Groundwater geochemistry and its implications for arsenic mobilization in shallow aquifers of the Hetao Basin, Inner Mongolia

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1 SCIENCE OF THE TOTAL ENVIRONMENT 393 (2008) available at Groundwater geochemistry and its implications for arsenic mobilization in shallow aquifers of the Hetao Basin, Inner Mongolia Huaming Guo, Suzhen Yang, Xiaohui Tang, Yuan Li, Zhaoli Shen School of Water Resources and Environment, China University of Geosciences, Beijing , PR China ARTICLE INFO ABSTRACT Article history: Received 17 October 2007 Received in revised form 14 December 2007 Accepted 17 December 2007 Available online 30 January 2008 Keywords: Reductive environment Reductive dissolution Release Sequential extraction Arsenic concentrations in shallow groundwaters from the Hetao Basin of Inner Mongolia range between 0.6 and 572 μg/l. High As groundwaters generally occur in the shallow alluvial lacustrine aquifers, which are mainly composed of black (or dark grey) fine sands in a reducing environment. They are characterized by high concentrations of dissolved Fe, Mn, HCO 3, P and S 2, and low concentrations of NO 3 and SO 2 4. Low SO 2 4 coupled with high S 2 suggests that SO 2 4 reduction has been an active process. In the reducing groundwaters, inorganic As(III) accounts for around 75% of total dissolved As. Total As contents in the sediments from three representative boreholes are observed to be mg/ (average of 18.9 mg/). The total As is mildly-strongly correlated with total Fe and total Mn, while a quite weak correlation exists between total As and total S, suggesting that the As is associated with Fe Mn oxides, rather than sulfides in the sediments. It is found in the sequential extraction that chemically active As is mainly bound to Fe Mn oxides, up to 3500 μg/. The mobilization of As under reducing conditions is believed to include reductive dissolution of Fe Mn oxides and reduction of adsorbed As. Although exchangeable As is labile and very vulnerable to hydrogeochemical condition, the contribution is relatively limited due to the low concentrations. The competition between As and other anions (such as HPO 2 4 ) for binding sites on Fe Mn oxides may also give rise to the release of As into groundwater. Slow groundwater movement helps accumulation of the released As in the groundwaters Elsevier B.V. All rights reserved. 1. Introduction Arsenic, which is ubiquitous in the environment, has been recognized as one of the most widespread and problematic water contaminants (Nickson et al., 1998; Smedley and Kinniburgh, 2002; Berg et al., 2007). Health problems associated with groundwater As have been documented in many parts of the world, including Bangladesh, India, China, Mexico, Hungary, Vietnam, Argentina and Chile (Smedley and Kinniburgh, 2002; Guo et al., 2003). Symptoms of chronic exposure to As in drinking water at concentrations significantly above 50 μg/l are numerous and often severe and commonly include skin diseases (pigmentation, dermal hyperkeratosis, skin cancer), many other cardiovascular, neurological, hematological, renal and respiratory diseases, as well as lung, bladder, liver, kidney and prostate cancers (Morton and Dunette, 1994; Das et al., 1995; Smith et al., 1998). The World Health Organization has set a provisional guideline limit of 10 µg/l for As in drinking water (World Health Organization, 1996) which was subsequently adopted by the European Union (European Commission, 1998) and the United States (EPA Office of Groundwater and Drinking Water, 2002), while many Corresponding author. Tel.: ; fax: address: hmguo@cugb.edu.cn (H. Guo) /$ see front matter 2007 Elsevier B.V. All rights reserved. doi: /j.scitotenv

2 132 SCIENCE OF THE TOTAL ENVIRONMENT 393 (2008) developing countries still adopt the value of 50 μg/l as their national standard. Since 2007, China has lowered the drinking water standard for As to 10 µg/l. Some areas in Northwest China, especially in Xinjiang, Inner Mongolia, and Shanxi, which have many wells with As concentrations above this limit, have been facing big challenges in finding alternative water resources and/or remediating high As water in a costeffective way. In the early 1980s, the cases of As poisoning were firstly recognised in Xinjiang Province. Wang (1984) found As concentrations up to 1200 μg/l in groundwaters. Luo et al. (2006) reported As concentrations of between 70 and 750 μg/l in deep artesian groundwaters (N200 m in depth) in the south part of the Zhunge'er Basin to the north of the Tianshan Mountains. Arsenic-affected areas mainly distribute between Aibi Lake (in the southwest of the Zhunge'er Basin) and Manasi River, paralleling the Tianshan Mountain (Wang et al., 2002). Shallow groundwaters had As concentrations between b10 and 68 μg/l. Artesian As groundwater has been used for drinking in the region since the 1960s, and subsequent chronic health problems have been identified (Wang and Huang, 1994). Endemic arsenism was firstly confirmed in the Datong Basin, Shanxi Province, in the early 1990s. It happened after local residents changed their drinking