Comparison of arsenic geochemical evolution in the Datong Basin (Shanxi) and Hetao Basin (Inner Mongolia), China

Applied Geochemistry 27 (2012) 2315–2323

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Comparison of arsenic geochemical evolution in the Datong Basin (Shanxi) and Hetao Basin (Inner Mongolia), China Ting Luo, Shan Hu, Jinli Cui, Haixia Tian, Chuanyong Jing ⇑ State Key Laboratory of Environmental Chemistry and Ecotoxicology, Research Center for Eco-Environmental Sciences, Chinese Academy of Sciences, Beijing 100085, China

a r t i c l e

i n f o

Article history: Received 9 June 2011 Accepted 13 August 2012 Available online 30 August 2012 Editorial handling by D. Polya

a b s t r a c t Insightful knowledge of geochemical processes controlling As mobility is fundamental to understanding the occurrence of elevated As in groundwater. A comparative study of As geochemistry was conducted in the Datong Basin (Shanxi) and Hetao Basin (Inner Mongolia), two strongly As-enriched areas in China. The results show that As concentrations ranged from <1–1160 lg L1 (n = 37) in the Datong Basin and <1–804 lg L1 (n = 62) in the Hetao Basin. The groundwater is of the Na-HCO3 type in the Datong Basin and Na-Cl-HCO3 type in the Hetao Basin. Silicate mineral weathering and cation exchange processes dominated the groundwater geochemistry in the two study areas. Principal component analysis of 99 groundwater samples using 12 geochemical parameters indicated positive correlations between concentrations of As and Fe/Mn in the Datong Basin, but no correlation of As and Fe/Mn in the Hetao Basin. Phosphate correlated well with As in the Datong Basin and Hetao Basin, suggesting phosphate competition might be another process affecting As concentrations in groundwater. High concentrations of As, Fe, and Mn occurred in the pe range 2 to 4. The results of this study further understanding of the similarities and differences of As occurrence and mobility at various locations in China. Ó 2012 Elsevier Ltd. All rights reserved.

1. Introduction Elevated concentrations of naturally-occurring As in groundwater in many countries including Argentina, Bangladesh and China are a cause of great public concern (Smedley and Kinniburgh, 2002). About 2 million people in China suffer from drinking As-tainted water and at least half a million are consuming groundwater with As concentrations >50 lg L1 (Guo et al., 2001; Yu et al., 2007). The Hetao (Inner Mongolia) and Datong (Shanxi) Basins are the two regions with the highest groundwater As concentrations and most reported endemic arsenicosis cases in China (Smedley et al., 2003; Yu et al., 2007). Since the 1980s, tube wells have been installed widely at a depth of 30–60 m in Shanxi (Zhu et al., 2006). However, even now, some villagers still drink As-enriched groundwater due to the cost and inconvenience of alternative drinking water supplies. Groundwater As occurrence is controlled mainly by redox potential, pH, microorganisms, and adsorption on a variety of Fe/ Mn oxides (Islam et al., 2004; Campbell et al., 2006). Reductive dissolution of oxide minerals in reducing environments due to microbial activities is widely accepted as a principal mechanism for As release (Oremland and Stolz, 2003; Islam et al., 2004). In addition, competitive ions such as phosphate and silicate may promote As release (Hongshao and Stanforth, 2001; Anawar et al., 2004). ⇑ Corresponding author. Tel./fax: +86 10 6284 9523. E-mail address: [email protected] (C. Jing). 0883-2927/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.apgeochem.2012.08.012

Therefore, the fate of groundwater As depends on complex biogeochemical processes and may be quite different in various geological formations (Mukherjee et al., 2009). Groundwater geochemistry has been extensively studied to determine the extent and evolution of As in Bangladesh (Zheng et al., 2005; Selim Reza et al., 2010), Croatia (Ujevic et al., 2010), Cambodia (Buschmann et al., 2007) and Pakistan (Baig et al., 2009) in the last few decades. Recently, several studies have reported the hydrology and concurrent As mobilization in the Hetao and Datong Basins in China (Smedley et al., 2003; Guo and Wang, 2004; Zhu et al., 2006; Deng et al., 2009; Guo et al., 2011). However, no work has attempted to explore the differences and similarities in groundwater geochemistry between these two areas. These two strongly enriched areas, the Hetao Basin in Inner Mongolia and the Datong Basin in Shanxi province, belong to the Cenozoic rift basin (Guo and Wang, 2004; Guo et al., 2008). In the Hetao Basin, shallow groundwater primarily occurs in the Quaternary alluvial, alluvial–pluvial and fluvial–lacustrine aquifers (Guo et al., 2008). In the Datong Basin, widely distributed As-rich rocks and coals are the original source of As in groundwater (Wang et al., 1998). The possible processes leading to the elevated As concentrations in groundwater are not fully understood, and the lack of knowledge about As mobilization processes in groundwater may hinder the development of appropriate water treatment technologies. The objectives of this study are (1) to elucidate the possible geochemical processes involved in As-enrichment of groundwater, and (2) to assess geochemical parameters correlated with As

