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Spatial variation in arsenic and fluoride concentrations of shallow groundwater from the town of Shahai in the Hetao basin, Inner Mongolia
Applied Geochemistry 27 (2012) 2187–2196
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Spatial variation in arsenic and fluoride concentrations of shallow groundwater from the town of Shahai in the Hetao basin, Inner Mongolia Huaming Guo ⇑, Yang Zhang, Lina Xing, Yongfeng Jia State Key Laboratory of Biogeology and Environmental Geology, China University of Geosciences, Beijing 100083, PR China School of Water Resources and Environment, China University of Geosciences, Beijing 100083, PR China
a r t i c l e
i n f o
Article history: Available online 2 February 2012
a b s t r a c t Twenty-nine wells were selected for groundwater sampling in the town of Shahai, in the Hetao basin, Inner Mongolia. Four multilevel samplers were installed for monitoring groundwater chemistry at depths of 2.5–20 m. Results show that groundwater As exhibits a large spatial variation, ranging between 0.96 and 720 lg/L, with 71% of samples exceeding the WHO drinking water guideline value (10 lg/L). Fluoride concentrations range between 0.30 and 2.57 mg/L. There is no significant correlation between As and F concentrations. Greater As concentrations were found with increasing well depth. However, F concentrations do not show a consistent trend with depth. Groundwater with relatively low Eh has high As concentrations, indicating that the reducing environment is the major factor controlling As mobilization. Low As concentrations (<10 lg/L) are found in groundwater at depths less than 10 m. High groundwater As concentration is associated with aquifers that have thick overlying clay layers. The clay layers, mainly occurring at depths <10 m, have low permeability and high organic C content. These strata restrict diffusion of atmospheric O2 into the aquifers, and lead to reducing conditions that favor As release. Sediment composition is an additional factor in determining dissolved As concentrations. In aquifers composed of yellowish-brown fine sands at depths around 10 m, groundwater generally has low As concentrations which is attributed to the high As adsorption capacity of the yellow–brown Fe oxyhydroxide coatings. Fluoride concentration is positively correlated with pH and negatively correlated with Ca2+ concentration. All groundwater samples are over-saturated with respect to calcite and under-saturated with respect to fluorite. Dissolution and precipitation of Ca minerals (such as fluorite and calcite), and F adsorption–desorption are likely controlling the concentration of F in groundwater. Ó 2012 Elsevier Ltd. All rights reserved.
1. Introduction The Hetao basin is located in an arid region of Inner Mongolia, PR China. Although the Yellow River lies to the south of the basin and its water is used for farmland irrigation, groundwater is the major source of drinking water. Before the 1980s, every village had 2–4 dug wells (3–5 m in depth) that provided drinking water. This shallow water usually had total dissolved solid (TDS) > 1000 mg/L, which caused residents to abandon dug wells for deeper hand pump tube wells (usually 15–30 m in depth) (Tang et al., 1996). The deeper groundwater generally has lower TDS, but commonly contains high As concentrations (>50 lg/L). The change in drinking water supplies has led to endemic As poisoning that was first recognized in 1990 (Gao, 1990). High As groundwater is widespread in the basin, which poses a significant health risk to more than one million residents (Guo et al., 2008a). It was reported ⇑ Corresponding author at: School of Water Resources and Environment, China University of Geosciences, Beijing 100083, PR China. Tel.: +86 10 8232 1366; fax: +86 10 8232 1081. E-mail address: [email protected] (H. Guo). 0883-2927/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved. doi:10.1016/j.apgeochem.2012.01.016
that the chronic As poisoning is affecting 24.8% of the basin population (Jin et al., 2003). In addition to abnormal As levels, high F concentrations are common in shallow groundwater in the Hetao basin (<30 m in depth) (Guo et al., 2008a; Deng et al., 2009). High dissolved F concentrations result in residents of the Hetao basin suffering from chronic fluorosis. The number of people suffering from dental fluorosis in the basin is as high as 1.9 million, and those with skeletal fluorosis is estimated to be more than 230,000 (Wang et al., 1999). Both groundwater As and F are believed to be of geogenic origin (Smedley et al., 2003; Guo et al., 2008a). Groundwater As and F concentrations are highly variable on a local and regional scale. Although As concentrations have been known to range from <1.0 to 572 lg/L over distances from tens of meters to kilometers (Guo et al., 2008a; Deng et al., 2009; He et al., 2009), the cause of the lateral heterogeneity in As concentration remains unknown. High As groundwater generally occurs in shallow alluvial–lacustrine aquifers (Guo et al., 2008a). The authors’ previous investigation showed that wells near irrigation channels and drainage channels produce low As groundwater (Guo et al., 2011). It has been generally accepted that tube wells
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2. Materials and methods
Mountains, NW of the study area, are mainly composed of Jurassic to Cretaceous metamorphic rocks (slate, gneiss and marble) (Guo et al., 2008a). Alluvial fans extend from the front of the mountain range to a broad plain (Fig. 1b). The plain comprises 75% of the study area and has a topographic gradient between 1/1000 and 1/5000. The plain is underlain by sediments with a thickness between 1500 and 8000 m that are mainly derived from the Langshan Mountains and from fluvial deposits of the old Yellow River. Quaternary fine-grained, lacustrine sediments are locally part of the basin fill (Guo et al., 2008a). Shallow groundwater mainly occurs in the Quaternary alluvial, alluvial–pluvial, and fluvial–lacustrine aquifers, which are unconfined or leaky-confined. The alluvial-pluvial unconfined aquifers mainly occur in the belt of alluvial fans, while fluvial–lacustrine leaky-confined aquifers are common in the low-lying area. Locally, the topmost part of the sedimentary sequence consists of alluvial deposits and lacustrine sediments that were deposited in oxbow lakes and small lagoons separated from the large lake that formerly occupied the basin (Guo et al., 2008a). Peat accumulations are also distributed within the sediment. High As groundwater is most common in fluvial to lacustrine aquifers. In the basin, the average annual precipitation is around 188 mm, which is much less than potential evaporation of 2000– 2500 mm. The depth of the water table ranges from 1.0 to 10 m. Groundwater in the basin-fill sediments is mainly recharged through the alluvial fans by water flowing through fractures along the mountain front, and by vertically infiltrating precipitation, ditch water (irrigation channels), and irrigation water in the flat plain. It is discharged mainly via evapotranspiration, drainage, and pumping. The main and secondary drainage channels were constructed to locally lower groundwater levels, thus reducing salinization due to evaporation. The general direction of groundwater flow is from north to south. The flow rate in the basin-fill sediment ranges between 0.002 and 0.2 m/d (Inner Mongolia Institute of Hydrogeology, 1982).
2.1. Study area
2.2. Sample collection
The study area of approximately 1000 km2 is located in the northwestern portion of the Hetao basin (Fig. 1a). The Langshan
Twenty-nine groundwater samples were collected from handpumped tube wells and electric-powered public water supply
(15–30 m in depth) have higher As concentrations than dug wells (3–5 m in depth). Nonetheless, the vertical distribution of As in these shallow aquifers has not been documented. High F groundwaters are found not only in bedrock aquifers (Chae et al., 2007; Reddy et al., 2010; Mamatha and Rao, 2010), but also in aquifers developed in young sediments (Wang and Cheng, 2001; Meenakshi et al., 2004; Deng et al., 2009). In both aquifers, rock weathering, evaporation, and mineral crystallization and precipitation are the main factors controlling F concentration (Reddy et al., 2010; Mamatha and Rao, 2010). Groundwater from sediments in inland basins with high F concentration typically has low Ca/Na ratio, high pH, high HCO3 , high salinity, and high sodicity (Wang and Cheng, 2001; Meenakshi et al., 2004; Rafique et al., 2009). As with As, spatial distribution of groundwater F has not been sufficiently investigated. Understanding spatial variability of As and F are crucial not only for understanding the biogeochemical–hydrological processes that control groundwater concentrations of As and F , but also to help develop strategies for sustainable use of shallow groundwater that will reduce exposure to As and F (Fendorf et al., 2010). Shahai is a typical town in the Hetao basin, where As and F concentrations range between 1.17 and 483 lg/L (median 67.7 lg/L), and between 0.08 and 7.87 mg/L (median 1.40 mg/L), respectively (Guo et al., 2008a). In the vicinity of the town, there are recognized areas of groundwater recharge, and discharge. Both irrigation channels and drainage channels, which are believed to affect As concentrations in shallow groundwater (Guo et al., 2011), are distributed across the region. The objectives of this study are to (1) characterize spatial distributions of groundwater As and F in shallow aquifers; (2) describe the vertical distributions of As and F concentrations in groundwaters; and (3) evaluate the factors controlling As and F distribution.
Fig. 1. Location of study area (a) and sampling sites (b).
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wells in May 2010 (Fig. 1b). The data for 12 samples published previously in Deng et al. (2009) were also considered in this study (Fig. 1b). The depths of these wells ranges between 13 and 100 m. Four multilevel well clusters (A–D) were installed in July, 2009 approximately parallel to the groundwater flow path along a 5 km transect (Fig. 1b). The multilevel samplers were installed following the procedure of Kim (2003) using bundles of polyethylene tubes fitted with 20 cm screened intervals. Each well cluster has 3–5 sampling ports placed at depths ranging from 2.5 to 20 m. Groundwater was sampled from each well after pumping (usually 20 min) until the flowing water showed a stabilized temperature, pH, DO (dissolved O2), and Eh. All samples were filtered through 0.45 lm membrane filters in the field. Water samples for major and trace element analysis were collected in 100 mL HNO3-washed polyethylene bottles, followed by addition of 6 M HNO3 to pH < 2. Aliquots for analysis of As species were preserved with 0.25 M EDTA (10%). Aliquots for dissolved organic C (DOC) analysis were stored in 30 mL amber glass bottles, followed by addition of 6 M H2SO4 (2%). Samples for analysis of As species and DOC were stored at 4 °C. Samples for anion analysis were not acidified. At the time of groundwater sampling, parameters, including water temperature, electrical conductivity (EC), pH and Eh, were measured using a multiparameter portable meter (HANNA, HI 9828), while concentrations of S2 , Fe(II), and alkalinity were determined using a portable spectrophotometer (HACH, DR2800). 2.3. Sample analysis Concentrations of major cations and trace elements were determined by ICP-AES and ICP-MS, respectively. Unacidified aliquots were analyzed for F , Cl , Br , NO3 ; NO2 ; SO24 concentrations by Ion Chromatography with a DX-120 (Dionex). A total carbon analyzer (TOC-Vwp, Shimadzu) was used to determine DOC concentrations. Concentrations of total dissolved As and Se were determined by atomic fluorescence spectrometry (AFS) with hydride generation (Price and Pichler, 2005). For most samples, charge imbalances were less than 5%. Arsenic species in groundwater samples were determined using an HPLC-ICP-MS. A high performance liquid chromatography (HPLC, 1100 Series, Agilent) consisting of a system controller, a solvent delivery module, a column oven and a 6-port injection valve was used. A reversed-phase C18 column (Capcell, Pak, 250 mm 4.6 mm, 5 lm particle size) was used for separation of As species. An ICP mass spectrometer (7500C, Agilent) was used as a detector, which was operated in the He mode to remove the ArCl interference.