water resources from dug wells (b10 m in depth) to tube wells (between 20 and 40 m in depth) with hand pump in the middle of 1980s. Wang et al. (1998) found that As concentrations ranged between 2.0 and 1300 μg/l in groundwaters. A recent investigation showed that 54.4% of the tested 3083 wells exceeded 50 μg/l (Wang et al., 2003b). High As groundwater was characterized by high ph between 7.09 and 8.71 (mean 8.19, n=15), high HPO 4 2 contents up to mg/l (mean 1.45 mg/l, n=15), low SO 4 2 concentrations generally less than 2.0 mg/l (Wang et al., 2004). The affected groundwater was found in alluvial lacustrine aquifers containing relatively high organic matter (up to 1.0% organic carbon). Arsenic was mainly present as inorganic As (III), accounting for 55 66% of total As, in the reductive aquifers (Guo et al., 2003). In Inner Mongolia, concentrations of As in excess of 50 μg/l have been found in groundwaters from aquifers in Keshenketeng County, the Hetao Basin, and the Tumote Basin (Sun, 1994; Luo et al., 1997). The As-affected areas covered about 3000 km 2 with more than 1 million people being at risk. More than 400,000 people were drinking high As groundwater (N50 µg/l) with 3000 confirmed arsenicosis patients in 776 villages. Arsenic-related diseases have been identified by Ma et al. (1995), including lung, skin and bladder cancer as well as prevalent keratosis and skin-pigmentation problems. Groundwater As in the Keshenketeng County was introduced by arsenopyrite mining, while in the basins of Hetao and Tumote it was generally believed to occur naturally in Holocene alluvial lacustrine aquifers (Ma et al., 1995; Tang et al., 1996; Smedley et al., 2003). In the Huhhot Basin, high As concentrations up to 1500 μg/l were found in the groundwaters, with 60 90% present as As(III), due to highly reducing conditions (Smedley et al., 2003; Wang et al., 2003a). The problem was worst in the lowest-lying parts of the basin. Groundwaters in some shallow dug wells also had relatively high As concentrations (up to 556 μg/l). Saline groundwaters were found in the shallow aquifers in parts of the region as a result of evaporative concentration and many had high F concentrations, although the F did not generally correlate with As (Smedley et al., 2003). In the Hetao Basin, As concentrations ranged between 1.1 and 969 μg/l in shallow groundwaters, with a significant proportion (up to 90%) of the As occurring as As(III) (Tang et al., 1996). Li and Li (1994) and Tang et al. (1996) proposed that groundwater As occurred naturally under reducing conditions. Contrastingly, Zhang et al. (2002) suggested that the As in groundwater of the Hetao Basin was released from higher elevations, where mining had been carried out for a long time, and was then transported from the mining district downgradient. Although the groundwater As problems and the severe health effects have been identified in several provinces from China, many of the As-affected groundwater areas have been poorly documented. Especially in the Hetao Basin, the causes Fig. 1 Location of the Hetao Basin, Inner Mongolia.

3 SCIENCE OF THE TOTAL ENVIRONMENT 393 (2008) of the high As groundwater are not well understood. This paper delineates the hydrogeochemistry of the shallow aquifers in the Hetao Basin of Inner Mongolia. The main objectives are to 1) characterize the hydrogeochemical setting of the area; 2) evaluate the nature and the extent of As enrichment in groundwaters; 3) assess the main geochemical factors controlling As mobilization. 2. Geological and hydrogeological characteristics 2.1. Geology and stratigraphy The basin is one of the Cenozoic rift basins, lying in the western part of Inner Mongolia, on the northern margins of the Yellow River, and on the eastern fringe of the Wuranbuh Desert (Fig. 1). Kidney-shaped, it lies between the Yellow River and the Langshan Mountains to the north, with an area of about 10,000 km 2. The alluvial basin has a gentle SE slope with elevation varying between around 1060 and 1007 m. The Langshan Mountains are mainly composed of a metamorphic complex (slate, gneiss and marble), generally of Jurassic to Cretaceous age, which is folded and fractured (Li and Li, 1994). The basin is also fault-bounded (Zhang et al., 1995). Sedimentary environment, palaeo-geography and lithology were affected by both paleo-climate and tectonic movement. During the Tertiary period, red (or deep brown) sediments occurred in an oxidative environment which accumulated great amounts of salinity (such as gypsum and calcite). In the Quaternary period, due to the closed geologic structure and continuous tectonic subsidence, inland lacustrine sediments with fine clast had locally been deposited and thick Mid-Cenozoic sedimentary formation had developed. The thickness of the sediment in the southeast of the basin ranges from 500 to 1500 m, and in the northwest of the basin from 7000 to 8000 m (Tang et al., 1996). Quaternary sediments are mainly derived from the Langshan Mountains and partly from fluvial deposits of the Yellow River (having developed in the middle Pliocene, Li and Yang, 1991), which are