linear equations that fit them (seeFigure 3(b)) is 0·13, notably more than the difference (0·04) between the theoretical Equations (6) and (10). R2 valuesof the two fits are greater than 0·9 and both trends plot a little above the theoretical line for dolomite. Interceptvalues in the samples from the limestone springs suggest the presence of ions not contributing to total hardness (e.g.nitrates or chlorides), which explains the slightly lower correlation coefficient for the limestone waters; it hints thatcontaminants are being discharged more effectively than in the dolomite areas.

An aggregate picture for carbonate karstFor what might be termed a ‘grand slam’ analysis of the utility of SpC measurement in limestone and dolomite karst,we assembled a total of 2309 published samples from springs, wells and streams, including those consideredabove. There are uncontaminated (SNC < 10%), moderately contaminated (10% < SNC < 20%) and contaminated(SNC > 20%) classes. They cover the climatic gamut, from polar Svalbard to tropical Cuba. The reliability andcomprehensiveness within the data sets varies substantially, as there are many individual samples where the ionbalance errors are >5% and/or only the Ca2+, Mg2+ and HCO3

− ions were measured. As a consequence of these factorsthis data set may be considered a ‘worst case’ test of the SpC versus total hardness correlation in carbonate karstregions.

The data are presented in Figure 4. The best fit line for 1949 samples where SpC < 600 µS/cm is

SpC = 1·86 TH + 31·5 R2 = 0·93 (17)

The slope of this line is very close to that for limestone karst waters with SNC < 10% and reliable, comprehensiveanalyses (Equation (13)), the difference being that the line is shifted up the SpC axis because the intercept valueis more than ten times greater. R2 is an acceptable 0·93. The one standard error of estimate band is small: forSpC = 200 it gives total hardness = 93 ± 2 mg/L; for SpC = 400, TH = 193 ± 2 mg/L; for SpC = 600, TH = 293 ±3 mg/L (Table III). Total hardness can be obtained with the equation:

TH = 0·54 SpC − 17 (18)

Where SpC < 600 µS/cm, we have shown that there is an increasing deviation from the theoretical cases for bothlimestone and dolomite as SpC increases from ~100 to 500 µS/cm. The precision of the SpC versus TH correlation ina given geographical area thus should be checked with a few comprehensive analyses of cation and anion loadingsbefore the field worker relies on SpC alone for e.g. limestone or dolomite dissolution rate estimates.

Where SpC is close to or exceeds 600 µS/cm in Figure 4 (dark circles, n = 360), the correlation coefficients falldrastically (R2 = 0·22 in this study) and the error bands become unacceptable. In the majority of cases this is becauseof the increasing abundance (stacking) of common and foreign ions from non-carbonate sources. SpC may also exceed~600 µS/cm in purely bicarbonate waters, due to rapid evaporation (as in the Barkly Tableland cited here) or toadditional dissolution by CO2 from deep sources (Argens, Dordogne, Baishuitai and Huanglong).

Table III. Mean and standard error in the SpC = f (TH) relationship from ananalysis of 1949 water samples where SpC ≤ 600 µS/cm, compared with thetheoretical relationship computed with PHREEQC

SpC [µµµµµS/cm] Total hardness Total hardnessfrom the best fit from the theoretical[mg/L CaCO3] curve [mg/L CaCO3]

100 43 ± 1 52200 93 ± 2 118300 143 ± 2 183400 193 ± 2 249500 243 ± 2 315600 293 ± 3 381

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Copyright © 2005 John Wiley & Sons, Ltd. Earth Surf. Process. Landforms 31, 221–234 (2006)

Figure 3. (a) Correlation between SpC and total hardness (expressed as mg/L CaCO3) for selected dolomite karsts: waters withSpC < 600 µS/cm (open triangles) and SpC > 600 µS/cm (dark triangles). PHREEQC curves for dolomite dissolution are markedby dashed lines for ion activities (lower) and ion concentrations (upper). (b) Correlation of SpC and total hardness (expressed asmg/L CaCO3) for springs and wells in Pennsylvania, draining limestone (open squares) and dolomite (open triangles); all waterswith SpC < 600 µS/cm. PHREEQC curves for dolomite dissolution are marked by dashed lines for ion activities (lower) and ionconcentrations (upper).

Specific conductivity in karst waters 231

Copyright © 2005 John Wiley & Sons, Ltd. Earth Surf. Process. Landforms 31, 221–234 (2006)

Figure 4. A ‘grand slam’ correlation between SpC and total hardness (expressed as mg/L CaCO3), utilizing 2309 publishedmeasurements of limestone and dolomite karst waters worldwide; waters with SpC < 600 µS/cm (open circles) and SpC > 600 µS/cm(dark circles). PHREEQC curves for calcite or aragonite dissolution are marked by dashed lines for ion activities (lower) and ionconcentrations (upper). The one standard error bands of best fits above SpC = 600 µS/cm are marked with short dashes.