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mobilization using principal component analysis (PCA). Ninetynine groundwater samples were collected from private tube wells in the Hetao and Datong Basins, and 23 geochemical parameters were determined for each sample. The results improve understanding of the As speciation and mobility in these two strongly enriched areas in China.

Lianhe, Langshannongchang, Mingqiang, Sizhi, Wuxingerdui, Xianfengqishe, Xinxinsidui and Zhaotan are representative of Asaffected districts. The major differences between these two studied areas are shown in Table 1.

2. Materials and methods

Thirty-seven and sixty-two samples were collected from existing tube wells in the Datong and Hetao Basins, respectively, in July 2010. The tube well water is used as a drinking water source without further treatment. At each sampling site, the geographical coordinates were recorded with a hand-held global positioning system, and well depth was obtained through interview with private well owners. The well depth ranged from 20 to 90 m in the Datong Basin, and the majority of the well depths were about 20–30 m in the Hetao Basin. Electrical conductivity (EC), oxidation–reduction potential (ORP), total dissolved solids (TDSs), and temperature of groundwater were monitored in situ in a flow-through cell using a multi-parameter water quality meter (U-52G, Horiba, Japan). The ORP reference electrode used Ag/AgCl with 3.33 mol L1 KCl as inner solution, and the reported Eh values were standardized against normal hydrogen electrodes by the meter. Prior to Eh measurements, calibration was carried out using Zobell’s solution (3  103 M potassium ferrocyanide and 3  103 M potassium ferricyanide in 0.1 M KCl) with a standard potential of +228 mV at 25 °C. The parameters were recorded until the readings reached a stable level after continuous pumping of the

2.1. Study sites Fig. 1 shows the locations of sampling sites. Shanyin County is situated in the SW of the Datong Basin with relatively stagnant zones of groundwater flow. It is located in the north of Shanxi province between the coordinates 39°110 and 39°470 N latitude, 112°250 and 113°040 E longitude. About 230,000 people live in this 1657 km2 area. The mean annual temperature is 7 °C, and the annual rainfall is about 362 mm. The villages Daying, Gucheng, Silizhuang, Xiyanchi are seriously affected by As, and are typical districts with endemic arsenicosis. Some of the villagers are still drinking high As groundwater without treatment. The Hetao Basin in Inner Mongolia lies about 500 km NW of Shanxi. This 10,000 km2 area is semiarid to arid with annual rainfall between 150 and 400 mm. Since the Qin Dynasty about 221 BC, the Yellow River has been used across this area for irrigation purposes. The infiltration of irrigation water has resulted in saline groundwater. Hongqierdui,

2.2. Sample collection

Fig. 1. The study areas and sampling locations in the Datong Basin and Hetao Basin, China. Inserts show sampling villages: Daying (DY), Gucheng (GC), Silizhuang (SL) and Xiyanchi (XY) in the Datong Basin; Hongqierdui (HQ), Lianhe (LH), Langshannongchang (LS), Mingqiang (MQ), Sizhi (SZ), Wuxingerdui (WX), Xianfengqishe (XF), Xinxinsidui (XX), Zhaotan (ZT) in the Hetao Basin.

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85–115%. About 10% of samples were analyzed in duplicate. Alkalinity was measured using the titration method. TOC was determined with a Phoenix 8000 TOC Analyzer (Tekmar-Dohrmann, US).

Table 1 Key differences between the Datong Basin and Hetao Basin.