mostly represented by sand, silt and clay. The fluvial sediments originating from the old Yellow river were mainly deposited in the southern part of the basin, with alluvial and pluvial sediments in the northern part. The lacustrine sediments at depths between 10 and 40 m below land surface (BLS), firstly developing in a former lake at the early stage of late Pleistocene (around 120 ka BP), usually occur in the northwest and the east of the basin (Li et al., 2005). Locally, the topmost part of the sedimentological sequence consists of lacustrine sediments of small lagoons separated from the former lake and alluvial deposits. The sediments are heterogeneous both vertically, with two sedimentological sequences from coarse grains to fine grains, and spatially with coarse grains in the south and fine grains in the north Hydrogeology Groundwaters mainly occur in the Quaternary alluvial, alluvial pluvial and fluvial lacustrine aquifers. The sediments are mainly composed of alluvial pluvial sand, sandy silt, lacustrine and fluvial lacustrine sandy silt, silty clay and clay rich in organic matter in the central part of the basin, fluvial sand and fine sand on the banks of rivers, and alluvial sand in the fan areas. The aquifers are generally multi-layered. The aquifer conditions vary from unconfined to leaky-confined in the shallow deposits and confined in the deeper ones. The Quaternary groundwater systems can be basically divided into two classes: the shallow aquifers (upper Pleistocene Holocene alluvial pluvial and alluvial lacustrine sands within a depth of around 100 m) and deeper aquifers (middle Pleistocene lacustrine sands occurring at depths greater than 100 m and separated from the upper water bearing formations by a thick blanket of clay). The shallow aquifers are further subdivided into alluvial pluvial unconfined aquifers and alluvial lacustrine leaky-confined aquifers. The alluvial pluvial aquifers mainly occur in the belt of alluvial fans and on the banks of rivers. The groundwater table ranges from 5 to 20 m. From the top to the front of the fans, the sediments decrease in grain size, with the specific capacity decreasing from 30 m 3 /(h m) to 10 m 3 /(h m). The quality of groundwater is bad at the transition zone between alluvial pluvial aquifers and alluvial lacustrine aquifers, with the presence of considerable dissolved H 2 S and CH 4. High As groundwaters are mainly present in this zone. The alluvial lacustrine aquifers mainly occur at depths between 10 and 100 m, which are usually leaky-confined or semi-confined. The quality of water varies in different parts of the basin, with total dissolved solid (TDS) between b1 and 10 g/l. Groundwater is recharged by vertically infiltrating meteoric water in the basin and laterally penetrating fracture water from marble, slate and gneiss along the mountain front, as well as a little leaked water from river and ditches, and irrigation return flow from farmland. It is discharged mainly via evapotranspiration and artificial abstraction. Groundwater movements are controlled by the surface topography which is very gentle over most of the study area. The general direction of groundwater flow in the northern part is from north to south, while that in the southern part is from south to north, but the flow rate is very slow. The region experiences an arid climate with an average annual precipitation of mm (mainly during July to September) and annual evaporation rates of about mm. Average annual temperatures range from 5.6 to 7.8 C. There are many ditches used for conducting the river water into farmland, some of which were built thousands of years ago. The well-irrigated condition has made the region one of the oldest food-producing areas. Due to the scarcity of efficient management, over-irrigation has been carried out for several decades in the region, which gives rise to 35% of surface soils being pickled. Most of lagoons, lakes and wetlands are over-salted. 3. Sampling and analysis Water samples were collected in August 2006 from 63 boreholes across the Hetao Basin. Three tap water samples were also taken from a small central water supply to investigate the drinking water quality in the villages where the transformation of water

4 Table 1 Chemical compositions of groundwater from shallow aquifers in the Hetao Basin No. Latitude ( E) Longitude ( N) Well depth (m) Temp ( C) Eh (mv) ph Na + K + Ca 2+ Mg b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b Fe Mn (μg/l) As Tot (μg/l) As(III) (μg/l) F Cl HCO 3 CO 3 2 NO 3 SO 4 2 S SCIENCE OF THE TOTAL ENVIRONMENT 393 (2008)

5 b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b SCIENCE OF THE TOTAL ENVIRONMENT 393 (2008)

