Geostatistical and geochemical analysis of surface water leakage into groundwater on a regional scale: a case study in the Liulin karst system, northwestern China

Journal of Hydrology 246 (2001) 223±234

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Geostatistical and geochemical analysis of surface water leakage into groundwater on a regional scale: a case study in the Liulin karst system, northwestern China Y. Wang*, T. Ma, Z. Luo Department of Hydrogeology and Environmental Engineering, China University of Geosciences, 430074 Wuhan, China Received 27 October 1999; revised 15 February 2001; accepted 6 March 2001

Abstract The Liulin karst system is typical of hydrogeological systems in northern China, with a group of springs as the dominant way of regional groundwater discharge. Surface water leakage into groundwater has been observed in six sections of the rivers in the study area. To extract hydrogeological information from hydrochemical data, 29 water samples were collected from the system. On a trilinear diagram, most of the groundwater samples are clustered around the surface waters, indicating the effect of leakage on their chemistry. R-mode factor analysis was made on seven variables (Na, Ca, Mg, SO4, Cl, HCO3, and NO3) of the samples and three principal factors were obtained: the F1 factor is composed of Ca, Mg and SO4 , the F2 of HCO3 and NO3, and the F3 of Na and Cl. These factors are then used as regionalized variables in ordinary Kriging for unbiased estimates of the spatial variations of their scores. Considering regional hydrogeological conditions, the hydrogeological implications of the spatial distribution of the factor scores as related to the effects of the surface leakage are discussed. To evaluate the geochemical processes, the geochemical modeling code NETPATH was employed. The modeling results show that mixing commonly occurs in the system and dolomite dissolution is more important than calcite dissolution. Dedolomitization (calcite precipitation and dolomite dissolution driven by anhydrite dissolution) is locally important, in the western ¯ank of the system where the surface water leakage has the least effect. q 2001 Elsevier Science B.V. All rights reserved. Keywords: Karst; Groundwater; Geochemistry; Geostatistics; Modeling; Surface water

1. Introduction Surface±subsurface water interaction plays an important role in controlling groundwater chemistry and dynamics in karst terrains. Understanding these processes is the basis for sustainable development, management and protection of water resources. Losses of stream¯ow into groundwater systems are * Corresponding author. Tel.: 186-2787-482829; fax: 186-2787481365. E-mail address: [email protected] (Y. Wang).

major processes of such interactions. Under natural conditions, there are basically three pathways of the losses. The ®rst and the second refer to the transformation of surface runoff into subsurface either via swallow holes at the margin of the karst, or via sinkholes as internal ¯ow (White, 1988). The third is the leakage through carbonate bedrock beneath the river bed, as often observed in the 19 major karst groundwater systems in Shanxi Province, northwestern China (including Liulin for the present study), which are the most important source of water supply for the province (Han et al., 1993). Such leakage

0022-1694/01/$ - see front matter q 2001 Elsevier Science B.V. All rights reserved. PII: S 0022-169 4(01)00376-6

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problems are dif®cult to be quantitatively studied, not only because of the extreme complexity of the dissolution cavities, but because of the regional scale at which the leakage takes place. Arti®cial tracer methods using purposefully drilled wells have been widely applied to detect the bypass routes of reservoir water around dams built in carbonate terrain (Silar, 1988). These methods are unsuitable for regions of several thousand square kilometers, where groundwater ¯ows in anisotropic and heterogeneous aquifers, as commonly observed in the karst systems in Shanxi. As an alternative approach to this problem, hydrochemistry of groundwaters as a natural tracer to demonstrate the leakage process is applied by us and discussed in the present paper. As a major diagnostic tool in groundwater hydrology, hydrogeochemical data have been used to identify recharge zones and ¯ow patterns, calculate recharge rates or mixing ratios, and to discern hydraulic connections between aquifers (Hem, 1989; Mayo et al., 1992; Mazor et al., 1993; Panno et al., 1994; Wang and Khaustov, 1997). Geostatistical analysis of geochemical data can often give some insights into the underlying factors controlling hydrogeological processes. For instance, Kriging has been found to be especially useful for analyzing regional scale hydrochemical data (Goovaerts et al., 1993). The objectives of the present study are: (1) to characterize the hydrogeochemical features of the Liulin karst groundwater system as a typical case in northern China; and (2) to show the effectiveness of combining geostatistical and geochemical analysis techniques to extract hydrological information about the leakage processes from hydrogeochemical data.