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have As concentrations of 96.7, 93.3, 364, 264 and 152 lg/L, respectively. Within 1 km of the drainage channels, groundwater contains low As concentrations. Wells 40 and 35, which are positioned 350 and 850 m from the main drainage, have As concentrations of 30.6 and 10.4 lg/L, respectively (Fig. 2). A notable exception to this generalization is Well 39, which is near the drainage channel (about 700 m), but yields water with As concentration up to 340 lg/L. This is attributed to the greater screening depth of well 39 (around 100 m), relative to the other wells. The deeper wells likely sample water below the influence of the drainage channel. In the SE portion of the study area, where the Yellow River fluvial sediments prevail, groundwater generally has low As concentrations (<50 lg/L) (Fig. 2). Dissolved sulfide and DOC in groundwater are considered as indicators of the redox state of the aquifers. More than 95% of all groundwater samples have measurable S2 . Sulfide concentrations show a great variability, ranging from <1.0 to 20 lg/L. The coexistance of S2 and SO24 indicates that SO24 reduction occurs in the aquifer system. The correlation between As and S2 is not significant. Concentrations of DOC in groundwater samples range between 0.85 and 21.3 mg/L with a median of 5.22 mg/L. There is no significant correlation between DOC and As. 3.2. Spatial variation of F concentration Groundwater pH is near-neutral to weakly alkaline (7.2–8.9). Sodium is the predominant cation, ranging from 29.6 to 1310 mg/L, while HCO3 or Cl is the predominant anion. Bicarbonate concentrations range between 166 and 1080 mg/L. Groundwaters have Cl concentrations between 33.0 and 2050 mg/L and the ratio of [Na+]/[Cl ] is approximately 1:1. Groundwaters are generally of Na–HCO3–(Cl), Na–(Mg)–Cl–HCO3, and Na–Mg–Cl–SO4 (HCO3) types. Fluoride concentrations range between 0.30 and 2.57 mg/L (median 1.02 mg/L), with 51.2% of the samples exceeding the Chinese Drinking Water Standard (1.0 mg/L). High F concentrations (>1.0 mg/L) were mainly observed in groundwater of Na(–Mg)– Cl–HCO3 type. Lateral heterogeneity in F concentration was also observed. For example, 0.88 mg/L was measured in Well 74 and 1.86 mg/L in Well 75 over a distance of 10 m (Fig. 3). High F groundwaters (>1.0 mg/L) are common at the distal end of the alluvial fans (Wells 51, 52, and 67) and near the main drainage channels (Wells 34, 35, 37, 39, 40, 73, 78, D22, and D66) (Fig. 3), where depths of the groundwater table range between 1.0 and 4.0 m. In these areas, evapotranspiration is significant, as evident by the 2H and 18O compositions of groundwater (Guo et al., 2011). Low F concentration was found in shallow groundwater
3. Results 3.1. Spatial variation of As concentration Concentrations of groundwater As range between 0.96 and 720 lg/L (median 30.2 lg/L), with 71% of samples exceeding the WHO drinking water guideline value (10 lg/L). Large variations in As concentration over short distances are common, as illustrated by the low As concentration in Well 74 with a depth of 22 m (3.18 lg/L), whereas groundwater in Well 75 with a depth of 20 m being 10 m from Well 74 contains 94.3 lg/L As (Fig. 2). Arsenic concentrations vary systematically relative to position on the alluvial fans. Wells, installed on the slopes of the alluvial fans (68, 69 and 52), produce water with low As concentrations (Fig. 2). At the distal edge of the alluvial fans, where the topography is generally flat, groundwater generally has high As concentrations (Fig. 2). Wells 67, 51, 66, D22, and D20 located in this area
Fig. 2. Spatial variation of As concentration of groundwaters in the study area.
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in the southeastern portion of the area (Fig. 3). The correlation between F and As concentrations is relatively poor (r2 = 0.44). 3.3. Arsenic concentrations at different depths Of the 41 groundwater samples, 4 samples were taken at depths of less than 15 m, 23 samples at depths between 15 and 30 m, and 14 samples at depths between 30 and 100 m. Generally, As concentration is dependent on depth (Fig. 4). For the samples taken at depths of less than 15 m, As concentrations are less than 50 lg/L. Over 52% of samples taken at depths between 15 and 30 m have As concentrations greater than 50 lg/L. Of the 14 samples taken at depths greater than 30 m, there are 6 samples (43%) having As concentration greater than 50 lg/L. Therefore, it is likely that high As groundwater is present at depths greater than 15 m. The high As groundwaters found in deep aquifers (40–100 m), which are currently used as domestic water supplies and for irrigation, must be the focus of additional investigations. Four multilevel samplers were used to monitor vertical variations in As concentration at depths between 2.5 and 20 at each of the four sites (Fig. 1). At all sites, clay/silty clay layers were intersected at depths of less than 10 m. The clay layers have low permeability and high organic C contents (1.0–4.6%). At site A, there is a yellowish brown clay layer between 4.2 and 7.6 m, gray fine sand between 9.2 and 11.5 m, and between 11.5 and 22.0 m. A similar yellowish brown clay layer was observed at depths between 4.0 and 7.0 m at site B, and between 3.4 and 6.2 m at site C. At sites B and C, yellowish brown fine sand was observed at depths between 9.0 and 12.0 m, and 8.0 and 11.5 m, respectively. In comparison, a thicker sequence of yellowish brown clay/gray clay was found at site D. Eight intervals of this clay were intersected: yellowish brown clay between 4.0 and 6.0 m, 7.0 and 7.5, 9.5 and 10.5 m, 11.2 and 12.0 m, and between 13.5 and 14.2 m; gray clay between 6.0 and 7.0 m, 12.0 and 12.4 m, and between 13.0 and 13.5 m. At all sites, light gray–gray fine sands were observed at depths greater than 12.0 m. Results are shown in Figs. 5–8. During the investigation, no water samples were collected from samplers 1-1 (Fig. 5a), 2-1 (Fig. 6a), 4-1, and 4-2 (Fig. 8a) due to technical problems. As in the broader well survey, As concentrations in the multilevel wells increase with increasing depth (Figs. 5b, 6b, 7b and 8b). Low As concentrations (<50 lg/L) are generally present in samples from depths <10 m (Figs. 6b and 7b), which coincides with the interval dominated by yellow–brown clay/silty clay layers (Figs. 5a, 6a, 7a and 8a). In addition, results suggest that increasing thickness of clay layers corresponds to increased As concentration in groundwater. At site D with a thicker sequence of yellowish brown clay/gray clay, groundwater As is higher, up to 465 lg/L.
Fig. 4. Depth distribution of As concentration in groundwater (gray shaded area indicates the depth interval where high As groundwater is most common).
Arsenic(III) is the dominant As species in all samples, accounting for more than 67% of total As. Vertical distribution of As(III) shows the same trend as total As (Figs. 5b, 6b, 7b and 8b). Waters from the multi-level samplers have lower As(III) concentration at <10 m than samples taken at depths around 20 m. For example, the As(III) concentration from Sampler 4-3 is 15.5 lg/L, being less than that from Sampler 4-5 (429 lg/L) (Fig. 8b). In addition to having low concentrations of As(III) and total As, samples at depths less than 10 m generally have low ratios of As(III) to total As.
3.4. Fluoride concentrations at different depths In the broader well survey, more than 57% of samples taken at depths of less than 20 m have F concentrations greater than 1.0 mg/L, while samples, taken at depths, between 20 and 30 m, have F concentrations ranging between 0.34 and 1.81 mg/L (median 0.88 mg/L). Of 14 samples taken at depths greater than 30 m, 8 samples have F concentrations greater than 1.0 mg/L. Groundwaters from the multi-level well clusters generally indicate a decrease in F concentration with increasing depth (Figs. 5b, 6b, 7b, 8b). Except for site C, where F concentration is relatively constant (0.89–1.16 mg/L), groundwater sampled at depths around 10 m have higher F concentrations than those at depths around 20 m. Nonetheless, considering all of the groundwater samples analyzed for this study, F concentrations do not define a consistent trend with depth.
4. Discussion 4.1. Importance of reducing conditions in As release
Fig. 3. Spatial variation of F concentration of groundwaters in the study area.