Province Studied well numbers Water type Depths (m) Highest As (lg L1) SO4, Cl and NO3 concentration Correlation between As and Fe Correlation between As and Mn

Datong Basin

Hetao Basin

Shanxi 37 Na-HCO3 20–90 1160 Low High High

Inner Mongolia 62 Na-Cl-HCO3 20–30 840 High None None

wells for 10–20 min. Then, groundwater samples were collected in clean polyethylene bottles. Sub-samples were passed through a 0.45-lm syringe filter and acidified immediately after collection. Soluble constituents (i.e. Al, Ca, Mg, Na, K, Pb, Si, Fe, and Mn) were measured in filtered and 2 3 acidified samples. Concentrations of F-, Cl-, NO 3 , SO4 , PO4 , and alkalinity were determined using filtered samples without acidification. Samples for the total organic C (TOC) analysis were collected in amber glass bottles to prevent light exposure, and were filtered and acidified with concentrated H2SO4 to pH < 2. All samples were stored at 4 °C until analysis. 2.3. Analytical methods Concentrations of dissolved Al, Ca, Mg, Na, K, Pb, Si, Fe, and Mn in groundwater samples were measured using an inductively coupled plasma optical emission spectrometer (ICP-OES, Perkin Elmer, US). Arsenic speciation and concentration were determined by high performance liquid chromatography atomic fluorescence spectrometry (HPLC-AFS, Jitian, China) with a detection limit of 2 3 1 lg L1. Anions including F, Cl, NO 3 , SO4 , and PO4 were measured using an ion chromatography system, DX-1100 (Dionex, US). For quality control, external standards and spiked samples were tested with the samples regularly to ensure the accuracy of the chemical analyses. The recovery range of added standards was

2.4. Statistical analysis Principal component analysis (PCA) can be used to investigate the processes which influence groundwater quality by examining chemical associations defined by one or more variables’ loadings on factors (Buschmann et al., 2007; Sikdar and Chakraborty, 2008). The principal components (PCs) are the eigenvectors of a variance–covariance matrix. Each PC-axis or factor (with high loadings on one or more variables) may represent an independent source of variation, and these PCs may give clues to the genesis of processes. A loading on one variable close to ±1 indicates a strong correlation between a variable and the factor. Variables which exhibited a loading >0.5 were considered significant. 3. Results and discussion 3.1. Groundwater chemistry The analyses of groundwater from the study areas are shown in Table 2 (The raw geochemistry data for each sampling well is shown in Table A1 and A2). The groundwater was slightly alkaline in the Datong Basin (pH 8.0) and Hetao Basin (pH 8.2). EC values ranged between 0.5 and 5.2 mS cm1 in the Datong Basin, and between 0.01 and 6.3 mS cm1 in the Hetao Basin. Approximately 91% of groundwater samples exceeded the drinking water standard of As at 10 lg L1. The highest As concentration was 1,160 lg L1 and 804 lg L1 in the Datong Basin and Hetao Basin, respectively. About 73% of total dissolved As was in the As(III) form in the two study areas. No organic As species was detected. The mean Eh value of 113 mV (Datong Basin) and 162 mV (Hetao Basin) indicated reducing conditions, and only 5% of samples had positive Eh values. High As groundwater samples in the Datong Basin contained low NO3 (0.5 mg L1 mean value) and

Table 2 Statistical summary of geochemical parameters of groundwater in the Datong Basin and Hetao Basin. Parameters

Total Diss. As (lg L1) %As(III) Alkalinity (CaCO3 mg L1) Depth (m) EC (mS cm1) ORP (mV) pH T (°C) TDS (mg L1) TOC (mg L1) Al (mg L1) Ca (mg L1) Cl (mg L1) F (mg L1) Fe (mg L1) K (mg L1) Mg (mg L1) Mn (mg L1) Na (mg L1) NO3 (mg L1) Pb (mg L1) PO4 (mg L1) Si (mg L1) SO4 (mg L1)

Datong Basin

Hetao Basin

Maximum

Minimum

Mean

Median

SD

Maximum

Minimum

Mean

Median

SD

1160 92 737 90 5.2 79 8.5 15.1 3290 17.5 0.22 40.4 631.8 5.6 0.35 10.9 112.5 0.23 941.4 5.8 NDa 3.6 12.2 490.2

<1 ND 252 20 0.5 242 7.7 10.1 298 1.4 0.14 6.4 7.9 0.4 <0.01 0.4 21.7 <0.01 40.0 <0.1 ND <0.1 0.03 0.4

280 73 384 36 1.1 113

260 76 350 30 0.7 145 8.0 11.8 421 4.0 0.14 17.8 26.4 0.8 0.10 0.7 30.4 <0.01 91.9 <0.1 ND 0.5 10.8 4.6

242 14 124 16 1 82 0.2 0.9 636 3.4 0.02 8.7 162. 1.1 0.10 1.7 22.7 0.06 177.1 1.3 / 0.9 2.2 92.1