at a given point (Rizzo and Dougherty, 1994). The concept of a regionalized variable is the basis of Kriging. Many parameters in karst hydrogeology studies can be regarded as regionalized variables, such as permeability, thickness of karsti®ed strata, dissolved component concentration, saturation index, etc., because these parameters not only show random spatial distribution patterns affected by many uncertain local factors in karsti®cation processes, but statistically re¯ect regional intrinsic processes controlling karst development. Various Kriging models have been developed, such as ordinary Kriging, factorial Kriging, and neural Kriging (Goovaerts, 1992, 1997; Rizzo and Dougherty, 1994). The principle of the ordinary Kriging model used in this study is as follows. Given n measurements of a regionalized variable Z(xi) (i ˆ 1,2,¼,n) in period t, the estimate of the variable using ordinary Kriging at an unmeasured point (x0) is a weighted sum of the available measurements Z p …x0 † ˆ

2.1. Geostatistical methods Factorial analysis and ordinary Kriging are combined in this study to extract hydrogeologic information related to surface runoff leakage in Liulin. Factorial analysis has been widely used in geoscience studies. Also, as one of the most widely used geostatistical methods in the hydrology community in recent years, Kriging is a powerful interpolating approach in unbiased estimation of the ®eld variables

iˆ1

li Z…xi †

…1†

where l i are the weights of the Kriging estimator that minimize the estimation variance. Supposing that the regionalized variable Z(xi) meets the intrinsic hypothesis, the ordinary Kriging equations in period t show n X iˆ1

li G…x i 2 xj † 1 m ˆ G…x i 2 x0 †

i ˆ 1; 2; ¼; n …2†

and n X iˆ1

2. Methods

n X

li ˆ 1

…3†

where G and m are the variogram and Lagrange multiplier, respectively. The variogram is a function of the distance between two measurement points: 2G…x i 2 xj † ˆ Var‰Z…xi † 2 Z…xj †Š

…4†

In practice, a stationarity hypothesis is usually made, and thus we have 2G…x i 2 xj † ˆ E{‰Z…xi † 2 Z…xj †Š2 }:

…5†

In the present study, we ®rst calculate the scores for principal factors using R-mode analysis of chemical

Y. Wang et al. / Journal of Hydrology 246 (2001) 223±234

compositions of karst groundwaters from Liulin. The three principal factors, composed of different major ions in the waters, are considered to have hydrogeological meaning. Each of the principal factors is in¯uenced both by regional hydrogeological conditions and by local geology and hydrogeology. Thus, these principal factors are regarded as regionalized variables. Ordinary Kriging is then applied for an unbiased estimate of the spatial variation of the principal factors. 2.2. Geochemical modeling Inverse geochemical modeling (Plummer, 1992) has been widely used in interpreting geochemical processes that account for the hydrochemical and isotopic evolution of groundwaters. One of the most powerful inverse geochemical modeling codes, NETPATH, is used in the present study. NETPATH 2.0 (Plummer et al., 1994) is an interactive Fortran 77 computer program that can be used to compute the mixing proportions of two to ®ve initial waters and net geochemical reactions accounting for the observed composition of a ®nal water. NETPATH contains two Fortran 77 codes: DB.FOR and NETPATH.FOR. In the present study, we ®rst use DB.FOR to input and edit the chemical and isotopic analysis data and then run WATEQFP included in DB to calculate mineral saturation indices. NETPATH.FOR is then run to calculate mixing ratios of surface water and karst groundwater and interpret geochemical reactions in the chemical evolution of natural waters in Liulin.