The groundwater in the study area has low Eh values ranging between 23 and 375 mv (median 63 mv). There is a negative relationship between As concentration and redox potential (Fig. 9a), indicating that reduction promotes As release from aquifer sediments as suggested by Guo et al. (2008a). The reducing condition is attributable to high concentrations of organic C in the sediments and groundwater. Contents of organic C range between 0.12% and 0.51% in the aquifer sands (unpublished data). Microbial degradation of natural organic matter entrained in the aquifer system is the redox driver in the study area, which is similar to other As-rich
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Fig. 5. Sedimentary lithologic logs (a), concentrations of As species and F (b), and concentrations of total dissolved Fe, Fe(II), and Eh (c) in site A of Fig. 1. Concentrations of As species are shown on a logarithmic scale.
groundwater systems (Ravenscroft et al., 2001; McArthur et al., 2004). Microbially mediated redox processes favor reduction in the sequence NO3 , Mn(IV), As(V), Fe(III) and SO24 (Stumm and Morgan, 1996; Mukherjee et al., 2008). All of these species are reduced within the sedimentary aquifers examined in this study. The low median concentration of NO3 (<0.01 mg/L) and the high concentrations of NH4–N (0.10–7.81 mg/L, median 2.29 mg/L) suggest that the aquifer conditions are capable of NO3 reduction. In environments where NO3 is stable, As is expected to be bound to ferric oxides in the sediments consistent with the low As concentration (10 lg/L) found in groundwater with NO3 concentrations greater than 10 mg/L (Fig. 9b). At the stage of Mn4+ reduction, As bound to Mn oxyhydroxides will be released, and is expected to be readsorbed in the presence of Fe-oxyhydroxides at circumneutral pH (Stüben et al., 2003). In the studied groundwaters with Mn concentrations greater than 150 lg/L, As concentrations are less than 30 lg/L. At an Eh favoring As(V) reduction to As(III), adsorbed As(V) is expected to be converted to As(III) and surficially bound As will be released to the groundwater. Preferential release of As(III) is expected because of the lower affinity of As(III) species to mineral surfaces than the more strongly bound As(V) species (Guo et al., 2007). In the studied groundwaters, the greater concentrations of As(III) can be explained by the release due to reductive desorption (Smedley and Kinniburgh, 2002). At redox conditions favoring Fe oxide reduction, As trapped within ferric oxyhydroxides will also be released (Nickson et al., 1998). Iron(II) formed by reduction can combine with S2 and HCO3 in the studied waters to form pyrite and siderite. This is consistent with the oversaturation of groundwaters with respect to pyrite and siderite, and explains the lack
of correlation between As concentration and Fe concentration (Fig. 9c), although it could be attributable to reduction and desorption of As without reduction of ferric oxyhydroxides. During SO24 reduction, S2 will be produced, which has been detected in most of the water samples. Since the studied groundwaters are oversaturated with respect to pyrite, part of the As may be immobilized in sulfide phases. Vertical variations of As concentration and measured Eh further support the importance of reducing conditions in As mobilization. Except for site C, As concentration increases with increasing depth, while Eh shows the opposite trend (Figs 5–8). At the same sites, it was observed that dissolved total Fe and Fe(II) generally increase with decreasing Eh and increasing depth. The increased dissolved Fe is attributed to the reduction of Fe oxides in the aquifers and suggests that that reductive dissolution of Fe oxides plays a major role in As mobilization. 4.2. Roles of low permeable layers on groundwater As distribution Arsenic contents of the sediments of the Hetao basin are not exceptionally high, most range between 7.3 and 40.6 mg/kg (average 17.6 mg/kg) (Guo et al., 2008a; He et al., 2009). Groundwater Eh shows a great spatial variation, with the range between 23 and 375 mv. Although a negative relationship between Eh and As concentration is observed, reducing conditions alone are insufficient to explain the extreme variation in groundwater As concentration at the local scale both laterally and vertically. Biogeochemical and hydrogeological processes associated with low permeablility layers, including lacustrine clay layers and peat deposits, are believed responsible for the observed heterogeneity (Guo et al., 2008b).
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Fig. 6. Sedimentary lithologic logs (a), concentrations of As species and F (b), and concentrations of total dissolved Fe, Fe(II), and Eh (c) in site B of Fig. 1. Concentrations of As species are shown on a logarithmic scale.
The authors’ previous investigation showed that the thickness of overlying clay layers is well correlated with groundwater As concentration (Guo et al., 2008b). In the areas where the thickness of overlying clay layers is >3.0 m, groundwater sampled at a depth of around 20.0 m in shallow aquifers with fine gray sands has high As concentrations (>400 lg/L). However, in the areas where the overlying clay layer is absent, groundwater As concentrations are low (<20 lg/L). The clay layers have low permeability and high organic C contents that are expected to restrict diffusion of atmospheric O2 into the aquifers, which will favor reducing conditions in the shallow aquifers (Guo et al., 2008a). The yellowish brown clay layers in this study have organic C contents between 1.0% and 4.6%, and low permeability, which is a very important contributor to As mobilization because they limit vertical recharge of oxygenated water. The permeability of surface layers is one of the key factors controlling groundwater As concentration. The aquifers underlying permeable layers of sandy soils can be flushed more rapidly than those underlying clay layers, and, therefore, contain lower As groundwater, as shown by studies in Bangladesh (Stute et al., 2007; Weinman et al., 2008). Peat deposits are major components of lacustrine sediments in the Hetao basin and occur within the aquifer sands as lentoid bodies and in clay layers with a thickness of around 20 cm. Natural organic matter in peaty strata can drive reductive dissolution of FeOOH and release of adsorbed As to groundwater (McArthur et al., 2004). The areal distribution of As concentration corresponds to the areal distribution of buried peat in the Bengal basin (Ravenscroft et al., 2001). Aziz et al. (2008) and Nath et al. (2010) also showed that As concentrations tend to be lower in shallow aquifers underlying sandy soils with higher hydraulic conductivity than below finer-grained soils (such as clay, peat deposits) with lower hydraulic conductivity.
In this study, high groundwater As concentrations are spatially associated with thick surface/sub-surface clay layers, where the clay layers commonly occur at depths less than 10 m. It is observed that groundwaters sampled at depths of around 20.0 m have high As concentrations (>100 lg/L) at sites where the thickness of overlying clay layers is greater than 3.4 m (Figs. 5, 6 and 8). The As concentration is up to 465 lg/L in groundwater underlying thick clay layers (around 7.0 m in thickness) at depths between 4 and 15 m at site D. However, at site C where the clay layer occurs at depths between 3.4 and 6.2 m, although organic C contents are comparable to those at other sites, groundwater As concentrations are low (mostly <52 lg/L). The lack of high As groundwater at this site is attributed to the Fe oxide-coated sand layer 1.0–1.5 m thick. In addition, the presence of yellowish brown fine sand at depths between 9.0 and 12.0 m at site B results in relatively low As concentrations (ranging between 2.73 and 110 lg/L). In addition to the depth and the stratigraphy, sediment reactivity is an important factor determining As concentration. Although clay layers are present at depths of less than 10 m, dissolved As concentrations at depths of around 10 m are lower than those at depths of around 20 m. The presence of yellowish-brown sediments at depths around 10 m may contribute to the difference, indicating that redox conditions were not sufficiently reducing to dissolve the Fe oxides that are associated with the release of adsorbed As. Burgess et al. (2010) observed that oxidized sediments have a high capacity for adsorbing dissolved As, and therefore lead to low As concentration in groundwater. The surface hydrology of the Hetao basin has also been shown to affect groundwater geochemistry, especially As concentration (Guo et al., 2011). Although the stratigraphy, the surface/subsurface lithology, and the abundance of ferric oxyhydroxides of site
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Fig. 7. Sedimentary lithologic logs (a), concentrations of As species and F (b), and concentrations of total dissolved Fe, Fe(II), and Eh (c) in site C of Fig. 1. Concentrations of As species are shown on a logarithmic scale.
Fig. 8. Sedimentary lithologic logs (a), concentrations of As species and F (b), and concentrations of total dissolved Fe, Fe(II), and Eh (c) in site D of Fig. 1. Concentrations of As species are shown on a logarithmic scale.
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Fig. 9. Variation of As concentration with Eh (a), NO3 (b), and Fe (c) of water samples.
A and site B are similar, their As concentrations are quite different (Figs. 5 and 6). Groundwater at site B, located immediately downgradient of a main drainage canal (about 200 m), has lower As concentrations compared to site A. The interaction between the surface water and the shallow groundwater caused by the fluctuation of the water level in the channel results in a pulse of O2-rich water that is expected to recharge the aquifer and raise the Eh, stabilizing ferric oxides and favoring As immobilization in the aquifers (Guo et al., 2011). 4.3. Dependence of F concentration on mineral dissolution and precipitation Groundwater F can originate from the dissolution of F-bearing minerals (Handa, 1975; Nordstrom et al., 1989; Shah and Danishwar, 2003). In metamorphic rocks such as those in the Langshan Mountains, F contents are commonly between 240 and 528 mg/ kg (data not shown). As an accessory mineral in metamorphic rocks, fluorite (CaF2) is a plausible source of F in groundwater. Dissolution of fluorite could play an important role in the supply of F from the recharge area and by dissolution in the sediment derived from the Langshan Mountains within the Hetao basin. Fluoride concentration is negatively correlated with the Ca2+ concentration, but positively correlated with pH in the study area (Fig. 10). Other researchers have also found a positive correlation of F concentration with pH and a negative correlation of F with Ca2+ (Nordstrom and Jenne, 1977; Jain, 2005; Chae et al., 2007). The negative correlation between Ca2+ and F suggests that fluorite solubility may limit F concentration (Kfluorite = 10 10.6 from
Parkhurst and Appelo, 1999). As shown in Fig. 11a, all water samples are undersaturated with respect to fluorite. When fluorite congruently dissolves, activities of Ca2+ and F are expected to follow line 1 in Fig. 11a. However, the Ca2+ and F activities of most samples fall on the right side of line 1, which indicates that Ca is added to the groundwater from a source other than fluorite dissolution. Other Ca minerals free of F, known to be present in the Hetao basin sediments such as, calcite (3.6–25%), and dolomite (<1.0– 10%) (Guo et al., 2008a), could dissolve to increase Ca2+ concentration. If calcite coexists with fluorite in the aquifers and both dissolve progressively with a ratio of 200 (calcite: fluorite), the activities of Ca2+ and F are predicted to follow line 2 in Fig. 11a. The positions of all water samples between line 1 and line 2 of Fig. 11a, demonstrate that F concentration can be limited by Ca2+ originating from fluorite dissolution and other Ca mineral dissolution (such as calcite). The activity of Ca2+ may decrease (line 3 of Fig. 11a) by mineral precipitation or cation exchange. Mineral saturation indices calculated with PHREEQC (Parkhurst and Appelo, 1999) show that all groundwater samples are oversaturated with respect to calcite (Fig. 11b). Precipitation of calcite would lower the dissolved Ca2+ concentration and favor dissolution of fluorite. The anticipated increase in the amount of soluble F in groundwater with formation of calcite is inconsistent with the lack of significant correlation between SIcalcite and F concentration (Fig. 11b). The lack of correlation may indicate that fluorite in aquifer sediments is heterogeneously distributed and, therefore, not a consistent contributor to groundwater F . Adsorption of F on minerals in the aquifers can be a major sink for F in groundwater (Chae et al., 2007). Sediments from shallow
Fig. 10. Variation of F concentration with Ca2+ concentration (a), and pH (b) of water samples.