804 97 905 32 6.3 65 8.8 17.5 3960 16.0 0.15 219.8 1644.5 3.8 0.38 9.3 264.7 1.16 946.4 20 ND 1.2 12.3 973.3

<1 ND 140 13 0.01 205 7.4 9.9 8 1.0 0.08 3.6 39.0 0.3 <0.01 1.3 17.2 <0.01 52.3 <0.1 ND <0.1 5.8 0.4

314 73 542 23 2.1 162

304 78 534 23 1.7 174 8.3 11.8 1080 5.8 0.12 17.2 238.8 0.9 0.02 3.2 45.0 <0.01 300.4 1.4 ND 0.4 7.8 92.9

214 20 160 5 1.3 45 0.3 1.6 799 3.3 0.02 39.1 308.0 0.7 0.09 1.7 63.9 0.15 170.5 3.4 / 0.2 1.6 214.2

11.7 694 4.9 0.15 19.4 92.7 1.2 0.07 1.0 39.5 0.03 165.9 0.5 ND 0.8 10.1 43.0

Note: Samples numbers were 37 in the Datong Basin and 62 in the Hetao Basin. ND: not detected. a Pb detection limit = 40 lg/L.

12.0 1339 6.3 0.12 34.3 327.8 1.1 0.06 3.8 71.3 0.04 327.1 2.4 ND 0.5 8.4 174.0

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adsorption with coexisting anions may contribute to As mobilization in the Hetao Basin. In addition, Cl concentrations were three times higher in the Hetao Basin (327.8 mg L1 mean value) than those in the Datong Basin (92.7 mg L1 mean value). Sodium was the predominant cation in the Datong Basin (165.9 mg L1) and in the Hetao Basin (327.1 mg L1). The distribution of major cations (Ca2+, Mg2+, Na+ and K+) and 2 anions (Cl, HCO 3 and SO4 ) in groundwater samples in the Datong Basin and Hetao Basin is shown in the Piper diagram (Fig. 2). The percentage of the major dissolved ions indicates that the type of groundwater was mainly of Na-HCO3 in the Datong Basin, and Na-Cl-HCO3 type in the Hetao Basin, which was in accord with previous studies (Guo et al., 2003, 2008, 2011). 3.2. Geochemical processes

Fig. 2. Piper diagram depicting the main geochemical type of groundwater in the Datong Basin ( ) and Hetao Basin ( ).

SO4 (43.0 mg L1 mean value) concentrations, indicating that the reducing condition is a major factor controlling the As, NO3 and SO4 concentrations. In contrast, concentrations of NO3 (2.4 mg L1 mean value) and SO4 (174.0 mg L1 mean value) in the Hetao Basin were considerably higher than those in the Datong Basin. The difference suggests that besides the redox conditions, competitive

Cation exchange and weathering of silicate minerals may be the major chemical processes in the groundwater of the two study areas (Figs. 3–5). Based on the stoichiometric analysis, the ratio of Na/Cl is greater than 1 for most groundwater samples in the two study areas (Fig. 3). In agreement with this observation, previous studies have reported similar trends for the Na/Cl ratio in the Datong Basin and Hetao Basin (Guo et al., 2003, 2011; Guo and Wang, 2005) (Fig. 3). To correlate concentrations of Na+ and divalent cations, the concentrations of Ca2+ and Mg2+ were justified 2 by subtracting anion concentrations (HCO 3 and SO4 ) associated with weathering processes for carbonate and silicate minerals (Mukherjee et al., 2009). The Na+ concentrations were corrected by subtracting the balanced concentration of Cl(Kortatsi et al., 2008; Mukherjee et al., 2009). The regression slope of normalized Na versus divalent cations was 0.9 (close to 1) in the Datong Ba-

45

45 DatongBasin

HetaoBasin 36 Na (meq/L)

Na (meq/L)

36 27 ratio=1 18

27

ratio=1

18 9

9 0

0 0

6

12

18

24

30

36

0

6

12

Cl (meq/L)

0 y = -0.9x + 0.6 r = 0.98

-5 -10 y = -0.7x -3.3 r = 0.88

-15 -20 -10

24

30

36

10

DatongBasin

Ca+Mg-HCO3-SO4 (meq/L)

Ca+Mg-HCO3-SO4 (meq/L)

10 5

18 Cl (meq/L)

HetaoBasin 5 0 y = -0.9x -0.3 r = 0.92

-5 -10 y = -0.7x -3.4 r= 0.92

-15 -20

-5

0 5 10 Na-Cl (meq/L)

15

20

-10

-5

0 5 10 Na-Cl (meq/L)

15

20

Fig. 3. Sodium as a function of chloride in groundwater samples of the Datong and Hetao Basins; The plot of (Ca + Mg-HCO3-SO4) against (Na-Cl) indicates cation exchange. The solid symbols represent the experimental data, and the open symbols represent the data from Guo et al. (2003) (D, s), Guo and Wang (2005) (e) in the Datong Basin, and Guo et al. (2011) (D, s) in the Hetao Basin. The solid and dashed lines represent the regression trend lines of the experimental data and previous data, respectively.