225

groundwater system's central region where the surface water leakage occurs were collected. Alkalinity, pH and temperature measurements were made immediately after sampling. For the pH and temperature measurements of water from wells, a ¯ow cell was used. Probes were placed directly into the current of a spring or a river to measure the pH and temperature. All samples were ®ltered through 0.2-micron cellulose acetate ®lters. For each sample, a 1-l polyethylene bottle was ®lled for anion analysis using IC (Dionex-120). Among the 29 samples, 16 representative samples were carefully selected, and for each sample a 100-ml polyethylene vial was ®lled and immediately acidi®ed using 1:1 nitric acid to pH , 2 for analysis of metallic elements by AAS. The 16 representative samples include: (1) one sample from each of the ®ve groups of the karst springs; (2) one sample from each of the four major rivers; and (3) seven samples from water supply wells that pump water from the middle Ordovician carbonate aquifer. The river water samples were collected upstream of the river section where leakage has been detected. Among the seven groundwater samples, four samples were collected from wells located around and downstream of the river leakage sections, two samples were from wells close to the Liulin springs, and one from a deep well (called the Jiajiagou Mine well, 700 m deep) located in a region without any rivers. In this way, the most important types of natural waters in Liulin were included in the sampling list and the groundwater samples were distributed over different hydrogeologic units of the Liulin karst system.

2.3. Leakage measurement Surface runoff leakage has been observed and measured in six sections of the rivers in the study area from 1990 to 1995. The measurement of the runoff discharge at the start and at the end of the section of a leaking river was made four times each year (in March, June, August and December) using a ¯ow meter. The leaked amount was then obtained by subtracting the discharge at the start with that at the end. 2.4. Sampling and hydrochemical measurements In August 1995, 29 water samples from the karst

3. Hydrogeology of the Liulin karst system Located in a semi-arid region, the Liulin karst system is hydrogeologically typical in northern China: a group of springs are the predominant way of regional groundwater discharge. Along a distance of less than 2 km of the Sanchuan River valley outcrop more than 100 springs with a total discharge of 2.3±6.1 m 3/s. These springs outcrop in ®ve groups and are collectively called the Liulin springs (Fig. 1). The northern and eastern boundaries of the system are composed of Archaen metamorphic rocks, while the southern and western boundaries consist of

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Fig. 1. Hydrogeology map and cross section of the Liulin karst groundwater system (modi®ed after Han et al., 1993). Water samples were collected from the central part of the system as delimited by the dashed lines.

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Fig. 2. Locations of the water samples and the start and ending points of the six leakage sections along the four rivers. The sections are labeled and highlighted with thicker lines than other reaches of the rivers. The map at the lower part is an enlarged view of the No. 6 leakage section and locations of the water samples (No. 7, 8, 25±29) from the discharge zone.

Carboniferous±Permian coal-bearing sandstone and shale (Fig. 1). Karst groundwaters in the middle Ordovician carbonate aquifers move towards the Liulin springs. Groundwater from the middle Ordovician karst aquifers has been one of the most important sources of water supply in northern China. The lithology of the aquifer is limestone and dolomite, with several thin layers or lenses of anhydrite at the middle of the carbonate aquifers. The anhydrite can be partly preserved under con®ned conditions only when the aquifers are overlain by thick, relatively impermeable Carboniferous±Permian coal-bearing sandstone and shale. Along the river channels where leakage occurs,

the middle Ordovician carbonates either outcrop or underlie alluvial sediments, creating favorable seepage conditions for the river waters.

4. Results and discussion 4.1. River water leakage Flow meter measurements over years have shown surface runoff leakage along six sections of the four rivers (Fig. 2 and Table 1). The largest leakage was observed at No. 3 leakage section along the

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Table 1 Average values of runoff and leakage measured in 1990±1995 (in m 3/s). The leakage section numbers are the same as labeled in Fig. 2 Leakage section no. 1

2

3

4

5

6

L (km) a River

13.8 Xiaodong-chuan

6.7 Nanchuan

14.7 Dadongchuan

9.8 Beichuan

2.0 Sanchuan

6.4 Sanchuan

Runoff Leakage (%) Runoff Leakage (%) Runoff Leakage (%) Runoff Leakage (%) Runoff Leakage (%) Runoff Leakage (%)