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Fig. 11. The relationship between the activities of Ca2+ and F (a), and F and SIcalcite (b). Line 1 shows the path of fluorite dissolution; Line 2 indicates the path of calcite and fluorite dissolution with a ratio of 200 (calcite: fluorite); Line 3 shows that the path of Ca2+ decrease associated with calcite precipitation and/or cation exchange.
on the higher TDS values of the uppermost <5 m water samples, evaporation is a more important influence on water composition at shallow depths than it is in the deeper groundwater (10– 20 m). Accordingly, evaporation may partly account for the high dissolved F concentrations. Evaporation, mineral dissolution and precipitation, and adsorption–desorption control F concentrations in the Hetao basin groundwater. The lateral distribution of F is attributed to its local sources and the spatial distribution of those processes. Near the drainage channels, where evaporation is significant, and desorption is expected due to high pH and HCO3 concentrations, high F groundwater is common. In the southeastern plain where the Yellow River fluvial sediments prevail, and surface water has been used for irrigation, groundwater has a relatively high Ca2+/pH ratio (>5) and F concentrations are normally lower than 1.0 mg/L. This contrast with the characteristics of groundwater and sediment source areas elsewhere in the basin, accounts for some of the spatial variability in F concentration within the study area. 5. Conclusions
Fig. 12. Ratio of Na+ to (Na+ + Ca2+) as a function of TDS in all samples from this study. The gray shaded area reflects weathering dominated by rocks. TDS values below the shaded area indicate precipitation dominance, and TDS values above the shaded area by evaporation and mineral precipitation.
aquifers in the Hetao basin have a high content of clay minerals (averagely 15%), including smectite, illite and kaolinite (Guo et al., 2008a). There is a strong pH dependence for F adsorption with greater adsorption to clays and oxyhydroxides at lower pH (Karthikeyan et al., 2009; Gao et al., 2009; Kumar et al., 2009; Sujana et al., 2009). The pH values (7.2–8.9) of the investigated groundwaters are greater than the point of zero charge (PZC) of most minerals (<7–8), which makes the surface charge of the solids neutral or slightly negative and depresses the adsorption of negatively charged F . Groundwater samples are plotted on a Gibbs’ diagram in Fig. 12 and the distribution of sample points is consistent with evaporation and mineral precipitation as the mechanisms controlling the major ion groundwater chemistry in the study area. Evaporation increases the TDS and pH of the groundwater because of concentration of various components and loss of CO2 (Handa, 1975). Based
Spatial variations in both As and F concentration are found in the NW area of the Hetao basin. Concentrations of groundwater As range between 0.96 and 720 lg/L (median 30.2 lg/L). Fluoride concentrations range between 0.30 and 2.58 mg/L (median 1.07 mg/L). There is no significant correlation between the As and F concentrations. Within depths <30 m, As concentrations increase with increasing depth. However, F concentrations do not define a consistent trend with depth. At depths less than 10 m, groundwater generally has low As concentration, and high F concentration. Redox conditions are the major factor controlling As concentration in shallow groundwater. Arsenic concentrations are negatively correlated with redox values. Shallow groundwaters are mostly at the stage of Fe oxide reduction, in which most NO3 is reduced. Reductive dissolution of ferric oxides is the major mechanism for As release in the study area. High groundwater As concentration is associated with the presence of thick surface/sub-surface clay layers. The clay layers with low permeability and high organic C content restrict diffusion of atmospheric O2 into the aquifers and supply electron donor compounds to subjacent aquifers. These conditions favor reducing and As release in the underlying shallow aquifers. In addition to the depth and the stratigraphy, the sediment reactivity is an important factor determining As concentration. Yellowish-brown fine sands at depths near 10 m lead to As immobilization and subsequently low dissolved As concentrations.
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The surface hydrology, which changes the local groundwater flow system, affects groundwater As concentrations. Moderate positive correlation between pH and F concentrations indicates that high pH may favor F desorption, while HCO3 may act as a sorption competitor. Fluoride concentration is mainly controlled by dissolution and precipitation of Ca minerals (such as fluorite and calcite), and F adsorption–desorption in the arid environment. Acknowledgements The study has been financially supported by the National Natural Science Foundation of China (Nos. 41172224 and 40872160), the Program for New Century Excellent Talents in University (No. NCET-07-0770), and the Chinese Universities Scientific Fund (No. 2010ZD04). References Aziz, Z., van Geen, A., Stute, M., Versteeg, R., Horneman, A., Zheng, Y., Goodbred, S., Steckler, M., Weinman, B., Gavrieli, I., Hoque, M.A., Shamsudduha, M., Ahmed, K.M., 2008. Impact of local recharge on arsenic concentrations in shallow aquifers inferred from the electromagnetic conductivity of soils in Araihazar. Bangladesh. Water Resour. Res. 44, W07416. doi:10.1029/2007WR006000. Burgess, W.G., Hoque, M.A., Michael, H.A., Voss, C.I., Breit, G.N., Ahmed, K.M., 2010. Vulnerability of deep groundwater in the Bengal aquifer system to contamination by arsenic. Nat. Geosci. 3, 83–87. Chae, G.T., Yun, S.T., Mayer, B., Kim, K.H., Kim, S.Y., Kwon, J.S., Kim, K.J., Koh, Y.K., 2007. Fluorine geochemistry in bedrock groundwater of South Korea. Sci. Total Environ. 385, 272–283. Deng, Y., Wang, Y., Ma, T., 2009. Isotope and minor element geochemistry of high arsenic groundwater from Hangjinhouqi, the Hetao plain, Inner Mongolia. Appl. Geochem. 24, 587–599. Fendorf, S., Michael, H.A., van Geen, A., 2010. Spatial and temporal variations of groundwater arsenic in south and Southeast Asia. Science 328, 1123–1127. Gao, H.X., 1990. The first discovered endemic arsenic poisoning in Inner Mongolia. Inner Mongolia J. Endem. Dis. Cont. Treat 15, 1 (in Chinese). Gao, S., Sun, R., Wei, Z.G., Zhao, H.Y., Li, H.X., Hu, F., 2009. Size-dependent defluoridation properties of synthetic hydroxyapatite. J. Fluorine Chem. 130, 550–556. Guo, H.M., Stüben, D., Berner, Z., 2007. Adsorption of arsenic(III) and arsenic(V) from groundwater using natural siderite as the adsorbent. J. Colloid Interface Sci. 315, 47–53. Guo, H.M., Yang, S.Z., Tang, X.H., Li, Y., Shen, Z.L., 2008a. Groundwater geochemistry and its implications for arsenic mobilization in shallow aquifers of the Hetao basin. Inner Mongolia. Sci. Total Environ. 393, 131–144. Guo, H.M., Yang, S.Z., Tang, X.H., Zhang, B., Li, Y., Shen, Z.L., 2008b. Distribution of high As groundwater and mechanisms of As mobilization in shallow aquifers from the Hetao basin, Inner Mongolia. Hydrogeol. Engin. Geol. 35 (s), 92–99 (in Chinese with English abstract). Guo, H.M., Zhang, B., Li, Y., Berner, Z., Tang, X.H., Norra, S., 2011. Hydrogeological and biogeochemical constrains of As mobilization in shallow aquifers from the Hetao basin, Inner Mongolia. Environ. Pollut. 159, 876–883. Handa, B.K., 1975. Geochemistry and genesis of fluoride-containing ground waters in India. Ground water 13, 275–281. He, J., Ma, T., Deng, Y.M., Yang, H., Wang, Y.X., 2009. Environmental geochemistry of high arsenic groundwater at western Hetao plain, Inner Mongolia. Front. Earth Sci. China 3 (1), 63–72. Inner Mongolia Institute of Hydrogeology, 1982. Hydrogeological Setting and Remediation Approaches of Soil Salinity in the Hetao basin, Inner Mongolia. Scientific Report (in Chinese). Jain, C.K., 2005. Fluoride contamination in ground water. In: Lehr, J., Keeley, J., Lehr, J.J. (Eds.), Water Encyclopedia. Wiley, New Jersey, pp. 130–135. Jin, Y.L., Liang, C.H., He, G.L., Cao, J.X., Ma, F., Wang, H.Z., Ying, B., Ji, R.D., 2003. Study on distribution of endemic arsenism in China. J. Hygiene Res. 23, 519–540 (in Chinese with English abstract). Karthikeyan, M., Satheesh Kumar, K.K., Elango, K.P., 2009. Conducting polymer/ alumina composites as viable adsorbents for the removal of fluoride ions from aqueous solution. J. Fluorine Chem. 130, 894–901. Kim, K., 2003. Long-term disturbance of ground water chemistry following well installation. Ground Water 41, 780–789. Kumar, E., Bhatnagar, A., Jia, M.Y., Jung, W., Lee, S.H., Kim, S.J., Lee, G., Song, H., Choi, J.Y., Yang, J.S., Jeon, B.H., 2009. Defluoridation from aqueous solutions by granular ferric hydroxide (GFH). Water Res. 43, 490–498.