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25 Datong Basin

20

y = 0.9 x + 1.9 r = 0.84

15 10 y = 0.9 x + 0.6 r = 0.74

5

Hetao Basin

Na+K+Ca+Mg-CI (meq/L)

Na+K+Ca+Mg-Cl (meq/L)

25

y = 1.0 x + 2.3 r = 0.77

20 15 10

y = 1.0 x -0.4 r = 0.79

5

0

0 0

5

10 15 20 Bicarbonate (meq/L)

25

0

5

10 15 20 Bicarbonate (meq/L)

25

Fig. 4. Changes of Cl-corrected total cations as a function of HCO3 in the Datong and Hetao Basins. The solid symbols represent the experimental data, and the open symbols represent the data from Guo et al. (2003) (h) in the Datong Basin, and Guo et al. (2011) (h) in the Hetao Basin. The solid and dashed lines represent the regression trend lines of the experimental data and previous data, respectively.

10

10

Hetao Basin

Datong Basin

Carbonate Weathering

Carbonate Weathering

1 Mg/Na

Mg/Na

1

Silicate Weathering

0.1

0.1

0.01

0.01

0.1

1 Ca/Na

10

100

0.01 0.01

100

1 Ca/Na

10

100

Hetao Basin

CarbonateWeathering

1

Silicate Weathering

0.1

1 Ca/Na

10

1

Silicate Weathering

0.1

0.01

0.1

CarbonateWeathering

10 HCO3/Na

10

HCO3/Na

0.1

100

Datong Basin

0.01

Silicate Weathering

100

0.01 0.01

0.1

1 Ca/Na

10

100

Fig. 5. The plots of Na-normalized (lM/lM) HCO 3 versus Ca, and Mg versus Ca show trends of weathering processes in the Datong and Hetao Basins. The solid symbols represent the experiment data, and the open symbols represent the data from Guo et al. (2003) (h) in the Datong Basin, and Guo et al. (2011) (h) in the Hetao Basin.

sin and Hetao Basin (Fig. 3). If cation exchange occurred in the aquifer, the slope should be around 1 (Mukherjee et al., 2009). The analytical results indicate that an ion exchange reaction occurred between Ca and Na in the two study areas. In previous studies the regression slope of normalized Na versus divalent cations was found to be about 0.7 in the Datong Basin and Hetao Basin (Guo et al., 2003, 2011), which is comparable to the present results. Furthermore, this cation exchange process is also observed in Ca-Mg-(Na)-HCO3 and Na-(Mg)-HCO3-(Cl) type groundwater in a strongly As-enriched area in Cambodia (Buschmann et al., 2007). Silicate weathering is the major dissolution process in the Datong and Hetao Basins (Figs. 4 and 5). The regression slope of total cations (Ca + Mg + Na + K-Cl) as a function of HCO3 is 0.9 and 1.0

in the Datong and Hetao Basins, respectively (Fig. 4). The concentrations of cations were corrected by subtracting the Cl concentration (Ca + Mg + Na + K-Cl) to remove the contributions of Cl-minerals such as NaCl and CaCl2 (Kim, 2003). Among major processes contributing to HCO3 concentrations in aquatic systems, dissolution of silicate and carbonate minerals is the only process that can concurrently increase concentrations of HCO3 and total cations with a nearly 1:1 ratio (Kim, 2003). Then, in a plot of Nanormalized concentrations of Ca versus HCO3 (Fig. 5) and Ca versus Mg (Fig. 5), the samples tend to fall within the global-average silicate weathering region rather than carbonate weathering region (Gaillardet et al., 1999; Mukherjee and Fryar, 2008). Therefore, dissolution of silicate minerals may be the predominant process

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Table 3 Principal component analysis of groundwater samples in the Datong Basin and Hetao Basin. Variables

PO4 Fe Mn Cl SO4 ORP NO3 Mg Ca Na HCO3 As % Eigenvalue Cum% a

Loading on PC axes (Datong Basin)