Conclusions

In predominantly bicarbonate karst waters, specific electric conductivity measurements supported with a few simpletitrations for total hardness as mg/L CaCO3 will provide karst geomorphologists with valuable and quite preciseinformation on the probable water chemical composition. Where SpC < 600 µS/cm, a clear distinction can usually bedrawn between waters having little contamination with foreign ions from non-carbonate sources (i.e. with anions otherthan bicarbonate totalling less than 10%) and those substantially contaminated waters with SNC > 20%. Our samplesof clean limestone waters plot very close to the regression line and within the field defined by the theoreticalPHREEQC curves for calcite and aragonite dissolution (Figure 1(a), (b)). When an SpC measurement and a THtitration place the sample outside of this field water, contamination must be suspected (Figure 2(a), (b)). This is alsoillustrated in Figure 5(a). Plotting the range of a test set of comprehensive (major ion) measurements on this figure willsuggest whether any further such analysis must be done. The best fit lines for clean and polluted limestone waters arequite separate where SpC is below 400 µS/cm, helping to distinguish these two conditions. Where SpC > 450 µS/cmthe clean limestone water line enters the error bands for polluted limestone waters, so that discrimination betweenthem is no longer possible.

In clean limestone waters with SpC < 600 µS/cm total hardness is close to 1/2SpC:

TH = 0·53 SpC − 1·6 (19)

which supports a claim often made by purveyors of conductivity meters. Where SpC > 600 µS/cm the total hardnessof clean waters is higher:

TH = 0·70 SpC − 90 (20)

232 W. E. Krawczyk and D. C. Ford

Copyright © 2005 John Wiley & Sons, Ltd. Earth Surf. Process. Landforms 31, 221–234 (2006)

Figure 5. (a) The best fit lines and envelopes for waters with SpC < 600 µS/cm: clean water contacting limestone (solid line)and dolomite (dashed–dotted line); polluted (SNC > 20%) water contacting limestone (dashed line, with standard error bars).(b) Waters with SpC > 600 µS/cm: regression line for non-polluted water (SNC < 10%) contacting limestone (solid line); onestandard error bands for polluted (SNC > 20%) water contacting limestone (an open field marked by dashed lines) and dolomite(shaded field).

Specific conductivity in karst waters 233

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It is not possible to distinguish between limestone and dolomite waters at SpC < 250 µS/cm, but in the range250 < SpC < 600 µS/cm, for a given TH value, SpC will be lower for water contacting dolomites alone and sampleswill plot around the dolomite line (Figure 5(a)). Points representing dolomite waters on the SpC = f(TH) diagramsplot around the regression line and are placed within the field bordered by theoretical PHREEQC curves for dolomitedissolution (Figure 3(a), (b)). As was the case with limestone waters, when SpC measurements and TH titration placethe sample outside this field contamination is likely.

Dolomite waters can be discriminated from polluted limestone waters with SNC > 20% everywhere belowSpC = 600 µS/cm, and from clean limestone waters above SpC = 250 µS/cm.

Where SpC > 600 µS/cm in purportedly bicarbonate waters, there will usually be ions from foreign sources presentin abundance. Error bands about mean statistical relationships then become very large, overlapping both the cleandolomite and the polluted limestone cases (Figure 5(b)). In such waters, limestone sources cannot be discriminatedfrom dolomite. To discriminate clean from polluted limestone waters, titration for bicarbonate should be undertaken;when it is nearly equal to total hardness (both values in meq/L), the water is not polluted but contains additionalcarbon dioxide.

To conclude with a historical note: in the 1960s the French geomorphologist, Jean Corbel, introduced measurementof water resistivity (the reciprocal of conductivity) to estimate dissolved limestone loads in water. His equation,transformed to specific conductivity at 25 °C, is

TDS = 0·62 SpC (21)

This equation has been used in karst denudation research for many years. The TDS estimates of Equation (21) yieldvalues 15–17% greater for clean limestone waters with SpC < 600 µS/cm than do the TH estimates from Equation(19). Similarly, TDS values from Equation (21) are 17–40% greater than TH values with Equation (18) (based oncomparison with 1949 water samples); the biggest disagreements arise where SpC is low, e.g. ~100 µS/cm.