Mamatha, P., Rao, S.M., 2010. Geochemistry of fluoride rich groundwater in Kolar and Tumkur Districts of Karnataka. Environ. Earth Sci. 61, 131–142. McArthur, J.M., Banerjee, D.M., Hudson-Edwards, K.A., Mishra, R., Purohit, R., Ravenscroft, P., Cronin, A., Howarth, R.J., Chatterjee, A., Talukder, T., Lowry, D., Houghton, S., Chadha, D.K., 2004. Natural organic matter in sedimentary basins and its relation to arsenic in anoxic ground water: the example of West Bengal and its worldwide implications. Appl. Geochem. 19, 1255–1293. Meenakshi, Garg, V.K., Kavita, Renuka, Malik, Anju, 2004. Groundwater quality in some villages of Haryana, India: focus on fluoride and fluorosis. J. Hazard. Mater. 106B, 85–97. Mukherjee, A., von Brömsse, M., Scanlon, B.R., Bhattacharya, P., Fryar, A.E., Aziz Hasan, Md., Ahmed, K.M., Chatterjee, D., Jacks, G., Sracek, O., 2008. Hydrogeochemical comparison and effects of overlapping redox zones on groundwater arsenic near the Western (Bhagirathi sub-basin, India) and Eastern (Meghna sub-basin, Bangladesh) margins of the Bengal Basin. J. Contam. Hydrol. 99, 31–48. Nath, B., Mallik, S.B., Stüben, D., Chatterjee, D., Charlet, L., 2010. Electrical resistivity investigation of the arsenic affected alluvial aquifers in West Bengal, India: usefulness in identifying the areas of low and high groundwater arsenic. Environ. Earth Sci. 60, 873–884. Nickson, R., McArthur, J., Burgess, W., Ahmed, K.M., Ravenscroft, P., Rahman, M., 1998. Arsenic poisoning of Bangladesh groundwater. Nature 395, 338. Nordstrom, D.K., Jenne, E.A., 1977. Fluorite solubility equilibria in selected geothermal waters. Geochim. Cosmochim. Acta 41, 175–188. Nordstrom, D.K., Ball, J.W., Donahoe, R.J., Whittemore, D., 1989. Groundwater chemistry and water–rock interactions at Stripa. Geochim. Cosmochim. Acta 53, 1727–1740. Parkhurst, D.L., Appelo, C.A.J., 1999. User’s guide to PHREEQC (Version 2)-a Computer Program for Speciation, Batch-reaction, One Dimensional Transport and Inverse Geochemical Calculation. US Geol. Surv. Water Resour. Invest. Rep., pp. 99–4259. Price, R.E., Pichler, T., 2005. Distribution, speciation and bioavailability of arsenic in a shallow-water submarine hydrothermal system, Tutum Bay, Ambitle Island. PNG. Chem. Geol. 224, 122–135. Rafique, T., Naseem, S., Usmani, T.H., Bashir, E., Khan, F.A., Bhanger, M.I., 2009. Geochemical factors controlling the occurrence of high fluoride groundwater in the Nagar Parkar area, Sindh. Pakistan. J. Hazard. Mater. 171, 424–430. Ravenscroft, P., McArthur, J.M., Hoque, B.A., 2001. Geochemical and palaeohydrological controls on pollution of groundwater by arsenic. In: Chappell, W.R., Abernathy, C.O., Calderon, R. (Eds.), Arsenic Exposure and Health Effects IV. Elsevier, Oxford, pp. 53–78. 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Applied Geochemistry journal homepage: www.elsevier.com/locate/apgeochem
Spatial variation in arsenic and fluoride concentrations of shallow groundwater from the town of Shahai in the Hetao basin, Inner Mongolia Huaming Guo ⇑, Yang Zhang, Lina Xing, Yongfeng Jia State Key Laboratory of Biogeology and Environmental Geology, China University of Geosciences, Beijing 100083, PR China School of Water Resources and Environment, China University of Geosciences, Beijing 100083, PR China
a r t i c l e
i n f o
Article history: Available online 2 February 2012
a b s t r a c t Twenty-nine wells were selected for groundwater sampling in the town of Shahai, in the Hetao basin, Inner Mongolia. Four multilevel samplers were installed for monitoring groundwater chemistry at depths of 2.5–20 m. Results show that groundwater As exhibits a large spatial variation, ranging between 0.96 and 720 lg/L, with 71% of samples exceeding the WHO drinking water guideline value (10 lg/L). Fluoride concentrations range between 0.30 and 2.57 mg/L. There is no significant correlation between As and F concentrations. Greater As concentrations were found with increasing well depth. However, F concentrations do not show a consistent trend with depth. Groundwater with relatively low Eh has high As concentrations, indicating that the reducing environment is the major factor controlling As mobilization. Low As concentrations (<10 lg/L) are found in groundwater at depths less than 10 m. High groundwater As concentration is associated with aquifers that have thick overlying clay layers. The clay layers, mainly occurring at depths <10 m, have low permeability and high organic C content. These strata restrict diffusion of atmospheric O2 into the aquifers, and lead to reducing conditions that favor As release. Sediment composition is an additional factor in determining dissolved As concentrations. In aquifers composed of yellowish-brown fine sands at depths around 10 m, groundwater generally has low As concentrations which is attributed to the high As adsorption capacity of the yellow–brown Fe oxyhydroxide coatings. Fluoride concentration is positively correlated with pH and negatively correlated with Ca2+ concentration. All groundwater samples are over-saturated with respect to calcite and under-saturated with respect to fluorite. Dissolution and precipitation of Ca minerals (such as fluorite and calcite), and F adsorption–desorption are likely controlling the concentration of F in groundwater. Ó 2012 Elsevier Ltd. All rights reserved.
1. Introduction The Hetao basin is located in an arid region of Inner Mongolia, PR China. Although the Yellow River lies to the south of the basin and its water is used for farmland irrigation, groundwater is the major source of drinking water. Before the 1980s, every village had 2–4 dug wells (3–5 m in depth) that provided drinking water. This shallow water usually had total dissolved solid (TDS) > 1000 mg/L, which caused residents to abandon dug wells for deeper hand pump tube wells (usually 15–30 m in depth) (Tang et al., 1996). The deeper groundwater generally has lower TDS, but commonly contains high As concentrations (>50 lg/L). The change in drinking water supplies has led to endemic As poisoning that was first recognized in 1990 (Gao, 1990). High As groundwater is widespread in the basin, which poses a significant health risk to more than one million residents (Guo et al., 2008a). It was reported ⇑ Corresponding author at: School of Water Resources and Environment, China University of Geosciences, Beijing 100083, PR China. Tel.: +86 10 8232 1366; fax: +86 10 8232 1081. E-mail address: [email protected] (H. Guo). 0883-2927/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved. doi:10.1016/j.apgeochem.2012.01.016
that the chronic As poisoning is affecting 24.8% of the basin population (Jin et al., 2003). In addition to abnormal As levels, high F concentrations are common in shallow groundwater in the Hetao basin (<30 m in depth) (Guo et al., 2008a; Deng et al., 2009). High dissolved F concentrations result in residents of the Hetao basin suffering from chronic fluorosis. The number of people suffering from dental fluorosis in the basin is as high as 1.9 million, and those with skeletal fluorosis is estimated to be more than 230,000 (Wang et al., 1999). Both groundwater As and F are believed to be of geogenic origin (Smedley et al., 2003; Guo et al., 2008a). Groundwater As and F concentrations are highly variable on a local and regional scale. Although As concentrations have been known to range from <1.0 to 572 lg/L over distances from tens of meters to kilometers (Guo et al., 2008a; Deng et al., 2009; He et al., 2009), the cause of the lateral heterogeneity in As concentration remains unknown. High As groundwater generally occurs in shallow alluvial–lacustrine aquifers (Guo et al., 2008a). The authors’ previous investigation showed that wells near irrigation channels and drainage channels produce low As groundwater (Guo et al., 2011). It has been generally accepted that tube wells
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2. Materials and methods
Mountains, NW of the study area, are mainly composed of Jurassic to Cretaceous metamorphic rocks (slate, gneiss and marble) (Guo et al., 2008a). Alluvial fans extend from the front of the mountain range to a broad plain (Fig. 1b). The plain comprises 75% of the study area and has a topographic gradient between 1/1000 and 1/5000. The plain is underlain by sediments with a thickness between 1500 and 8000 m that are mainly derived from the Langshan Mountains and from fluvial deposits of the old Yellow River. Quaternary fine-grained, lacustrine sediments are locally part of the basin fill (Guo et al., 2008a). Shallow groundwater mainly occurs in the Quaternary alluvial, alluvial–pluvial, and fluvial–lacustrine aquifers, which are unconfined or leaky-confined. The alluvial-pluvial unconfined aquifers mainly occur in the belt of alluvial fans, while fluvial–lacustrine leaky-confined aquifers are common in the low-lying area. Locally, the topmost part of the sedimentary sequence consists of alluvial deposits and lacustrine sediments that were deposited in oxbow lakes and small lagoons separated from the large lake that formerly occupied the basin (Guo et al., 2008a). Peat accumulations are also distributed within the sediment. High As groundwater is most common in fluvial to lacustrine aquifers. In the basin, the average annual precipitation is around 188 mm, which is much less than potential evaporation of 2000– 2500 mm. The depth of the water table ranges from 1.0 to 10 m. Groundwater in the basin-fill sediments is mainly recharged through the alluvial fans by water flowing through fractures along the mountain front, and by vertically infiltrating precipitation, ditch water (irrigation channels), and irrigation water in the flat plain. It is discharged mainly via evapotranspiration, drainage, and pumping. The main and secondary drainage channels were constructed to locally lower groundwater levels, thus reducing salinization due to evaporation. The general direction of groundwater flow is from north to south. The flow rate in the basin-fill sediment ranges between 0.002 and 0.2 m/d (Inner Mongolia Institute of Hydrogeology, 1982).
2.1. Study area
2.2. Sample collection
The study area of approximately 1000 km2 is located in the northwestern portion of the Hetao basin (Fig. 1a). The Langshan
Twenty-nine groundwater samples were collected from handpumped tube wells and electric-powered public water supply
(15–30 m in depth) have higher As concentrations than dug wells (3–5 m in depth). Nonetheless, the vertical distribution of As in these shallow aquifers has not been documented. High F groundwaters are found not only in bedrock aquifers (Chae et al., 2007; Reddy et al., 2010; Mamatha and Rao, 2010), but also in aquifers developed in young sediments (Wang and Cheng, 2001; Meenakshi et al., 2004; Deng et al., 2009). In both aquifers, rock weathering, evaporation, and mineral crystallization and precipitation are the main factors controlling F concentration (Reddy et al., 2010; Mamatha and Rao, 2010). Groundwater from sediments in inland basins with high F concentration typically has low Ca/Na ratio, high pH, high HCO3 , high salinity, and high sodicity (Wang and Cheng, 2001; Meenakshi et al., 2004; Rafique et al., 2009). As with As, spatial distribution of groundwater F has not been sufficiently investigated. Understanding spatial variability of As and F are crucial not only for understanding the biogeochemical–hydrological processes that control groundwater concentrations of As and F , but also to help develop strategies for sustainable use of shallow groundwater that will reduce exposure to As and F (Fendorf et al., 2010). Shahai is a typical town in the Hetao basin, where As and F concentrations range between 1.17 and 483 lg/L (median 67.7 lg/L), and between 0.08 and 7.87 mg/L (median 1.40 mg/L), respectively (Guo et al., 2008a). In the vicinity of the town, there are recognized areas of groundwater recharge, and discharge. Both irrigation channels and drainage channels, which are believed to affect As concentrations in shallow groundwater (Guo et al., 2011), are distributed across the region. The objectives of this study are to (1) characterize spatial distributions of groundwater As and F in shallow aquifers; (2) describe the vertical distributions of As and F concentrations in groundwaters; and (3) evaluate the factors controlling As and F distribution.
Fig. 1. Location of study area (a) and sampling sites (b).