Loading on PC axes (Hetao Basin)

PC1

PC2

PC3

PC1

PC2

PC3

PC4

0.46 0.26 0.48 0.93a 0.87a 0.37 0.64a 0.96a 0.65a 0.83a 0.55a 0.06 41.8 41.8

0.23 0.51a 0.75a 0.25 0.27 0.62a 0.46 0.17 0.19 0.31 0.41 0.79a 22.1 63.9

0.69a 0.05 0.15 0.13 0.17 0.40 0.01 0.02 0.42 0.40 0.48 0.22 13.9 77.8

0.45 0.20 0.70a 0.77a 0.86a 0.34 0.47 0.86a 0.90a 0.66a 0.61a 0.05 37.9 37.9

0.19 0.04 0.44 0.47 0.19 0.75a 0.26 0.07 0.22 0.44 0.24 0.56a 18.1 56.0

0.55a 0.22 0.27 0.02 0.09 0.37 0.00 0.35 0.12 0.43 0.49 0.48 13.3 69.3

0.33 0.87a 0.12 0.05 0.13 0.03 0.36 0.06 0.16 0.10 0.08 0.12 9.0 78.3

Variables with significant loadings.

0.4 Datong Basin

Fe concentration (mg/L)

Fe concentration (mg/L)

0.4

0.3

0.2 r = 0.45

0.1

0 0.3 0.6 0.9 As concentration (mg/L)

0.1

0

1.2

0.4

0.3 0.6 0.9 As concentration (mg/L)

1.2

0.4 Datong Basin

Mn concentration (mg/L)

Mn concentration (mg/L)

0.2

0 0

0.3

0.2 r = 0.46

0.1

0

Hetao Basin

0.3

0.2

0.1

0 0

0.3 0.6 0.9 As concentration (mg/L)

1.2

0

0.3 0.6 0.9 As concentration (mg/L)

1.2

1.6 Phosphate concentration (mg/L)

4 Phosphate concentration (mg/L)

DatongBasin Hetao Hetao Basin Basin

0.3

Datong Basin 3

r = 0.43

2

r = 0.37

1

0

Hetao Basin 1.2

r = 0.50

0.8

0.4

0 0

0.2 0.4 0.6 0.8 As concentration (mg/L)

1

0

0.2 0.4 0.6 0.8 As concentration (mg/L)

1

Fig. 6. The correlation between As and, Fe, Mn and PO4 in groundwater in the Datong Basin and Basins. The solid symbols represent the experimental data, and the open symbols represent the data from Guo et al. (2003) (s) in the Datong Basin, and Guo et al. (2011) (s) in the Hetao Basin. The solid and dashed lines represent the regression trend lines of the experimental data and previous data, respectively.

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1200

DatongBasin

As concentration (µg/L)

As concentration (µg/L)

1200

900

600

300

0

HetaoBasin 900

600

300

0 -5

-4

-3

-2

-1

-5

0

-4

-3

-1

0

0.4

DatongBasin Fe concentration (mg/L)

Fe concentration (mg/L)

0.4

0.3

0.2

0.1

Hetao Basin 0.3

0.2

0.1

0

0 -5

-4

-3

-2

-1

0

-5

-4

-3

pe

-2

-1

0

pe

0.4

0.4

DatongBasin

Mn concentration (mg/L)

Mn concentration (mg/L)

-2 pe

pe

0.3

0.2

0.1

HetaoBasin 0.3

0.2

0.1

0

0 -5

-4

-3

-2

-1

0

pe

-5

-4

-3

-2

-1

0

pe

Fig. 7. Observed concentrations of As(III) (open square), As(V) (solid square) as a function of pe in the Datong and Hetao Basins. Changes of Fe and Mn concentrations as a function of pe in the Datong Basin and Hetao Basin. The black symbols represent the experimental data, and the blue symbols represent the data from Xie et al. (2009) ( ) in the Datong Basin, and Guo et al. (2011) ( ) in the Hetao Basin. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

controlling the major ion chemistry in the Datong and Hetao Basins. Previous studies are consistent with the present results (Figs. 4 and 5). As a comparison, both silicate weathering and carbonate dissolution are two primary processes controlling groundwater geochemistry in the Malda district of West Bengal (Sikdar and Chakraborty, 2008). 3.3. Principal component analysis The PCA results with 12 geochemical parameters from each of the 99 groundwater samples in the Datong and Hetao Basins are presented in Table 3. Three major factors (PC1, PC2, and PC3) were found in groundwater samples in the Datong Basin which could explain 77.8% of the variability. PC1 accounted for 41.8% of the variance in the whole data and was correlated with Cl, SO4, NO3, Mg, Ca, Na, and HCO3. Therefore, PC1 may reflect the major cations and anions resulting from mineral weathering and water–rock