0.329 0.053 (16.1) 0.267 0.021 (7.9) 0.140 0.014 (10.0) 0.252 0.023 (9.1) 0.178 0.041 (23.0) 0.320 0.045 (14.1)

0.265 0.035 (13.2) 0.110 0.020 (18,2) 0.210 0.042 (20.0) 1.030 0.080 (7.8) 1.040 0.099 (9.5) 0.332 0.045 (13.6)

0.430 0.235 (54.7) 0.268 0.194 (72.4) 0.187 0.148 (79.1) 0.164 0.090 (54.9) 0.095 0.087 (91.6) 0.136 0.112 (82.4)

2.160 0.162 (7.5) 1.660 0.350 (21.1) 3.210 0.356 (11.1) 1.390 0.300 (21.6) 0.530 0.140 (28.0) 1.620 0.225 (13.9)

3.250 0.139 (4.3) 1.530 0.155 (10.1) 2.280 0.103 (4.5) 2.010 0.117 (5.8) 1.700 0.033 (1.9) 1.470 0.224 (15.2)

4.050 0.260 (6.4) 2.090 0.049 (2.3) 0.550 0.077 (14.0) 1.505 0.090 (5.98) 1.738 0.030 (1.7) 1.230 0.113 (9.2)

a

Year

1990 1990 1991 1991 1992 1992 1993 1993 1994 1994 1995 1995

L, length of the leakage section. The start and end of the sections are shown in Fig. 2.

Dadongchuan River, with 55±92% of the river water leaking via ®ssures and pores in carbonate rocks of the riverbed. 4.2. Hydrochemistry Locations of the 29 samples are shown in Fig. 2. The major ion concentrations of the 29 samples are listed in Table 2. The hydrochemical properties and trace element contents of the 16 representative samples are listed in Table 3, and these samples were plotted onto a Piper diagram (Fig. 3) It can be seen from Tables 2 and 3 and Fig. 3 that the surface waters (samples 21±24) have the lowest TDS values and belong to Ca-HCO3 waters. Far from the surface waters on the trilinear plot are samples 16 and 29 with highest chloride and K, Sr, Si and F contents. These two samples are located in the northwestern part of the system, where the middle Ordovician aquifers are overlain by the relatively impermeable Carboniferous±Permian clastic rocks (Fig. 1). It is interesting to note that the chloride content of sample 29 (the Liujiageda spring, Fig. 2) in the discharge zone is lower than that of sample 16 upstream of the groundwater ¯ow path. If no mixing/ dilution happened to water sample 16 along its ¯ow path to the discharge zone, its chloride concentration should increase, following a normal major ion

evolution sequence under isolated hydrogeologic conditions (Chebotarev, 1955). The decrease in chloride content is, therefore, most probably related to the mixing/dilution with the surface water and groundwaters of other ¯ow paths before its discharge to the surface. As can be seen from Table 2, the surface waters (samples 21±24) and groundwater samples from other ¯ow paths (samples 1±15, and samples 17±20) have much lower chloride concentration as compared with that of sample 16. Most of the other groundwater samples are clustered around the surface waters, indicating the effect of leakage on their chemistry (Fig. 3). It is interesting that the ®ve groups of springs show different chemical compositions. Among them, the Longmenhui and Shangqinglong springs (samples 25 and 26), outcropping on the southern bank of the Sanchuan River (Fig. 2), have lowest TDS and Ca± HCO3 as major ions. As compared with the other three springs, these two springs are obviously strongly diluted by leaked surface waters before discharge. 4.3. Factorial and Kriging analysis R-mode factor analysis was made on the seven variables (Na, Ca, Mg, SO4, Cl, HCO3, and NO3) of the 29 samples. The three eigen values of R are: r1 ˆ 3.45, r2 ˆ 1.75, and r3 ˆ 1.08, with a cumulative percentage

Y. Wang et al. / Journal of Hydrology 246 (2001) 223±234

229

Table 2 Major ion concentration of surface waters and karst groundwaters from middle Ordovician aquifers (in mg/l) No.