AcknowledgementsWe thank Michel Bakalowicz for chemical analyses of French springs, Russell Drysdale for chemical data from the Apuan Alps,Cartaro Grande and Cartaro Piccole and Christopher Spötl for Obir Cave chemical analyses, Austria. Two anonymous refereesassisted in refining the paper. This work was supported by karst research projects of the University of Silesia to Krawczyk andresearch grants to Ford from the Natural Sciences and Engineering Research Council of Canada.

References

Andreo B, Carrasco F. 1999. Application of geochemistry and radioactivity in the hydrogeological investigation of carbonate aquifers (SierraBlanca and Mijas, southern Spain). Applied Geochemistry 14: 283–299.

Andreu JM, Ceron JC, Pulido-Bosch A, Estevez 1999. Algunos procesos hidrogeochimicos en la Cueva de Canalobre y en el acuifero delCabeco D’Or (Alicante). In Contribucion del Studio Ciencifico de las Cavidades Karsticas al Conocimiento Geologico, Andreo B,Carasco F, Duran JJ (eds). Patronato de la Cueva de Nerja: Nerja; 413–423.

Bayari CS, Kazanci N, Koyuncu H, Caglar S, Gokce D. 1995. Determination of the origin of the waters of Koycegiz Lake, Turkey. Journalof Hydrology 166: 171–191.

Crowther J. 1989. Groundwater chemistry and cation budgets of tropical karst outcrops, peninsular Malaysia, I. Calcium and magnesium.Journal of Hydrology 107: 169–192.

Drysdale RN, Taylor MP, Ihlenfeld C. 2002. Factors controlling the chemical evolution of travertine-depositing rivers of the Barkly karst,northern Australia. Hydrological Processes 16: 2941–2962.

Eith C, Kolb M, Seubert A, Viehweger K-H. 2001. Practical Ion Chromatography. Metrohm: Herisau.Enyedy-Goldner SR. 1994. The Karst Geomorphology of Manitoulin Island. MSc thesis, McMaster University.Ford DC, Williams PW. 1989. Karst geomorphology and hydrology. Chapman and Hall: London.Han GL, Liu CQ. 2004. Water geochemistry controlled by carbonate dissolution: a study of the river waters draining karst-dominated terrain,

Guizhou Province, China. Chemical Geology 204(1/2): 1–21.Hibszer B, Krawczyk WE. 2003. Zanieczyszczenia chemiczne wybranych 7ródel w dolinie Pradnika i Saspówki (Ojcowski Park Narodowy).

Jakotc i podatnotc wód podziemnych na zanieczyszczenie. Prace Wydzia¬u Nauk o Ziemi Uniwersytetu Tlåskiego 22: 53–62.Hibszer B, Krawczyk WE, Partyka J, Domagalska P. 2001. Charakterystyka chemiczna wód powierzchniowych i podziemnych w dolinie

Pradnika. In Badania naukowe w po¬udniowej cz´tci Wy8yny Krakowsko-Cz´stochowskiej, Materialy konferencyjne, Ojców; 48–53.Hill CA. 1995. Sulfur redox reactions: hydrocarbons, native sulfur, Mississippi Valley-type deposits, and sulfuric acid karst in the Delaware

Basin, New Mexico and Texas. Environmental Geology 25: 16–23.Kattan Z. 1997. Environmental isotope study of the major karst springs in Damascus limestone aquifer systems: case of the Figeh and

Barada springs. Journal of Hydrology 193: 161–182.

234 W. E. Krawczyk and D. C. Ford

Copyright © 2005 John Wiley & Sons, Ltd. Earth Surf. Process. Landforms 31, 221–234 (2006)

Krauskopf KB, Bird DK. 1995. Introduction to Geochemistry, 3rd ed. McGraw-Hill: New York.Krawczyk WE. 1993. Wlasno6ci fizykochemiczne wód doliny Kle6nicy i Moravy. In IV sympozium o krasu krkonossko-jesenicke soustavy,

Rehak J (ed.). Knihoma Ceske speoleologicke splecnosti, Svazek 24: 46–55.Krawczyk WE. 1994. Denudacja chemiczna w wybranych zlewniach SW Spitsbergenu. PhD thesis, University of Silesia.Krawczyk WE. 1996. Manual for karst water analysis. International Journal of Speleology. Handbook 1 – Physical Speleology.Krawczyk WE, Glowacki P, Nied9wied9 T. 2002. Charakterystyka chemiczna opadów atmosferycznych w rejonie Hornsundu (SW Spitsbergen)

latem 2000 r. na tle cyrkulacji atmosferycznych. In Polish Polar Studies. Funkcjonowanie i monitoring geoekosystemów obszarówpolarnych, Kostrzewski A, Rachlewicz G (eds). PoznaN; 187–202.