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wells in May 2010 (Fig. 1b). The data for 12 samples published previously in Deng et al. (2009) were also considered in this study (Fig. 1b). The depths of these wells ranges between 13 and 100 m. Four multilevel well clusters (A–D) were installed in July, 2009 approximately parallel to the groundwater flow path along a 5 km transect (Fig. 1b). The multilevel samplers were installed following the procedure of Kim (2003) using bundles of polyethylene tubes fitted with 20 cm screened intervals. Each well cluster has 3–5 sampling ports placed at depths ranging from 2.5 to 20 m. Groundwater was sampled from each well after pumping (usually 20 min) until the flowing water showed a stabilized temperature, pH, DO (dissolved O2), and Eh. All samples were filtered through 0.45 lm membrane filters in the field. Water samples for major and trace element analysis were collected in 100 mL HNO3-washed polyethylene bottles, followed by addition of 6 M HNO3 to pH < 2. Aliquots for analysis of As species were preserved with 0.25 M EDTA (10%). Aliquots for dissolved organic C (DOC) analysis were stored in 30 mL amber glass bottles, followed by addition of 6 M H2SO4 (2%). Samples for analysis of As species and DOC were stored at 4 °C. Samples for anion analysis were not acidified. At the time of groundwater sampling, parameters, including water temperature, electrical conductivity (EC), pH and Eh, were measured using a multiparameter portable meter (HANNA, HI 9828), while concentrations of S2 , Fe(II), and alkalinity were determined using a portable spectrophotometer (HACH, DR2800). 2.3. Sample analysis Concentrations of major cations and trace elements were determined by ICP-AES and ICP-MS, respectively. Unacidified aliquots were analyzed for F , Cl , Br , NO3 ; NO2 ; SO24 concentrations by Ion Chromatography with a DX-120 (Dionex). A total carbon analyzer (TOC-Vwp, Shimadzu) was used to determine DOC concentrations. Concentrations of total dissolved As and Se were determined by atomic fluorescence spectrometry (AFS) with hydride generation (Price and Pichler, 2005). For most samples, charge imbalances were less than 5%. Arsenic species in groundwater samples were determined using an HPLC-ICP-MS. A high performance liquid chromatography (HPLC, 1100 Series, Agilent) consisting of a system controller, a solvent delivery module, a column oven and a 6-port injection valve was used. A reversed-phase C18 column (Capcell, Pak, 250 mm 4.6 mm, 5 lm particle size) was used for separation of As species. An ICP mass spectrometer (7500C, Agilent) was used as a detector, which was operated in the He mode to remove the ArCl interference.
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have As concentrations of 96.7, 93.3, 364, 264 and 152 lg/L, respectively. Within 1 km of the drainage channels, groundwater contains low As concentrations. Wells 40 and 35, which are positioned 350 and 850 m from the main drainage, have As concentrations of 30.6 and 10.4 lg/L, respectively (Fig. 2). A notable exception to this generalization is Well 39, which is near the drainage channel (about 700 m), but yields water with As concentration up to 340 lg/L. This is attributed to the greater screening depth of well 39 (around 100 m), relative to the other wells. The deeper wells likely sample water below the influence of the drainage channel. In the SE portion of the study area, where the Yellow River fluvial sediments prevail, groundwater generally has low As concentrations (<50 lg/L) (Fig. 2). Dissolved sulfide and DOC in groundwater are considered as indicators of the redox state of the aquifers. More than 95% of all groundwater samples have measurable S2 . Sulfide concentrations show a great variability, ranging from <1.0 to 20 lg/L. The coexistance of S2 and SO24 indicates that SO24 reduction occurs in the aquifer system. The correlation between As and S2 is not significant. Concentrations of DOC in groundwater samples range between 0.85 and 21.3 mg/L with a median of 5.22 mg/L. There is no significant correlation between DOC and As. 3.2. Spatial variation of F concentration Groundwater pH is near-neutral to weakly alkaline (7.2–8.9). Sodium is the predominant cation, ranging from 29.6 to 1310 mg/L, while HCO3 or Cl is the predominant anion. Bicarbonate concentrations range between 166 and 1080 mg/L. Groundwaters have Cl concentrations between 33.0 and 2050 mg/L and the ratio of [Na+]/[Cl ] is approximately 1:1. Groundwaters are generally of Na–HCO3–(Cl), Na–(Mg)–Cl–HCO3, and Na–Mg–Cl–SO4 (HCO3) types. Fluoride concentrations range between 0.30 and 2.57 mg/L (median 1.02 mg/L), with 51.2% of the samples exceeding the Chinese Drinking Water Standard (1.0 mg/L). High F concentrations (>1.0 mg/L) were mainly observed in groundwater of Na(–Mg)– Cl–HCO3 type. Lateral heterogeneity in F concentration was also observed. For example, 0.88 mg/L was measured in Well 74 and 1.86 mg/L in Well 75 over a distance of 10 m (Fig. 3). High F groundwaters (>1.0 mg/L) are common at the distal end of the alluvial fans (Wells 51, 52, and 67) and near the main drainage channels (Wells 34, 35, 37, 39, 40, 73, 78, D22, and D66) (Fig. 3), where depths of the groundwater table range between 1.0 and 4.0 m. In these areas, evapotranspiration is significant, as evident by the 2H and 18O compositions of groundwater (Guo et al., 2011). Low F concentration was found in shallow groundwater
3. Results 3.1. Spatial variation of As concentration Concentrations of groundwater As range between 0.96 and 720 lg/L (median 30.2 lg/L), with 71% of samples exceeding the WHO drinking water guideline value (10 lg/L). Large variations in As concentration over short distances are common, as illustrated by the low As concentration in Well 74 with a depth of 22 m (3.18 lg/L), whereas groundwater in Well 75 with a depth of 20 m being 10 m from Well 74 contains 94.3 lg/L As (Fig. 2). Arsenic concentrations vary systematically relative to position on the alluvial fans. Wells, installed on the slopes of the alluvial fans (68, 69 and 52), produce water with low As concentrations (Fig. 2). At the distal edge of the alluvial fans, where the topography is generally flat, groundwater generally has high As concentrations (Fig. 2). Wells 67, 51, 66, D22, and D20 located in this area
Fig. 2. Spatial variation of As concentration of groundwaters in the study area.
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in the southeastern portion of the area (Fig. 3). The correlation between F and As concentrations is relatively poor (r2 = 0.44). 3.3. Arsenic concentrations at different depths Of the 41 groundwater samples, 4 samples were taken at depths of less than 15 m, 23 samples at depths between 15 and 30 m, and 14 samples at depths between 30 and 100 m. Generally, As concentration is dependent on depth (Fig. 4). For the samples taken at depths of less than 15 m, As concentrations are less than 50 lg/L. Over 52% of samples taken at depths between 15 and 30 m have As concentrations greater than 50 lg/L. Of the 14 samples taken at depths greater than 30 m, there are 6 samples (43%) having As concentration greater than 50 lg/L. Therefore, it is likely that high As groundwater is present at depths greater than 15 m. The high As groundwaters found in deep aquifers (40–100 m), which are currently used as domestic water supplies and for irrigation, must be the focus of additional investigations. Four multilevel samplers were used to monitor vertical variations in As concentration at depths between 2.5 and 20 at each of the four sites (Fig. 1). At all sites, clay/silty clay layers were intersected at depths of less than 10 m. The clay layers have low permeability and high organic C contents (1.0–4.6%). At site A, there is a yellowish brown clay layer between 4.2 and 7.6 m, gray fine sand between 9.2 and 11.5 m, and between 11.5 and 22.0 m. A similar yellowish brown clay layer was observed at depths between 4.0 and 7.0 m at site B, and between 3.4 and 6.2 m at site C. At sites B and C, yellowish brown fine sand was observed at depths between 9.0 and 12.0 m, and 8.0 and 11.5 m, respectively. In comparison, a thicker sequence of yellowish brown clay/gray clay was found at site D. Eight intervals of this clay were intersected: yellowish brown clay between 4.0 and 6.0 m, 7.0 and 7.5, 9.5 and 10.5 m, 11.2 and 12.0 m, and between 13.5 and 14.2 m; gray clay between 6.0 and 7.0 m, 12.0 and 12.4 m, and between 13.0 and 13.5 m. At all sites, light gray–gray fine sands were observed at depths greater than 12.0 m. Results are shown in Figs. 5–8. During the investigation, no water samples were collected from samplers 1-1 (Fig. 5a), 2-1 (Fig. 6a), 4-1, and 4-2 (Fig. 8a) due to technical problems. As in the broader well survey, As concentrations in the multilevel wells increase with increasing depth (Figs. 5b, 6b, 7b and 8b). Low As concentrations (<50 lg/L) are generally present in samples from depths <10 m (Figs. 6b and 7b), which coincides with the interval dominated by yellow–brown clay/silty clay layers (Figs. 5a, 6a, 7a and 8a). In addition, results suggest that increasing thickness of clay layers corresponds to increased As concentration in groundwater. At site D with a thicker sequence of yellowish brown clay/gray clay, groundwater As is higher, up to 465 lg/L.
Fig. 4. Depth distribution of As concentration in groundwater (gray shaded area indicates the depth interval where high As groundwater is most common).
Arsenic(III) is the dominant As species in all samples, accounting for more than 67% of total As. Vertical distribution of As(III) shows the same trend as total As (Figs. 5b, 6b, 7b and 8b). Waters from the multi-level samplers have lower As(III) concentration at <10 m than samples taken at depths around 20 m. For example, the As(III) concentration from Sampler 4-3 is 15.5 lg/L, being less than that from Sampler 4-5 (429 lg/L) (Fig. 8b). In addition to having low concentrations of As(III) and total As, samples at depths less than 10 m generally have low ratios of As(III) to total As.
3.4. Fluoride concentrations at different depths In the broader well survey, more than 57% of samples taken at depths of less than 20 m have F concentrations greater than 1.0 mg/L, while samples, taken at depths, between 20 and 30 m, have F concentrations ranging between 0.34 and 1.81 mg/L (median 0.88 mg/L). Of 14 samples taken at depths greater than 30 m, 8 samples have F concentrations greater than 1.0 mg/L. Groundwaters from the multi-level well clusters generally indicate a decrease in F concentration with increasing depth (Figs. 5b, 6b, 7b, 8b). Except for site C, where F concentration is relatively constant (0.89–1.16 mg/L), groundwater sampled at depths around 10 m have higher F concentrations than those at depths around 20 m. Nonetheless, considering all of the groundwater samples analyzed for this study, F concentrations do not define a consistent trend with depth.