Site name

Water type

Na 1 (1K 1)

Ca 21

Mg 21

Cl 2

SO22 4

HCO2 3

NO2 3

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29

The Chenjiawan village The Wucheng village The Youpinfang village The Longmenhui village The Longmenta village The Lijiawan village The Yangjiagang village The Yangjiagang village The Changwan village The Qujiagou village The Zhongyang Food Inc. The Zhike village The Sanjiaozhuang village The Shang'ancun village Zhongyang Fertilizer The Jiajiagou Coal Mine The Liulin Coal Mine The Xinwu Coal Mine The Shanglouqiao village The Datuhe village The Beichuan River The Dadongchuan River The Xiaodongchuan River The Nanchuan River The Longmenhui Spring The Shangqinglong Spring The Yangjiagang Spring The Zaidong Spring The Liujiageda Spring

Well Well Well Well Well Well Well Well Well Well Well Well Well Well Well Well Well Well Well Well River River River River Spring Spring Spring Spring Spring

5.9 5.4 19.6 88.4 62.8 41.8 81.7 5.3 20.0 46.0 31.7 10.5 8.9 31.0 24.0 370.0 45.0 37.5 23.0 21.0 12.0 13.0 10.5 9.0 47.8 57.5 111.0 75.0 190.0

56.1 61.1 66.1 287.6 59.1 57.1 214.4 73.1 46.1 96.2 53.1 73.1 72.1 21.0 103.9 154.5 87.4 75.6 56.9 97.7 49.9 30.6 38.7 44.7 64.1 65.1 76.4 67.9 112.0

14.5 15.8 23.7 86.3 22.5 18.9 57.8 24.3 21.9 28.6 17.6 16.4 15.2 60.9 25.9 53.0 29.7 24.7 17.3 22.2 10.5 13.6 9.8 17.3 21.5 25.9 28.9 23.9 35.0

8.9 8.9 19.5 113.5 67.4 39.0 104.6 56.7 17.7 46.1 26.6 10.6 12.4 25.9 19.5 667.9 44.7 34.7 16.0 19.5 11.7 7.1 8.5 9.9 45.2 61.9 132.6 127.0 397.0

4.8 7.2 55.2 934.2 69.6 60.0 622.0 108.1 72.1 165.7 57.6 36.0 28.8 63.4 168.1 241.1 141.2 99.9 43.7 146.0 29.3 26.9 26.4 38.9 62.8 77.3 136.4 165.0 168.0

241.0 259.3 262.4 204.4 247.1 238.0 219.7 244.1 183.1 256.3 201.5 274.6 262.4 257.5 286.5 285.6 274.6 271.5 240.1 252.0 189.8 156.8 156.8 193.4 265.5 262.7 274.6 284.0 210.0

4.0 4.0 3.5 2.0 8.0 2.5 0.2 7.0 12.0 3.0 8.0 10.0 8.0 13.5 16.1 7.3 8.1 11.2 13.1 11.6 3.5 3.6 1.7 2.6 9.6 9.3 12.7 10.0 7.5

Table 3 Hydrochemical properties and minor element contents of the 16 representative samples (in mg/l except temperature and pH) (the sample numbers are the same as in Table 2. The Cd and As contents of all the samples are lower than 0.005 and 0.01 mg/l, respectively. nd, Not determined) No.