6 136 SCIENCE OF THE TOTAL ENVIRONMENT 393 (2008) resource has been finished. Parameters, including water temperature, ph, Eh, and sulfide, were measured at the time of groundwater sampling. Alkalinity was performed on the sampling day and measured by the Gran titration method. Water samples were also collected for subsequent laboratory analysis. Water samples for major and trace element analysis were filtered through 0.45 µm Millipore filters on site into 100 ml HNO 3 -washed polyethylene bottles. Those for analysis of trace elements were acidified to ph 1 using 6 M HNO 3. Arsenic(III) in the water samples was separated in the field immediately following filtration with As-speciation cartridges (Metalsoft Center, Highland Park, NJ) (Meng and Wang, 1998; Meng et al., 2002). The cartridge packed with 2.5 g of aluminosilicate adsorbent selectively removed As(V) but did not adsorb As(III) in a ph range 4 to 9. Arsenic (III) was separated from As(V) by passing approximately 50 ml of sample through the cartridge at a flow rate of 60±30 ml/min using a syringe. The concentration of As(V) was determined indirectly as the difference between total As (As Tot ) and As(III) in the filtered sample. Samples for anion analysis were left unacidified. Concentrations of major cations and trace elements were determined by ICP-AES and ICP-MS, respectively. Unacidified aliquots were analyzed for F,Cl,NO 3,NO 2,NH 4 +, HPO 3 2,SO 4 2 by Ion Chromatography with an instrument model DX-120 (Dionex). Concentrations of As and Se were determined by atomic fluorescence spectrometry (AFS) with hydride generation. In most cases, analytical charge imbalances were less than 5%. In addition, three representative boreholes, including two in As-affected areas (BH1 in Langshan and BH3 in Shahai) and one in an As-free area (BH2 in Shanba) (Fig. 1), were drilled to take sediment samples from different depths up to 23 m BLS. These were dried, disaggregated, and milled for XRD analysis. An aliquot was subjected to a mixed acid (HNO 3 HClO 4 HF) digestion before analysis of a range of trace elements by ICP- MS. The same procedure was followed to measure chemical compositions of China National Geochemical Standard Materials (GSS1, GSS8, GSD 9 and GSD 10). Differences between concentrations determined by the above procedure and the certified values were less than 5%. Analysis of total element concentration can determine the degree of trace element enrichment in sediments, while element speciation provides an insight into transformation and mobility of trace element species. Speciation analysis is of major importance in environmental research since it can provide crucial information about the ecotoxicological characteristics (reactivity, bioavailability and toxicity) of contaminants (Van Herreweghe et al., 2003). Sequential chemical extraction was carried out to determine As-speciation in sediments using a procedure developed by Tessier et al. (1979), with few modifications. We used the original wet sediments; drying of the sediment samples was avoided since this would cause erroneous results (Buykx et al., 2000). Sediment As is generally considered to be present in the following forms: exchangeable, carbonate-bound, Fe Mn oxide-bound, organic matter and sulfide-bound, and residual. Exchangeable phase (F1) was extracted with 0.5 M Mg(NO 3 ) 2 at room temperature, carbonate-bound phase (F2) with 1 M NaOAc at room temperature, Fe Mn oxide-bound phase (F3) with 0.08 M NH 2 OH HCl at 96 C, organic matter and sulfide-bound phase (F4) with 0.02 M HNO 3 and 300 ml/l H 2 O 2 at 85 C and then 3.2 M NH 4 OAc in 20 ml/l HNO 3 at room temperature, and mineral matrix phase (F5) with concentrated HNO 3 at 105 C. 4. Results and discussion 4.1. Groundwater chemistry The results for chemical composition of 63 groundwaters sampled from selected wells in the study area are shown in Table 1. All boreholes abstracted groundwater from the shallow aquifer with depths between 5 and 80 m. Wells for groundwater sampling also include one dug well (No. 19 in Table 1) Characteristics of groundwater chemistry The groundwaters are generally of Na Cl HCO 3 type with high salinity in the study area mainly located in the centre of the basin, which is a result of evaporation exacerbated by the effects of irrigation losses. This is demonstrated by the plot of Na + versus Cl (Fig. 2), where the ratio of [Na + ]/[Cl ] is approximately 1:1. Concentrations of Cl reach up to 3900 mg/l, and Na + up to 2660 mg/l. Water temperatures vary between 9.8 and 14.7 C, which has no distinct correlation with well depths (r 2 =32.4%). This likely indicates that the geothermal gradient is not the only factor controlling the temperature of groundwater from shallow aquifers (b80 m). Solar radiation may also affect the temperature of shallow groundwaters. The groundwaters generally reflect the moderate reducing environment of the aquifers, which are lithologically characterized by the finer lacustrine sediments. Redox potentials range from 153 to 83 mv. Nitrate concentrations are mostly below detection limit (b0.01 mg/l) with a median value of b0.01 mg/l (Table 1). There are two groundwater samples with high NO 3 concentrations, which are possibly due to the influences of pollution, mainly from agricultural sources. A few groundwaters contain high NO 2, reaching up to 26.1 mg/l, with 5% of analyzed samples exceeding the WHO guideline value for acute exposure to NO 2 in drinking water (910 μg/l). In Fig. 2 Variation of Na + with Cl - in shallow groundwaters from the Hetao Basin, Inner Mongolia.