interactions in the aquifer. PC2 contributed 22.1% of the total variance with high loadings on Fe, As, Mn and ORP, which may indicate a strong impact of redox processes on elevated As concentrations in groundwater. Statistically significant correlations existed between As and Fe (r = 0.45, p < 0.01), and As and Mn (r = 0.46, p < 0.01) (Fig. 6). Guo et al. (2003) also observed that there was a positive correlation between total As and, Fe and Mn concentrations in groundwater of the Datong Basin (Fig. 6). The correlation results indicate that the reductive dissolution of Asassociated Fe/Mn oxides under reducing conditions may trigger As release in the Datong Basin. PC3 contributed 13.9% of the total variance with a high loading on PO4. This factor may indicate the application of agricultural phosphate fertilizers and pesticides in the Datong Basin. The anions PO4, HCO3 and silicate are wellknown As competitors for available sorption sites (Su and Puls, 2001; Smedley and Kinniburgh, 2002). A statistically significant correlation was observed between As and PO4 (r = 0.37, p < 0.05),

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and is consistent with a previous report (Guo et al., 2003) (r = 0.43, p < 0.05) (Fig. 6), which may suggest that competitive adsorption behaviors of As with P may be an another important controlling factor in As desorption in the Datong Basin. In clear contrast to the Datong Basin results, four eigenvalues were needed to account for 78.3% of the total variability for the Hetao Basin samples (Table 3). PC1 with 37.9% of the total variance had significant loadings on Mn, Cl, SO4, Mg, Ca, Na, and HCO3. Similar to the results in the Datong Basin, PC1 was attributed to the likely mineral weathering. PC2 had high loadings on As and ORP, and contributed 18.1% of the total variance. Statistical analyses were also conducted on As and Fe/Mn, but no significant correlation was detected (Fig. 6). The difference in the As and Fe/Mn correlations in the Datong and Hetao Basins suggest that more complicated processes may control the As occurrence in the Hetao Basin. Such processes may include mobilization of As(III) via direct microbial utilization of As(V) as an electron acceptor (Oremland and Stolz, 2003; Lloyd and Oremland, 2006). This direct As(V) reduction process would not have significant impacts on Fe mobilization, and it is frequently reported as a reason for elevated As concentrations under reducing conditions (Ahmann et al., 1997). Although there was no correlation between total As and Fe/Mn in a previous study (Fig. 6), Guo et al. (2011) suggested that As would be released by reductive dissolution of Fe/Mn oxyhydroxides in the reducing conditions. The PC3 and PC4 components had high loadings on PO4 and Fe, respectively. Similar to the results in the Datong Basin, the correlation between As and PO4 (r = 0.50, p < 0.01) (Fig. 6) may indicate that competitive adsorption between As and PO4 also occurred in the Hetao Basin (Sun et al., 2006). High P concentrations up to 3.54 mg L1 have also been observed in groundwater from the Hetao Basin (Guo et al., 2008). This competition phenomenon has been reported previously at La Pampa and Santiago del Estero in Argentina and at Kandal in Cambodia on agricultural lands with extensive phosphate fertilizer applications (Smedley et al., 2002; Bhattacharya et al., 2006; Rowland et al., 2008). The high loadings on Fe in PC4 could be attributed to the drainage derived from mining activities (Palma et al., 2010). There are abundant mineral resources such as iron ores and pyrites in the study area of the Hetao Basin in Inner Mongolia (http://baike.baidu.com/view/44905.htm#8). 3.4. Impact of redox potential on groundwater As occurrence Reducing conditions with low redox potentials can facilitate As release in groundwater (Bhattacharya et al., 1997; McArthur et al., 2001; Rowland et al., 2007; Mladenov et al., 2010). The elevated As concentrations in the groundwater of the Datong and Hetao Basins were influenced very strongly by ORP values (Fig. 7) and the elevated As occurred in the pe range 4 to 2. Under reducing conditions, As(III) was over 76% and 78% of total dissolved As in groundwater samples in the Datong and Hetao Basins, respectively (Table 2, Fig. 7). In line with the redox zonation of As occurrence, high concentrations of Fe and Mn were observed in the pe range 4 to 2 (Fig. 7). The pe range at which elevated As and, Fe and Mn in groundwater occurred is in line with previous studies in the Datong and Hetao Basins (Xie et al., 2009; Guo et al., 2011) (Fig. 7). Thermodynamic calculations, considering aqueous, adsorption, precipitation, and redox reactions, were used to simulate As mobilization in a previous study (Meng et al., 2001). The study suggested that the pe range could be divided into three redox zones, which are defined as adsorption, mobilization and reductive fixation zones. In the pe range 4 to 2, As(III) was the predominant soluble As species. Soluble As concentrations were decreased at pe > 2 due to the oxidation of As(III) to As(V), which subsequently adsorbed on Fe/Mn oxides. On the other hand, the decrease of