T (8C)

pH

TDS

K

Sr

Fe

Zn

Br

H2SiO3

F

14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29

11.5 12.0 25.0 17.0 18.0 12.0 16.0 21.5 23.0 24.5 18.0 25.0 18.5 18.5 17.0 21.0

7.0 7.0 7.2 7.2 6.5 7.0 7.0 7.0 6.5 6.5 7.0 7.6 7.3 7.0 7.3 7.2

487 638 1833 642 567 423 581 319 263 265 325 386 562 787 815 1218

1.58 1.83 6.70 1.69 1.55 1.69 1.58 1.92 1.41 1.30 1.73 1.90 1.19 2.73 3.35 3.70

0.46 0.62 2.51 0.72 0.53 0.35 0.51 0.26 0.29 0.24 0.25 0.57 0.59 0.95 0.64 1.25

0.03 0.36 0.10 , 0.03 0.01 0.47 0.20 0.40 0.32 0.72 0.11 0.00 0.00 0.152 nd nd

, 0.01 0.01 , 0.01 , 0.01 , 0.01 0.03 0.03 , 0.01 , 0.01 , 0.01 , 0.01 0.04 0.04 0.02 0.01 , 0.01

, 0.10 0.15 0.30 0.15 0.15 0.15 0.10 , 0.10 , 0.10 , 0.10 , 0.10 0.19 0.16 0.25 0.35 0.40

14.95 13.23 15.29 12.38 13.23 13.41 11.87 13.58 13.38 14.09 9.81 13.02 22.02 13.58 24.21 25.11

0.5 0.3 1.2 0.6 0.4 0.4 0.3 0.5 0.4 0.8 0.2 0.7 0.6 0.7 1.0 1.0

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Y. Wang et al. / Journal of Hydrology 246 (2001) 223±234

Fig. 3. Trilinear hydrochemical diagram of the natural waters from Liulin. The symbols and sample numbers are the same as those in Fig. 2.

variable contribution of 89.72%. Correspondingly, three principal factors F1, F2 and F3 were obtained (Fig. 4): the F1 factor is composed of Ca, Mg, and SO4, the F2 of HCO3 and NO3, and the F3 of Na and Cl. With the scores for the principal factors being calculated, ordinary Kriging was then applied for

unbiased estimates of the spatial variations of the scores and the results are given in Fig. 5. The high values of the F1 factor are found in the western region where the Ordovician aquifers are partly covered by Carboniferous±Permian coalbearing strata [Fig. 5(a)]. The karst groundwaters in

Fig. 4. Loadings of the three principal factors: (a) F1 ±F2 (unrotated); (b) F1 ±F3 (unrotated); (c) F1 ±F2 (after varimax rotation); (d) F1 ±F3 (after varimax rotation).

Y. Wang et al. / Journal of Hydrology 246 (2001) 223±234

231 21

21

SO22 4 .

the region are enriched in Ca , Mg and Two geochemical processes should be responsible for the high sulfate concentrations. One is the oxidation of sul®de minerals such as pyrite in the coal-bearing strata, producing hydrogen ion: 4FeS2 1 15O2 1 14H2 O ˆ 4Fe…OH†3 1 8SO22 4 1 16H1 ; and thus promoting carbonate dissolution: H1 1 CaCO3 ˆ Ca21 1 HCO2 3 : The process has been intensi®ed when more oxygen enters into the subsurface from the atmosphere due to strong and long-term abstraction of mine drainage in coal mining areas such as Jiajiagou where sample 16 was collected. The other process is the dissolution of anhydrite in the middle Ordovician carbonates that has been partly preserved over geologic history owing to the sealing effect of the relatively impermeable Carboniferous±Permian formations. Therefore, the spatial distribution of the F1 factor is an indicator of the hydrogeological condition of the karst regions where hydrochemistry of groundwaters is strongly affected by the coal-mining activity and anhydrite dissolution. The high values of the F2 factor (particularly NO3) are around the densely populated cityÐLisi [site 14, Fig. 5(b)]. Direct drainage of large amounts of municipal and industrial wastewaters into the rivers over decades has caused serious contamination of the surface waters. According to a water quality survey in 1990, surface waters around the city have been polluted by nitrate (The Water Resource Management Commission of the Luliang Prefecture, 1990). The increase in nitrate concentration of the local groundwaters is obviously related to the high nitrate content of the leaked surface waters. Since the F2 factor is composed of NO2 3 and HCO2 , it should be an indicator of the hydrochemical 3 effect of surface runoff leakage on groundwaters. As shown in Fig. 5(c), the high scores of the F3 factor are found in the northwestern ¯ank of the karst groundwater system, where the Ordovician Fig. 5. Spatial distribution of the principal factors F1 (Ca, Mg, SO4) (a), F2 (HCO3, NO3) (b), and F3 (Na, Cl) (c). Dots and numbers aside are sampling points and sample numbers corresponding to those in Fig. 2.