4. Discussion 4.1. Importance of reducing conditions in As release
Fig. 3. Spatial variation of F concentration of groundwaters in the study area.
The groundwater in the study area has low Eh values ranging between 23 and 375 mv (median 63 mv). There is a negative relationship between As concentration and redox potential (Fig. 9a), indicating that reduction promotes As release from aquifer sediments as suggested by Guo et al. (2008a). The reducing condition is attributable to high concentrations of organic C in the sediments and groundwater. Contents of organic C range between 0.12% and 0.51% in the aquifer sands (unpublished data). Microbial degradation of natural organic matter entrained in the aquifer system is the redox driver in the study area, which is similar to other As-rich
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Fig. 5. Sedimentary lithologic logs (a), concentrations of As species and F (b), and concentrations of total dissolved Fe, Fe(II), and Eh (c) in site A of Fig. 1. Concentrations of As species are shown on a logarithmic scale.
groundwater systems (Ravenscroft et al., 2001; McArthur et al., 2004). Microbially mediated redox processes favor reduction in the sequence NO3 , Mn(IV), As(V), Fe(III) and SO24 (Stumm and Morgan, 1996; Mukherjee et al., 2008). All of these species are reduced within the sedimentary aquifers examined in this study. The low median concentration of NO3 (<0.01 mg/L) and the high concentrations of NH4–N (0.10–7.81 mg/L, median 2.29 mg/L) suggest that the aquifer conditions are capable of NO3 reduction. In environments where NO3 is stable, As is expected to be bound to ferric oxides in the sediments consistent with the low As concentration (10 lg/L) found in groundwater with NO3 concentrations greater than 10 mg/L (Fig. 9b). At the stage of Mn4+ reduction, As bound to Mn oxyhydroxides will be released, and is expected to be readsorbed in the presence of Fe-oxyhydroxides at circumneutral pH (Stüben et al., 2003). In the studied groundwaters with Mn concentrations greater than 150 lg/L, As concentrations are less than 30 lg/L. At an Eh favoring As(V) reduction to As(III), adsorbed As(V) is expected to be converted to As(III) and surficially bound As will be released to the groundwater. Preferential release of As(III) is expected because of the lower affinity of As(III) species to mineral surfaces than the more strongly bound As(V) species (Guo et al., 2007). In the studied groundwaters, the greater concentrations of As(III) can be explained by the release due to reductive desorption (Smedley and Kinniburgh, 2002). At redox conditions favoring Fe oxide reduction, As trapped within ferric oxyhydroxides will also be released (Nickson et al., 1998). Iron(II) formed by reduction can combine with S2 and HCO3 in the studied waters to form pyrite and siderite. This is consistent with the oversaturation of groundwaters with respect to pyrite and siderite, and explains the lack
of correlation between As concentration and Fe concentration (Fig. 9c), although it could be attributable to reduction and desorption of As without reduction of ferric oxyhydroxides. During SO24 reduction, S2 will be produced, which has been detected in most of the water samples. Since the studied groundwaters are oversaturated with respect to pyrite, part of the As may be immobilized in sulfide phases. Vertical variations of As concentration and measured Eh further support the importance of reducing conditions in As mobilization. Except for site C, As concentration increases with increasing depth, while Eh shows the opposite trend (Figs 5–8). At the same sites, it was observed that dissolved total Fe and Fe(II) generally increase with decreasing Eh and increasing depth. The increased dissolved Fe is attributed to the reduction of Fe oxides in the aquifers and suggests that that reductive dissolution of Fe oxides plays a major role in As mobilization. 4.2. Roles of low permeable layers on groundwater As distribution Arsenic contents of the sediments of the Hetao basin are not exceptionally high, most range between 7.3 and 40.6 mg/kg (average 17.6 mg/kg) (Guo et al., 2008a; He et al., 2009). Groundwater Eh shows a great spatial variation, with the range between 23 and 375 mv. Although a negative relationship between Eh and As concentration is observed, reducing conditions alone are insufficient to explain the extreme variation in groundwater As concentration at the local scale both laterally and vertically. Biogeochemical and hydrogeological processes associated with low permeablility layers, including lacustrine clay layers and peat deposits, are believed responsible for the observed heterogeneity (Guo et al., 2008b).
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Fig. 6. Sedimentary lithologic logs (a), concentrations of As species and F (b), and concentrations of total dissolved Fe, Fe(II), and Eh (c) in site B of Fig. 1. Concentrations of As species are shown on a logarithmic scale.
The authors’ previous investigation showed that the thickness of overlying clay layers is well correlated with groundwater As concentration (Guo et al., 2008b). In the areas where the thickness of overlying clay layers is >3.0 m, groundwater sampled at a depth of around 20.0 m in shallow aquifers with fine gray sands has high As concentrations (>400 lg/L). However, in the areas where the overlying clay layer is absent, groundwater As concentrations are low (<20 lg/L). The clay layers have low permeability and high organic C contents that are expected to restrict diffusion of atmospheric O2 into the aquifers, which will favor reducing conditions in the shallow aquifers (Guo et al., 2008a). The yellowish brown clay layers in this study have organic C contents between 1.0% and 4.6%, and low permeability, which is a very important contributor to As mobilization because they limit vertical recharge of oxygenated water. The permeability of surface layers is one of the key factors controlling groundwater As concentration. The aquifers underlying permeable layers of sandy soils can be flushed more rapidly than those underlying clay layers, and, therefore, contain lower As groundwater, as shown by studies in Bangladesh (Stute et al., 2007; Weinman et al., 2008). Peat deposits are major components of lacustrine sediments in the Hetao basin and occur within the aquifer sands as lentoid bodies and in clay layers with a thickness of around 20 cm. Natural organic matter in peaty strata can drive reductive dissolution of FeOOH and release of adsorbed As to groundwater (McArthur et al., 2004). The areal distribution of As concentration corresponds to the areal distribution of buried peat in the Bengal basin (Ravenscroft et al., 2001). Aziz et al. (2008) and Nath et al. (2010) also showed that As concentrations tend to be lower in shallow aquifers underlying sandy soils with higher hydraulic conductivity than below finer-grained soils (such as clay, peat deposits) with lower hydraulic conductivity.
In this study, high groundwater As concentrations are spatially associated with thick surface/sub-surface clay layers, where the clay layers commonly occur at depths less than 10 m. It is observed that groundwaters sampled at depths of around 20.0 m have high As concentrations (>100 lg/L) at sites where the thickness of overlying clay layers is greater than 3.4 m (Figs. 5, 6 and 8). The As concentration is up to 465 lg/L in groundwater underlying thick clay layers (around 7.0 m in thickness) at depths between 4 and 15 m at site D. However, at site C where the clay layer occurs at depths between 3.4 and 6.2 m, although organic C contents are comparable to those at other sites, groundwater As concentrations are low (mostly <52 lg/L). The lack of high As groundwater at this site is attributed to the Fe oxide-coated sand layer 1.0–1.5 m thick. In addition, the presence of yellowish brown fine sand at depths between 9.0 and 12.0 m at site B results in relatively low As concentrations (ranging between 2.73 and 110 lg/L). In addition to the depth and the stratigraphy, sediment reactivity is an important factor determining As concentration. Although clay layers are present at depths of less than 10 m, dissolved As concentrations at depths of around 10 m are lower than those at depths of around 20 m. The presence of yellowish-brown sediments at depths around 10 m may contribute to the difference, indicating that redox conditions were not sufficiently reducing to dissolve the Fe oxides that are associated with the release of adsorbed As. Burgess et al. (2010) observed that oxidized sediments have a high capacity for adsorbing dissolved As, and therefore lead to low As concentration in groundwater. The surface hydrology of the Hetao basin has also been shown to affect groundwater geochemistry, especially As concentration (Guo et al., 2011). Although the stratigraphy, the surface/subsurface lithology, and the abundance of ferric oxyhydroxides of site
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Fig. 7. Sedimentary lithologic logs (a), concentrations of As species and F (b), and concentrations of total dissolved Fe, Fe(II), and Eh (c) in site C of Fig. 1. Concentrations of As species are shown on a logarithmic scale.
Fig. 8. Sedimentary lithologic logs (a), concentrations of As species and F (b), and concentrations of total dissolved Fe, Fe(II), and Eh (c) in site D of Fig. 1. Concentrations of As species are shown on a logarithmic scale.
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Fig. 9. Variation of As concentration with Eh (a), NO3 (b), and Fe (c) of water samples.
A and site B are similar, their As concentrations are quite different (Figs. 5 and 6). Groundwater at site B, located immediately downgradient of a main drainage canal (about 200 m), has lower As concentrations compared to site A. The interaction between the surface water and the shallow groundwater caused by the fluctuation of the water level in the channel results in a pulse of O2-rich water that is expected to recharge the aquifer and raise the Eh, stabilizing ferric oxides and favoring As immobilization in the aquifers (Guo et al., 2011). 4.3. Dependence of F concentration on mineral dissolution and precipitation Groundwater F can originate from the dissolution of F-bearing minerals (Handa, 1975; Nordstrom et al., 1989; Shah and Danishwar, 2003). In metamorphic rocks such as those in the Langshan Mountains, F contents are commonly between 240 and 528 mg/ kg (data not shown). As an accessory mineral in metamorphic rocks, fluorite (CaF2) is a plausible source of F in groundwater. Dissolution of fluorite could play an important role in the supply of F from the recharge area and by dissolution in the sediment derived from the Langshan Mountains within the Hetao basin. Fluoride concentration is negatively correlated with the Ca2+ concentration, but positively correlated with pH in the study area (Fig. 10). Other researchers have also found a positive correlation of F concentration with pH and a negative correlation of F with Ca2+ (Nordstrom and Jenne, 1977; Jain, 2005; Chae et al., 2007). The negative correlation between Ca2+ and F suggests that fluorite solubility may limit F concentration (Kfluorite = 10 10.6 from
Parkhurst and Appelo, 1999). As shown in Fig. 11a, all water samples are undersaturated with respect to fluorite. When fluorite congruently dissolves, activities of Ca2+ and F are expected to follow line 1 in Fig. 11a. However, the Ca2+ and F activities of most samples fall on the right side of line 1, which indicates that Ca is added to the groundwater from a source other than fluorite dissolution. Other Ca minerals free of F, known to be present in the Hetao basin sediments such as, calcite (3.6–25%), and dolomite (<1.0– 10%) (Guo et al., 2008a), could dissolve to increase Ca2+ concentration. If calcite coexists with fluorite in the aquifers and both dissolve progressively with a ratio of 200 (calcite: fluorite), the activities of Ca2+ and F are predicted to follow line 2 in Fig. 11a. The positions of all water samples between line 1 and line 2 of Fig. 11a, demonstrate that F concentration can be limited by Ca2+ originating from fluorite dissolution and other Ca mineral dissolution (such as calcite). The activity of Ca2+ may decrease (line 3 of Fig. 11a) by mineral precipitation or cation exchange. Mineral saturation indices calculated with PHREEQC (Parkhurst and Appelo, 1999) show that all groundwater samples are oversaturated with respect to calcite (Fig. 11b). Precipitation of calcite would lower the dissolved Ca2+ concentration and favor dissolution of fluorite. The anticipated increase in the amount of soluble F in groundwater with formation of calcite is inconsistent with the lack of significant correlation between SIcalcite and F concentration (Fig. 11b). The lack of correlation may indicate that fluorite in aquifer sediments is heterogeneously distributed and, therefore, not a consistent contributor to groundwater F . Adsorption of F on minerals in the aquifers can be a major sink for F in groundwater (Chae et al., 2007). Sediments from shallow
Fig. 10. Variation of F concentration with Ca2+ concentration (a), and pH (b) of water samples.