7 SCIENCE OF THE TOTAL ENVIRONMENT 393 (2008) comparison with NO 3, the groundwaters contain more SO 2 4, with a median value of 224 mg/l. The lower concentrations of NO 3 suggest that denitrification has been an active process in the aquifers. Under anaerobic conditions, concentrations of total Fe (Fe Tot ) and Mn are high, reaching up to 5.90 mg/l and 1270 μg/ L, respectively. Of the samples collected, 64% exceed the WHO guideline value of 0.3 mg/l Fe, and 14.3% exceed the value of 500 μg/l Mn. 78% of the collected samples contain detectable S 2 (detection limit of 0.01 mg/l), with a maximum of 0.12 mg/l in the As-affected aquifers. The occurrence of S 2, together with relatively low SO 2 4 concentration in the reducing environment, suggests that SO 2 4 reduction has also taken place in the aquifers. This indicates that moderate reducing conditions exist in the As-affected aquifers, which is supported by the negative Eh values observed (Table 1). Groundwater ph is near-neutral to weak alkaline ( ). Concentrations of HCO 3 are very high, reaching up to 1600 mg/l. Carbonate (CO 2 3 ) is also observed in three groundwater samples, with concentrations up to 26.4 mg/l. Dissolved organic matter may be an active source for HCO 3 in groundwater. Bicarbonate (HCO 3 ) presenting in the reducing aquifers from the neighbouring Huhhot Basin is believed to substantially derive from the oxidation of organic matter in the sediments and in solutions (Smedley et al., 2003). One feature of particular interest is the relatively high concentration of dissolved organic matter in the groundwater. A few unfiltered groundwater samples analyzed for total organic carbon (TOC) show that the average concentration is around 650 mg/l. Groundwaters are mostly discoloured and frothy, possibly indicating that humic acid is present. The occurrence of humic acid in As drinking water has been speculatively linked to the development of some chronic health problems, including blackfoot disease in Taiwan (Yu et al., 2002). Concentrations of P are also high in the As-affected groundwaters, up to 3.54 mg/l. Despite these high concentrations, the groundwaters are undersaturated with respect to vivianite and hydroxyapatite. The sources of P in the groundwater are possibly solid organic matter and Fe oxides, which would release phosphate in the reducing conditions (Holdren and Amstrong, 1980). Saturation indices (SI) for selected minerals, calculated by the hydrogeochemical code PHREEQC (Parkhurst and Appelo, 1999) using groundwater chemical data, show that most of the groundwaters are supersaturated with respect to calcite and dolomite. This suggests that the elevated HCO 3 concentrations are not only controlled by the dissolution of the carbonates in the aquifer. It may partly originate from the oxidation of organic matter in the sediment and in solution. In addition, positive SI values are observed for most of the groundwaters with respect to siderite (average 0.24) and barite (average 0.51). The precipitation of siderite and barite is an important hydrogeochemical process in the As-affected aquifers. All of the groundwaters are undersaturated with respect to pyrite and gypsum Spatial distribution of arsenic Arsenic concentrations in the groundwater range between 0.58 and 572 μg/l, with 71% of the selected samples exceeding the Chinese Drinking Water Standard of 10 μg/l (Ministry of Health of PR China, 2006). The regional distribution of As in the groundwaters is shown in Fig. 3. High As groundwaters are mainly localized in five hotspots, including Shuangmiao- Sandaoqiao, Shahai-Manhui, Bainaobao-Langshan, Taerhu and Shengfeng. Dark grey fine sand layers are universally found in these As-affected aquifers, where groundwaters are anaerobic. The sediments were possibly deposited in the estuary of lakes during the later Pleistocene, where water flow became stagnant. It could be proved by the presence of organic matter-enriched clay in the sand layers. In comparison, the As-free aquifers are mainly composed of light-brown fine sands, which were deposited in the fluvial environment. It is found that the shallow groundwaters close to the Yellow River generally contain lower As concentration. Arsenic(III) concentrations are variable in the shallow groundwater, ranging between 0.35 and 456 μg/l (median 29.2 μg/l). As(III) is the major As species. Of the analyzed samples, 83% have ratios of As(III) to the total concentration greater than 60% (Fig. 4). The remaining constituents are dominated by As(V), although methylated As species (MMAA, DMAA) may occur in the organic matter-enriched water. Organic As species were previously detected in groundwaters from the Hetao Basin, and found to be minor constituents, accounting for ~7% of the total concentration (Lin et al., 1999). The prevalence of As(III) over As(V) also indicates that the shallow groundwaters are anaerobic. In the anaerobic groundwaters, the NO 3 concentration is very low. Of the analyzed samples, 61% have NO 3 concentration of Fig. 3 Distribution of total As in the groundwaters from the shallow aquifers of the Hetao Basin, Inner Mongolia.