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aquifer is con®ned by the thick overlying Carboniferous±Permian clastic rocks. The regional ground water ¯ow in this part of the aquifer directs towards the Liujiageda spring (sample 29). As a result, the spring is least affected by surface water leakage and consequently has the highest temperature and TDS among the ®ve groups of springs, and a hydrochemical type (Na±Ca±Cl water) different from the other springs (Tables 2 and 3 and Fig. 3). The abnormality of the F3 factor scores in the northwestern ¯ank implies that it is an indicator of a hydrogeologically isolated environment. Hydraulic interconnection of karst waters in this environment with surface waters and shallow groundwaters is quite weak due to the sealing effect of the thick overlying Carboniferous±Permian clastic rocks. 4.4. Geochemical process analysis Results of Saturation Index calculation using WATEQFP contained in NETPATH are plotted onto Fig. 6. The karst waters except those in the northwestern ¯ank are slightly supersaturated or at equilibrium with respect to calcite [Fig. 6(a)]. All the waters, except those in the northwestern and northeastern ¯ank, are undersaturated with respect to dolomite [Fig. 6(b)], and all the water samples are undersaturated with respect to anhydrite [Fig. 6(c)]. The major reason for such a widespread undersaturation state with respect to the predominant mineral phases of the aquifer matrix should be the dilution caused by the mixing of karst groundwater with dilute, leaked surface water, which commonly has a low TDS and a Ca±HCO3 major ion composition. Modeling the hydrochemical evolution along ¯ow paths using NETPATH further supports this conclusion. To run NETPATH, we ®rst selected the constraints and models (Table 4). Calcite, dolomite and anhydrite are the three major minerals of the Ordovician aquifers, and correspondingly, are constrained by the dominant ions C, Ca, Mg, and S. The carbonate rocks of the aquifer outcrop in some parts of the Liulin karst system (Fig. 1), making it an open or semi-open Fig. 6. Contour maps of SI values of the karst groundwaters from the middle Ordovician aquifer: (a)SIcalcite, (b)SIdolomite, (c)SIgypsum. Dots and numbers aside are sampling points and sample numbers corresponding to those in Fig. 2.

Y. Wang et al. / Journal of Hydrology 246 (2001) 223±234 Table 4 Selected constraints, phases and parameters in geochemical modeling using NETPATH (1, dissolution only) Constraints

Phases

Parameters

Carbon, calcium, magnesium, sulfur

Calcite, dolomite, 1anhydrite, carbon dioxide

Dilution/evaporation: yes

233

Samples 10 and 16 were collected from the northwestern ¯ank of the Liulin karst system, corresponding to a hydrogeologically isolated environment as discussed above. As a matter of fact, anhydrite was found when the Qujiagou and Jiajiagou wells were drilled. Dedolomitization may proceed when anhydrite preserved in the aquifer strata is dissolved. 5. Conclusions

system in many places. Therefore, carbon dioxide is an important phase for the system. Some representative results of modeling using NETPATH are tabulated in Table 5. A positive value refers to the mass entering the solution, and a negative value the mass leaving. For all the ¯ow paths modeled, mixing ratios between two or three end-members have been calculated. A typical example can be seen in the last two ¯ow paths of Table 5: the mixing ratio of karst groundwater from the Shanglouqiao well (sample 19) is quite high or even predominant in the ®nal water, up to 93 percent as along the Shang'ancun (sample 14)±Shanglouqiao (sample 19)±Chenjiawan (sample 1) ! Longmenhui (sample 25) ¯ow path. This is of course again a clear indication of the vital importance of surface water leakage for the water resource in the Liulin karst groundwater system. Geochemical reactions along the ¯ow paths can be postulated from the modeling results. Dolomite dissolution is more important than calcite dissolution. Comparing Fig. 6(a) with Fig. 6(b), we can also see that calcite is basically at equilibrium in the system and even supersaturated in some places. But dedolomitization, namely dolomite dissolution and calcite precipitation caused by anhydrite dissolution, has been found in our modeling only along the Qujiagou (sample 10) ! Jiajiagou (sample 16) ¯ow path.