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Fig. 11. The relationship between the activities of Ca2+ and F (a), and F and SIcalcite (b). Line 1 shows the path of fluorite dissolution; Line 2 indicates the path of calcite and fluorite dissolution with a ratio of 200 (calcite: fluorite); Line 3 shows that the path of Ca2+ decrease associated with calcite precipitation and/or cation exchange.
on the higher TDS values of the uppermost <5 m water samples, evaporation is a more important influence on water composition at shallow depths than it is in the deeper groundwater (10– 20 m). Accordingly, evaporation may partly account for the high dissolved F concentrations. Evaporation, mineral dissolution and precipitation, and adsorption–desorption control F concentrations in the Hetao basin groundwater. The lateral distribution of F is attributed to its local sources and the spatial distribution of those processes. Near the drainage channels, where evaporation is significant, and desorption is expected due to high pH and HCO3 concentrations, high F groundwater is common. In the southeastern plain where the Yellow River fluvial sediments prevail, and surface water has been used for irrigation, groundwater has a relatively high Ca2+/pH ratio (>5) and F concentrations are normally lower than 1.0 mg/L. This contrast with the characteristics of groundwater and sediment source areas elsewhere in the basin, accounts for some of the spatial variability in F concentration within the study area. 5. Conclusions
Fig. 12. Ratio of Na+ to (Na+ + Ca2+) as a function of TDS in all samples from this study. The gray shaded area reflects weathering dominated by rocks. TDS values below the shaded area indicate precipitation dominance, and TDS values above the shaded area by evaporation and mineral precipitation.
aquifers in the Hetao basin have a high content of clay minerals (averagely 15%), including smectite, illite and kaolinite (Guo et al., 2008a). There is a strong pH dependence for F adsorption with greater adsorption to clays and oxyhydroxides at lower pH (Karthikeyan et al., 2009; Gao et al., 2009; Kumar et al., 2009; Sujana et al., 2009). The pH values (7.2–8.9) of the investigated groundwaters are greater than the point of zero charge (PZC) of most minerals (<7–8), which makes the surface charge of the solids neutral or slightly negative and depresses the adsorption of negatively charged F . Groundwater samples are plotted on a Gibbs’ diagram in Fig. 12 and the distribution of sample points is consistent with evaporation and mineral precipitation as the mechanisms controlling the major ion groundwater chemistry in the study area. Evaporation increases the TDS and pH of the groundwater because of concentration of various components and loss of CO2 (Handa, 1975). Based
Spatial variations in both As and F concentration are found in the NW area of the Hetao basin. Concentrations of groundwater As range between 0.96 and 720 lg/L (median 30.2 lg/L). Fluoride concentrations range between 0.30 and 2.58 mg/L (median 1.07 mg/L). There is no significant correlation between the As and F concentrations. Within depths <30 m, As concentrations increase with increasing depth. However, F concentrations do not define a consistent trend with depth. At depths less than 10 m, groundwater generally has low As concentration, and high F concentration. Redox conditions are the major factor controlling As concentration in shallow groundwater. Arsenic concentrations are negatively correlated with redox values. Shallow groundwaters are mostly at the stage of Fe oxide reduction, in which most NO3 is reduced. Reductive dissolution of ferric oxides is the major mechanism for As release in the study area. High groundwater As concentration is associated with the presence of thick surface/sub-surface clay layers. The clay layers with low permeability and high organic C content restrict diffusion of atmospheric O2 into the aquifers and supply electron donor compounds to subjacent aquifers. These conditions favor reducing and As release in the underlying shallow aquifers. In addition to the depth and the stratigraphy, the sediment reactivity is an important factor determining As concentration. Yellowish-brown fine sands at depths near 10 m lead to As immobilization and subsequently low dissolved As concentrations.
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The surface hydrology, which changes the local groundwater flow system, affects groundwater As concentrations. Moderate positive correlation between pH and F concentrations indicates that high pH may favor F desorption, while HCO3 may act as a sorption competitor. Fluoride concentration is mainly controlled by dissolution and precipitation of Ca minerals (such as fluorite and calcite), and F adsorption–desorption in the arid environment. Acknowledgements The study has been financially supported by the National Natural Science Foundation of China (Nos. 41172224 and 40872160), the Program for New Century Excellent Talents in University (No. NCET-07-0770), and the Chinese Universities Scientific Fund (No. 2010ZD04). References Aziz, Z., van Geen, A., Stute, M., Versteeg, R., Horneman, A., Zheng, Y., Goodbred, S., Steckler, M., Weinman, B., Gavrieli, I., Hoque, M.A., Shamsudduha, M., Ahmed, K.M., 2008. Impact of local recharge on arsenic concentrations in shallow aquifers inferred from the electromagnetic conductivity of soils in Araihazar. Bangladesh. Water Resour. Res. 44, W07416. doi:10.1029/2007WR006000. Burgess, W.G., Hoque, M.A., Michael, H.A., Voss, C.I., Breit, G.N., Ahmed, K.M., 2010. Vulnerability of deep groundwater in the Bengal aquifer system to contamination by arsenic. Nat. Geosci. 3, 83–87. Chae, G.T., Yun, S.T., Mayer, B., Kim, K.H., Kim, S.Y., Kwon, J.S., Kim, K.J., Koh, Y.K., 2007. Fluorine geochemistry in bedrock groundwater of South Korea. Sci. Total Environ. 385, 272–283. Deng, Y., Wang, Y., Ma, T., 2009. Isotope and minor element geochemistry of high arsenic groundwater from Hangjinhouqi, the Hetao plain, Inner Mongolia. Appl. Geochem. 24, 587–599. Fendorf, S., Michael, H.A., van Geen, A., 2010. Spatial and temporal variations of groundwater arsenic in south and Southeast Asia. Science 328, 1123–1127. Gao, H.X., 1990. The first discovered endemic arsenic poisoning in Inner Mongolia. Inner Mongolia J. Endem. Dis. Cont. Treat 15, 1 (in Chinese). Gao, S., Sun, R., Wei, Z.G., Zhao, H.Y., Li, H.X., Hu, F., 2009. Size-dependent defluoridation properties of synthetic hydroxyapatite. J. Fluorine Chem. 130, 550–556. Guo, H.M., Stüben, D., Berner, Z., 2007. Adsorption of arsenic(III) and arsenic(V) from groundwater using natural siderite as the adsorbent. J. Colloid Interface Sci. 315, 47–53. Guo, H.M., Yang, S.Z., Tang, X.H., Li, Y., Shen, Z.L., 2008a. Groundwater geochemistry and its implications for arsenic mobilization in shallow aquifers of the Hetao basin. Inner Mongolia. Sci. Total Environ. 393, 131–144. Guo, H.M., Yang, S.Z., Tang, X.H., Zhang, B., Li, Y., Shen, Z.L., 2008b. Distribution of high As groundwater and mechanisms of As mobilization in shallow aquifers from the Hetao basin, Inner Mongolia. Hydrogeol. Engin. Geol. 35 (s), 92–99 (in Chinese with English abstract). Guo, H.M., Zhang, B., Li, Y., Berner, Z., Tang, X.H., Norra, S., 2011. Hydrogeological and biogeochemical constrains of As mobilization in shallow aquifers from the Hetao basin, Inner Mongolia. Environ. Pollut. 159, 876–883. Handa, B.K., 1975. Geochemistry and genesis of fluoride-containing ground waters in India. Ground water 13, 275–281. He, J., Ma, T., Deng, Y.M., Yang, H., Wang, Y.X., 2009. Environmental geochemistry of high arsenic groundwater at western Hetao plain, Inner Mongolia. Front. Earth Sci. China 3 (1), 63–72. Inner Mongolia Institute of Hydrogeology, 1982. Hydrogeological Setting and Remediation Approaches of Soil Salinity in the Hetao basin, Inner Mongolia. Scientific Report (in Chinese). Jain, C.K., 2005. Fluoride contamination in ground water. In: Lehr, J., Keeley, J., Lehr, J.J. (Eds.), Water Encyclopedia. Wiley, New Jersey, pp. 130–135. Jin, Y.L., Liang, C.H., He, G.L., Cao, J.X., Ma, F., Wang, H.Z., Ying, B., Ji, R.D., 2003. Study on distribution of endemic arsenism in China. J. Hygiene Res. 23, 519–540 (in Chinese with English abstract). Karthikeyan, M., Satheesh Kumar, K.K., Elango, K.P., 2009. Conducting polymer/ alumina composites as viable adsorbents for the removal of fluoride ions from aqueous solution. J. Fluorine Chem. 130, 894–901. Kim, K., 2003. Long-term disturbance of ground water chemistry following well installation. Ground Water 41, 780–789. Kumar, E., Bhatnagar, A., Jia, M.Y., Jung, W., Lee, S.H., Kim, S.J., Lee, G., Song, H., Choi, J.Y., Yang, J.S., Jeon, B.H., 2009. Defluoridation from aqueous solutions by granular ferric hydroxide (GFH). Water Res. 43, 490–498.
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