8 138 SCIENCE OF THE TOTAL ENVIRONMENT 393 (2008) Fig. 4 Histogram of As(III)/As Tot (%) ratios in shallow groundwaters from Hetao Basin. less than 0.01 mg/l, which suggests that the reduction of NO 3 has perhaps occurred throughout the aquifers. However, Fig. 5 A shows that a relatively low correlation exists between As and NO 3 (r 2 = 10.5%). It implies that, besides NO 3, other electron acceptors are available in the system to allow oxidation of organic matter, which is widely believed to be an important contributor to the generation of anoxic conditions in the Asaffected anaerobic aquifers (Stüben et al., 2003; Ahmed et al., 2004; Agusa et al., 2006; Varsányi and Kovács, 2006). Groundwater SO 4 2 would readily act as the electron acceptor, which can be proved by the strong negative correlation (r 2 = 87%) between As and SO 4 2 in the groundwater samples with SI barite b0andno 3 concentration b0.01 mg/l (Fig. 5B). Because the precipitation of barite controls the SO 4 2 concentration in the groundwater as well, a weak negative correlation ( r 2 = 8.1%) exists between As and SO 4 2 in the case of SI barite N0(Fig. 5B). Arsenic exhibits a good positive correlation (r 2 =76.7%) with HCO 3 in the case of SI siderite b0, while a relatively low correlation (r 2 = 16.7%) exists in the case of SI siderite N0(Fig. 5C). Dissolved inorganic carbon is mainly derived from organic matter in the reducing aquifers from the neighbouring Huhhot Basin (Smedley et al., 2003), which may also occur in the reducing Hetao Basin aquifers. Therefore, the strong positive correlation indicates the oxidation of organic matter has possibly caused the increase in HCO 3 concentration in the groundwaters (Ravenscroft et al., 2001; Ahmed et al., 2004), the decrease in redox potential, and consequently the mobilization of As in the aquifers. In the case of SI siderite N0, the precipitation of siderite has decreased HCO 3 concentration and resulted in the poor correlation between As and HCO 3. The reductive dissolution of Fe(III) oxyhydroxides is believed to be a cause of As release and has caused coincident increases in dissolved Fe 2+, which can be speculated from a good correlation between Fe Tot and As Tot (r 2 =66%) in the groundwaters with SI siderite b0 (Fig. 5D). However, the precipitation of Fe 2+ as siderite (FeCO 3 ) has been an important factor, producing a relatively low correlation (r 2 = 30.5%) between As Tot and Fe Tot in the reducing groundwaters with SI siderite N0(Fig. 5D) Sediment geochemistry The chemical compositions of the sediments sampled from three representative boreholes (BH2 in the As-free area, BH1 and BH3 in the As-affected areas) are given in Table 2. The sediments range in texture from fine sand to clay and in colour from brown through grey to black. The SiO 2 concentrations range between 48 and 76 wt.%, with high concentrations in the sand samples and low Fig. 5 Variation of total As (As Tot ) with NO 3,SO 4 2, HCO 3, and Fe Tot in shallow groundwaters from the Hetao Basin.

9 SCIENCE OF THE TOTAL ENVIRONMENT 393 (2008) Table 2 Chemical compositions of sediment samples collected from three representative boreholes Bore Depth Lithology SiO 2 Al 2 O 3 Fe 2 O 3 MgO CaO Na 2 O K 2 O As Ba F Mn Mo P S Sr No (m) % % % % % % % mg/ mg/ mg/ mg/ mg/ mg/ mg/ mg/ Brown clay Brown silty clay Brown clay Dark grey silty clay Yellow silty clay Pale green silty clay Yellow silty clay Brown silty clay Grey silt Yellow silty clay Dark grey fine sand Black fine sand Black fine sand Black fine sand Deep green silt Average Brown silty clay Yellow brown silty clay Yellow clay Brown silt Brown silty clay Yellow fine sand Brown silty clay Yellow silt Deep yellow sand Deep yellow silt Deep yellow silty fine sand 18.5 Deep yellow clayey fine sand 21.0 Pale grey fine sand Average Yellow silty clay Yellowish brown silt Silty clay Grey silty clay Grey clay Yellow silty clay Yellowish brown clay Brown clay Grey silty clay Brown clay Grey silty clay Brown silty clay Grey silty clay Dark grey clay Dark grey clayey silt Dark grey clay Black fine sand Dark grey fine sand Black fine sand Black silty clay Average concentrations in the clay samples. The Fe 2 O 3 concentrations lie in the range wt.%, which are close to the values in the sediments from As-affected areas in Bangladesh (BGS and DPHE, 2002). The high concentrations are found in clay or silty clay, which usually enriches organic C with the origin of lacustrine environment (McArthur et al., 2004). Although total organic C was not determined in the selected samples, the contents are likely to be relatively high as in the sediments from other areas with a similar sedimentary environment (Guo et al., 2003; Smedley et al., 2003). The results of XRD analyses show that fine sands contain considerable amounts of quartz (up to 61.9%), while clay