The results obtained in the present study show that coupling geostatistical techniques with hydrogeochemical analysis is an effective approach in regional karst hydrology studies. The major conclusions of the study are as follows: 1. Surface water leakage into the Ordovician carbonate aquifers occurs in most parts of the Liulin karst groundwater system. In places such as No. 3 leakage section along the Dadongchuan River, as much as 55±92% (up to 0.235 m 3/s) of the surface runoff leaked into the subsurface. The leakage has widespread in¯uence on the dynamics and chemistry of the karst groundwaters. 2. Hydrochemical data contain important information about the impact of the leakage on the groundwater resource. The buried karst groundwaters in the western ¯ank of the system having the highest TDS and K, Sr, Si and F contents and the Liujiageda springs discharging these waters are least affected by the leakage. 3. Quantitative analysis of the available hydrochemical data using geostatistical techniques can help elucidate the hydrogeological features of the system studied. The spatial variations of the three principal factors have distinct patterns and are related to the regional hydrogeological settings. The F1 factor is an indicator of the hydrogeological

Table 5 Geochemical modeling results using NETPATH. The ¯ow paths were selected according to the regional groundwater ¯ow directions. The water sample(s) before and after the arrow (¯ow direction) correspond to initial and ®nal waters, respectively Flow path (sample number)

(1)±(15) ! (5)

(10) ! (16)

(19)±(20) ! (6)

(14)±(19)±(1) ! (25)

(14)±(19)±(1) ! (26)

Calcite Dolomite Anhydrite Mixing ratio

2 0.522 0.204 0.000 0.57(1):0.43(15)

2 0.701 1.962 1.889

2 0.148 0.191 0.000 0.70(19):0.30(20)

0.000 0.010 0.154 0.07(14):0.93(19):0.00(1)

0.000 2 0.890 0.000 0.41(14):0.59(19):0.00(1)

234

Y. Wang et al. / Journal of Hydrology 246 (2001) 223±234

condition of the buried karst regions where the hydrochemistry of groundwaters is strongly affected by the coal-mining activity and anhydrite dissolution; the F2 factor is an indicator of the hydrochemical effect of surface runoff leakage on groundwaters; and the F3 factor is an indicator of a hydrogeologically isolated environment, least affected by surface water leakage. 4. Geochemical modeling results show a widespread undersaturation state of the waters from the Ordovician aquifers with respect to the major mineral phases (calcite, dolomite and anhydrite). This results from the dilution effect and short residence time of the leaked surface water, usually with low TDS and a Ca±HCO3 major ion composition. Major geochemical reactions occurring in the system include mixing and carbonate dissolution. Dolomite dissolution is stronger than calcite dissolution. Dedolomitization only occurs in the western ¯ank of the system where the leakage has the least effect. It is worth noting that more detailed geochemical data are needed to get a better description of the effect of surface water leakage on the karst groundwaters. For instance, sulfur and carbon isotopes can provide quantitative information about geochemical reactions, as shown by Plummer et al. (1990) in their work on the Madison aquifer in the US. Time series data are also extremely important for monitoring the change of the karst groundwater resource.

Acknowledgements The research was ®nancially supported by National Natural Science Foundation of China (No.49832005) and the Ministry of Science and Technology of China (Grant No.95-pre-39). Dr L. Niel Plummer of USGS is appreciated for sending us the NETPATH code. Dr Eric J. Reardon of University of Waterloo is acknowledged for giving valuable supervision and providing computing facilities when Yanxin Wang stayed in Waterloo as a visiting scientist and prepared the manuscript in 1998±1999. The manuscript greatly bene®ted from the constructive comments from two anonymous reviewers and Dr Marios Sophocleous.

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