soluble As concentrations when pe < 4 can be attributed to the precipitation of As-containing minerals such as orpiment, realgar and arsenopyrite. 4. Conclusions High As groundwater is often found in shallow aquifers in the Datong Basin (Shanxi) and Hetao Basin (Inner Mongolia) in China. To understand the mechanisms triggering the enrichment of naturally-occurring As, geochemical processes controlling As in groundwater were studied. The results show that the groundwater is generally of the Na-HCO3 type in the Datong Basin and Na-ClHCO3 type in the Hetao Basin. The major ion composition in both study areas is regulated by concurrent silicate weathering and cation exchange processes. The PCA results show high loadings on Fe, Mn, redox potential, and As in one cluster in the Datong Basin. The association indicates that reductive dissolution of Fe and Mn oxides releases the sorbed As into groundwater. On the other hand, PCA results show that there are high loadings on As and redox potential in one cluster with no correlation of As with Feand Mn in the Hetao Basin, indicating multiple processes may control As mobilization in Inner Mongolia. In addition, PO4 has a good correlation with As in the Datong and Hetao Basins, which suggests that competition between PO4 and As may be another process affecting the As concentration in groundwater. Reduction of As, Fe and Mn occurred in the pe range 4 to 2. The results of this study may help further the understanding of As occurrence and mobilization in China. Because the effectiveness of As removal techniques depends largely on groundwater geochemistry, the research could serve as a foundation for selection of a suitable and cost-effective As removal technology. Acknowledgement We acknowledge the financial support of the National Natural Science Foundation of China (20977098, 20921063, 20890112). Appendix A. Supplementary material Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.apgeochem.2012. 08.012. References Ahmann, D., Krumholz, L.R., Hemond, H.F., Lovley, D.R., Morel, F.M.M., 1997. Microbial mobilization of arsenic from sediments of the Aberjona Watershed. Environ. Sci. Technol. 31, 2923–2930. Anawar, H.M., Akai, J., Sakugawa, H., 2004. Mobilization of arsenic from subsurface sediments by effect of bicarbonate ions in groundwater. Chemosphere 54, 753– 762. Baig, J.A., Kazi, T.G., Arain, M.B., Afridi, H.I., Kandhro, G.A., Sarfraz, R.A., Jamal, M.K., Shah, A.Q., 2009. Evaluation of arsenic and other physico-chemical parameters of surface and ground water of Jamshoro, Pakistan. J. Hazard. Mater. 166, 662– 669. Bhattacharya, P., Chatterjee, D., Jacks, G., 1997. Occurrence of arsenic-contaminated groundwater in alluvial aquifers from delta plains, eastern India: options for safe drinking water supply. Int. J. Water Res. 13, 79–92. Bhattacharya, P., Claesson, M., Bundschuh, J., Sracek, O., Fagerberg, J., Jacks, G., Martin, R.A., Storniolo, A.D., Thir, J.M., 2006. Distribution and mobility of arsenic in the Rio Dulce alluvial aquifers in Santiago del Estero Province, Argentina. Sci. Total Environ. 358, 97–120. Buschmann, J., Berg, M., Stengel, C., Sampson, M.L., 2007. Arsenic and manganese contamination of drinking water resources in Cambodia: coincidence of risk areas with low relief topography. Environ. Sci. Technol. 41, 2146–2152. Campbell, K.M., Malasarn, D., Saltikov, C.W., Newman, D.K., Hering, J.G., 2006. Simultaneous microbial reduction of iron(III) and arsenic(V) in suspensions of hydrous ferric oxide. Environ. Sci. Technol. 40, 5950–5955. Deng, Y.M., Wang, Y.X., Ma, T., 2009. Isotope and minor element geochemistry of high arsenic groundwater from Hangjinhouqi, the Hetao Plain, Inner Mongolia. Appl. Geochem. 24, 587–599.

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