Krawczyk WE, Opolka-Gadek J. 1992. Interpretacja analiz chemicznych wody na przykladzie zlewni Potoku Ko6cieliskiego (Tatry Zachodnie).In Metody hydrochemiczne w geomorfologii dynamicznej, Kostrzewski A, Pulina M (eds). Prace Naukowe Uniwersytetu Ílaskiego:Katowice; 40–64. Instytut Badan Czwartorzedu i Geoekologii Universytetu in Adama Mickiewicza.

Langmuir D. 1971. The geochemistry of some carbonate ground waters in central Pennsylvania. Geochimica et Cosmochimica Acta 35:1023–1045.

Liu Z, Svensson U, Dreybrodt W, Yuan D, Buhmann D. 1995. Hydrodynamic control of inorganic calcite precipitation in Huanglong Ravine,China: field measurements and theoretical prediction of deposition rates. Geochimica et Cosmochimica Acta 59 (15): 3087–3097.

Liu Z, Zhang M, Li Q, You S. 2003. Hydrochemical and isotope characteristics of spring water and travertine in the Baishuitai area (SWChina) and their meaning for paleoenvironmental reconstruction. Environmental Geology 44: 698–704.

Marjolet G, Salado J. 1976. Contribution à l’Étude de l’aquifère karstique de la source du Lez (Hérault). Thèse 3ème cycle, Université desSciences et Techniques du Languedoc: Montpellier.

Martin C. 1991. État des recherches sur l’hydrologie, l’hydrochemie et la travertinisation actuelle dans le bassin supérieur de l’Argens (Var).Etudes de Geographie Physique Travaux 1991, XX, Universite d’Aix-Marseille II.

Ozler HM, Zirhhoglu O, Akdemir S, Sayak M. 2001. Karst hydrology and water quality of Yoldondu, Kusluk and Dilmetas Karst Springs,Van – Turkey. In Present State and Future Trends of Karst Studies. Gunay G, Johnson KS, Ford DC, Johnson AI (eds), Proceedings of theSixth International Symposium and Field Seminar, Marmaris, Turkey, UNESCO IHP-V Technical Documents in Hydrology 49; 1.

Panno SV, Hackley KC, Hwang HH, Kelly WR. 2001. Determination of the sources of nitrate contamination in karst springs using isotopicand chemical indicators. Chemical Geology 179: 113–128.

Parkhurst DL, Appelo CAJ. 1999. User’s guide to PHREEQC (Version 2) – a computer program for speciation, batch reaction, one-dimensional transport, and inverse geochemical calculations, U.S. Geological Survey Water-Resources Investigation Report 99-4259.

Stumm W, Morgan JJ. 1996. Aquatic chemistry. Chemical equilibria and rates in natural waters, 3rd ed. Wiley: New York.Íwieca A. 1998. Wp¬yw czynników antropogenicznych na rzeczny odp¬yw roztworów i zawiesin na miFdzyrzeczu Wis¬y i Bugu. Wydawnictwo

Uniwersytetu Marii-Curie Sklodowskiej: Lublin.Tarcan G. 2001. Hydrogeology and hydrogeochemistry of the Gulbahce bay hydrothermal karst system, Izmir, Turkey. In Present State and

Future Trends of Karst Studies. Gunay G, Johnson KS, Ford DC, Johnson AI (eds), Proceedings of the Sixth International Symposium andField Seminar, Marmaris, Turkey, UNESCO IHP-V Technical Documents in Hydrology 49; 1.

Urbani, F. 1995. Composicion fisico-quimica de las aguas karsticas de la zona de Bironga-Capaya, estado Miranda. Bolletin SociedadVenezolana Espeleologia 29: 1–6.

Vallejo M, Duran JJ. 2001. Mixture of deep and meteoric waters in a karst spring: an analytical study using hydrochemical, isotopicand radioactive techniques. In Present State and Future Trends of Karst Studies. Gunay G, Johnson KS, Ford DC, Johnson AI (eds),Proceedings of the Sixth International Symposium and Field Seminar, Marmaris, Turkey, UNESCO IHP-V Technical Documents inHydrology 49; 1.

Whitaker FF, Smart PL. 1994. Bacterial mediation of organic matter: a major control on groundwater geochemistry and porosity generationin oceanic carbonate terrains. In Breakthroughs in Karst Geomicrobiology and Redox Chemistry. Sasowsky ID, Palmer MV (eds), KarstWaters Institute of America, Special Publication 1; 72–74.

White WB. 1988. Geomorphology and Hydrology of Karst Terrain. Oxford University Press: Oxford.Worthington SRH, Ford DC. 1995. High sulfate concentrations in limestone springs: an important factor in conduit initiation? Environmental

Geology 25: 9–15.