10 140 SCIENCE OF THE TOTAL ENVIRONMENT 393 (2008) Fig. 6 Vertical profiles of total As and As dissolved by the sequence of extractions that target different solid phases. (A) BH1 with high As groundwater at depths between 14 and 23.5 m; (B) BH2 with low As groundwater; (C) BH3 with high As groundwater between 18.4 and 22.9 m. sediments are mainly composed of clay minerals (up to 59.6%). All sediments exhibit various contents of calcite (3.6%~25.0%) and dolomite (b1.0% 10%). Siderite and pyrite are found in several silty clay sediments, indicating that they occur in a strongly reductive environment. Sand sediments from BH1 and BH3 have a higher content of clay minerals (average 15.0%), in comparison with those from BH2 (average 13.0%). Although a big difference in colour has been observed between aquifer fine sands from BH1 (or BH3) (dark grey or black) and those from BH2 (deep yellow), no significant variations are found in their mineralogical compositions. Concentrations of the sediment As range from mg/ (average of 18.9 mg/). Although they are not unusually high compared with average values found in sediments from elsewhere, the As distributions in the sediments from the regions with high As groundwater are different from those with low As groundwater (Table 2; Fig. 6). The As concentrations from the As-affected areas lie in the range mg/ (average of 20.8 mg/), which are a little higher in comparison with those in the As-free area (average of 13.6 mg/). Therefore, the As contents of the sediment may be a factor affecting the As concentrations in the groundwater since the sediment is the major As resource in the aquifer. Gao (1999) observed that As concentrations were of the same order ( mg/) in the sediments from Haiziyan in the Hetao Basin. The As is mildly-strongly correlated with total Fe (r 2 =60%) and total Mn (r 2 = 55%). The sample with the highest As concentration (73.3 mg/) is a black clay with the highest measured Fe 2 O 3 concentration (6.75 wt.%) in BH3. A quite weak correlation exists between total As and total S (r 2 =8%). This suggests that the As is associated with Fe oxides (or Mn oxides), rather than sulfides in the sediments. It is well known that Fe oxides (such as goethite: Sun and Doner, 1998; ferrihydrite: Raven et al., 1998; hematite: Guo et al., 2007) readily scavenge great amounts of As. The redox condition of the fine sand aquifers in BH1 and BH3 is distinct from that in BH2. The black fine sand together with organic matter is found in the aquifers from BH1 and BH3, which implies that it occurs in a moderate reductive environment, while the yellow deep yellow sand is observed in BH2 of weak oxidative environment. High As groundwaters also occur in the reductive alluvial aquifer comprising of fine sand, relatively higher percentage matrix (silt and clay) and organic matter in Bangladesh (Ravenscroft et al., 2001; Ahmed et al., 2004). Although sequential leaching methods are based on the assumption that selected solvents attack and dissolve particular forms of trace elements, which are specific mineralogical occurrences or operationally defined, the sequential leaching of samples by means of chemical extractants is often used for identification and evaluation of the availability of trace metals in sediment samples (Carbonell-Barrachina et al., 1999; Keon et al., 2001; Huggins et al., 2002; Van Herreweghe et al., 2003; Kelderman and Osman, 2007). Chemically active As forms (exchangeable, bound to carbonates, bound to Fe Mn oxides, bound to insoluble organic and sulfides) were investigated by the method of sequential extraction. The vertical profiles of As dissolved by the sequence of extractions are shown in Fig. 6. The active forms exhibit a good correlation with total As (r 2 =0.67), and generally increase with the increase in depths between 0 and ~18 m in the sediments from those three boreholes (Fig. 6). The peak appears at depths of 21.5, 17.5 and 16.3 m in BH1, BH2 and BH3, respectively. It is generally

11 SCIENCE OF THE TOTAL ENVIRONMENT 393 (2008) coincident with the location of the aquifers where high As wells are located in the As-affected areas. However, high As groundwater does not occur at BH2 although 2960 μg/ of chemically active As is observed at the depth of 17.5 m in the fine sand aquifer. The possible reason is that the active As forms keep relatively stable in the weakly oxidative aquifer, which can be proved by the yellow deep yellow colour of the aquifer sediments. This speculation is also supported by the higher relative concentrations of As bound to Fe Mn oxides in the aquifer sands from BH2 (74%) in comparison with those from BH1 and BH3 (around 67%). Of the chemically active fractions, 60 86% of As is bound to Fe Mn oxides (F3) in the sediments. Relatively higher percentages are observed in the fine sands from BH2 due to the higher redox potential. There is still a certain amount of Fe Mn oxides and oxyhydroxides in the sediments from BH1 and BH3 that may adsorb As, although most of the Fe and Mn possibly exists in a low valence state under reductive conditions. With regard to BH1, the maximum bound As of 2500 μg/ is observed in the fine sand, in which the high As wells with hand pump are mainly located in. The clay and the silt (silty sand) from BH3 contain much As bound to Fe Mn oxides, up to 3500 μg/. When the redox potential decreases and/or ph increases, the bound As will be released into groundwaters (Feijtel et al., 1998; Keon et al., 2001; Wang and Guo, 2004). Although the F3 fraction is operationally defined for the As bound to both Fe oxides and Mn oxides, the release of As from this fraction may be sensitive to the concentration of Fe oxides and the concentration of Mn oxides in the reductive environment. It has long been known that the reduction of Mn oxides is thermodynamically more favourable than is the reduction of FeOOH (Edmonds, 1986). The As originally adsorbed to Mn oxides in the F3 fraction is firstly released during reduction. The released As would be resorbed to FeOOH rather than be transported into groundwater (Stüben et al., 2003). Exchangeable As (F1), the As bound to carbonates (F2) and the As bound to insoluble organic and sulfides (F4) account for 4 20% of the chemically active form. The exchangeable As is vulnerable to ionic activity or ph, and could readily be mobilized by an increase in ionic strength of the aqueous solution (Keon et al., 2001). This fraction is assumed to consist of easily exchangeable, outer-sphere complexes of As (Wenzel et al., 2001). A big difference in the exchangeable As is observed between the sediments from the As-affected areas and those from the As-free area. The average exchangeable As in the sediments from BH1 is 120 μg/, while from BH2 it is only 60 μg/. Therefore, the exchangeable As may directly reflect the groundwater As in the aquifer, although a relatively low contribution of the fraction has been observed. In comparison with F1, the contribution of F2 is much greater in the high ph environment (ph ). Under those weakly alkaline conditions, this binding fraction is quite stable. Only when the carbonate minerals are dissolved will the As be released into groundwaters. For example, a lowering of sediment ph may give rise to mobilisation of As bound to carbonates (Calmano et al., 1993). A proportionally higher contribution of the organic matter and sulfide-bound As (F4) is observed in the reductive sediments from BH1 and BH3 (~24.5%), in comparison with that in the sediments from BH2 (~13.3%). Moore et al. (1988), Mok and Wai (1994) and Guo et al. (1997) also found that the As has a high affinity for sulphidic and organic substances in sediments under reducing conditions Implications for As mobilization The chemical characteristics of high As groundwaters and the regional distribution of dissolved As in the Hetao Basin indicate that mobilization in groundwater takes place under moderate reducing conditions, where Fe and Mn have been reduced to Fe (II) and Mn(II) forms, and the reductions of NO 3 and SO 4 2 have occurred. Investigations of the sediment stratigraphy and lithology across the basin show that lacustrine layers mainly composed of argillaceous materials widely occur in the uppermost subsurface sediments (b8 m). This geological setting will have restricted diffusion of atmospheric O 2 into the aquifers. Solid organic matter is also likely to be concentrated in the finegrained lacustrine sediments. In the young unconsolidated sediments, the organic matter should be fresh and may be a dominant source of electrons, which is thought to be a prime cause of the reducing conditions observed in the As-affected aquifers. Besides, the dissolved organic matter is present with relatively high concentrations in the groundwater, oxidation of which is another of the main factors in the initiation of reducing conditions (Bauer and Blodau, 2006). High concentrations of dissolved organic matter have also been recorded in a number of other As-affected areas (Varsányi and Kovács, 2006; Yu et al., 2002; Guo et al., 2003). Further impacts of dissolved humic substances have been proposed. Azcue and Nriagu (1995) reported that the possibility exists for binding of As to humic acids. The presence of negatively charged DOC also enhances the desorption of As from binding sites through electrostatic effects (Bauer and Blodau, 2006). The competitive effects of other anions may also be important factors. It is well known that phosphate competes strongly with As (especially arsenate) for available exchange sites on Fe oxides (Roy et al., 1986). Because concentrations of P are high in most of the groundwaters, this may be an additional factor in promoting As mobilization. High P concentrations have also been observed in groundwaters from Shanyin, China (Guo et al., 2003), Bangladesh (BGS and DPHE, 2002), and Argentina (Smedley et al., 2002). Although concentrations of HCO 3 are very high, it may play a minimal role in affecting the concentrations of As in groundwater (McArthur et al., 2004). The direct source of the As in the groundwater is believed to be the chemically active As fraction in the sediments. The weakly bound As is relatively labile, and may be readily released under favourable hydrogeochemical conditions. For example, an increase of ionic strength would lead to mobilization of exchangeable As. As noted above, the chemically active As is mainly bound to Fe Mn oxides, which suggests that Fe Mn oxides are a major potential source. Potential mechanisms for As release include reductive dissolution of Fe Mn oxides under reducing conditions (Nickson et al., 1998; Islam et al., 2004). The groundwater data, indicating that As concentrations increase with the increase in dissolved Fe(II) concentrations in the groundwaters with SI siderite b0, strongly support this implication. However, it is observed that a high proportion of dissolved As exists as As(III) in the high As groundwaters, suggesting that