Applied Geochemistry 41 (2014) 196–217

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Applied Geochemistry

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Review A review of high arsenic groundwater in Mainland and , : Distribution, characteristics and geochemical processes ⇑ Huaming Guo a,b, , Dongguang Wen c, Zeyun Liu b, Yongfeng Jia a,b, Qi Guo a,b a State Key Laboratory of Biogeology and Environmental Geology, China University of Geosciences, 100083, PR China b School of Water Resources and Environment, China University of Geosciences, Beijing 100083, PR China c China Geological Survey, 24 Huangsi Dajie, Xicheng , Beijing 100037, PR China article info abstract

Article history: China is a typical high-As region, where 20 provinces have high As groundwaters among 34 provinces. Received 15 October 2013 These groundwaters usually occur in both arid–semiarid inland basins and river deltas. In the inland Accepted 23 December 2013 basins, mainly distributed in the northwest of China, shallow groundwaters usually have high As concen- Available online 28 December 2013 trations in alluvial lacustrine or lacustrine sediment aquifers, while high As groundwater mainly occurs in Editorial handling by M. Kersten fluvial–marine sedimentary aquifers in the river deltas, which have been affected by transgression. In both the inland basins and the river deltas, high As groundwaters, mainly occurring in reducing condi- tions, are characterized by high Fe and Mn concentrations, high pH and HCO3 concentration, and rela- 2 tively low NO3 and SO4 concentrations. Although As contents are well correlated to Fe/Mn contents in the aquifer sediments, groundwater As concentrations are generally independent of sediment As con- tents. Redox processes, microbe-related reduction, and desorption processes are the major geochemical processes for As enrichment in groundwaters. In reducing conditions, both reductive dissolution of Fe oxides and reductive desorption of As are believed to result in As mobilization, which would be catalyzed by indigenous microbes. Although decomposition of the low-molecular weight organic matter during microbe metabolization would also release the colloid-bound As into groundwater, the cycling of colloi- dal As still needs to be further investigated during redox processes. Besides, high pH and high HCO3 lead to As desorption from adsorption sites in the aquifer systems. However, the contribution of competitive desorption to high As concentrations is still unknown and remains to be discovered, relative to reductive dissolution of Fe oxides, especially in the inland basins. Ó 2014 Elsevier Ltd. All rights reserved.

Contents

1. Introduction ...... 197 2. Distribution of high As groundwater ...... 197 2.1. High As groundwater in arid–semiarid inland basins...... 198 2.1.1. The basin and the Huhhot basin of ...... 198 2.1.2. The basin, the and the basins of province ...... 200 2.1.3. The Songnen basin of province ...... 200 2.1.4. The basin of province...... 201 2.1.5. The Dzungaria basin of province ...... 201 2.2. High As groundwater in river deltas ...... 201 2.2.1. The river delta of and ...... 201 2.2.2. The delta of province ...... 201 2.2.3. The delta of province ...... 201 2.2.4. The Chianan alluvial plain of Taiwan ...... 201 2.2.5. The Lanyang alluvial plain of Taiwan ...... 202 2.3. Others (mountain areas) ...... 202 3. Hydrogeology of the aquifers ...... 202

⇑ 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 2014 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.apgeochem.2013.12.016 H. Guo et al. / Applied Geochemistry 41 (2014) 196–217 197

3.1. Sedimentary aquifers ...... 202 3.1.1. Inland basins...... 202 3.1.2. River deltas ...... 204 3.2. Geothermal related aquifers ...... 205 4. Aqueous chemistry of high As groundwater ...... 205 4.1. Major ions ...... 205 4.2. Redox sensitive species...... 207 4.3. Arsenic species ...... 208 4.4. Colloid particles...... 209 4.5. Dynamic variation in As concentration ...... 209 5. Mobilization of As in aquifers ...... 210 5.1. Sources of arsenic ...... 210 5.2. Redox processes ...... 211 5.3. Roles of microbes in As mobilization...... 212 5.4. Competitive desorption...... 212 6. Conclusive remarks ...... 213 Acknowledgements ...... 213 References ...... 213

1. Introduction (Harvey et al., 2002; Ng et al., 2003; van Geen et al., 2003; McArthur et al., 2004; Guo et al., 2008a). Hydrogeological and Arsenic is a ubiquitous element in the earth’s crust, which has biogeochemical studies showed that source of dissolved organic notoriously been known as a toxicant and carcinogen for the gen- carbon, microbial diversity, sedimentation sequences and ground- eral population. It has been ranked among the top 20 most hazard- water hydraulics are the major contributors for spatial and tempo- ous, high priority substances by the Agency for Toxic Substances ral variation in As concentrations (Harvey et al., 2002; van Geen and Disease Registry (ATSDR, 2005), which can cause acute and et al., 2003; McArthur et al., 2004, 2011; Guo et al., 2012, 2013b; chronic health effects on human (Gray et al., 1989; WHO, 1996; Fendorf et al., 2010). Saha, 2003). Acute poisoning symptoms include gastrointestinal Among these high As groundwater countries, China is one of the discomfort, vomiting, coma and even death, which usually occur largest countries, where high As groundwaters have been found in within 30 min of ingestion, while the chronic effects commonly in- both inland basins experiencing an arid/semiarid continental cli- clude skin diseases (pigmentation, dermal hyperkeratosis, and skin mate, and river deltas experiencing a humid tropical climate cancer), many other cardiovascular, neurological, hematological, (Guo et al., 2008a; Chen, 1998). In the Mainland of China, high renal and respiratory diseases, skin cancer, and other internal can- As groundwaters (>10 lg/L) have been found in 19 provinces. cers (Morton and Dunette, 1994). These provinces include , Beijing, , Guangdong, , Elevated As levels of aqueous environment have resulted from , , Inner Mongolia, Jilin, , , Ningxia, both natural processes and anthropogenic activities (Bissen and , Shandong, Shanxi, , , Xinjiang, and Frimmel, 2003). Most As problems are the result of mobilization (Jin et al., 2003). Estimation based on a statistical risk model shows under natural conditions, including weathering reactions, biologi- that 19.6 million residents are at risk of being affected by the cal activities and volcanic emissions (Smedley and Kinniburgh, consumption of high-As groundwater throughout China 2002). Arsenic concentration in geothermal water is up to 50 mg/ (Rodríguez-Lado et al., 2013). The number of residents at risk L(Ellis and Mahon, 1977), while natural groundwaters in As-rich may be underestimated since the model mainly considers the allu- provinces have high As concentration of 1500 lg/L (Guo et al., vial sediment aquifers. According, the results must be confirmed 2008a). Ingestion of high As drinking water is a major pathway with additional field investigations. for As to enter the human body, and therefore poses the significant This paper summarizes distribution and chemical characteris- treat to human health. High As concentrations have widely been tics of high As groundwater, hydrogeological settings of high As found in potable groundwaters, which have received much concern groundwater aquifers, and systematically reviews geochemical from both governmental and scientific levels (Guo et al., 2007a). In processes controlling As distribution in typical areas of China. order to protect public health, the World Health Organization has set a provisional guideline limit of 10 lg/L for As in drinking water (WHO, 1996), which has been subsequently adopted by the Euro- 2. Distribution of high As groundwater pean Union (European Commission, 1998), the United States (EPA Office of Groundwater and Drinking Water, 2002), and China High As groundwaters (>10 lg/L) have been widely found in (Ministry of Health of PR China, 2006). However, although China is China (Fig. 1). Generally, there are two types of areas where high seeking to reduce its limit in line with the WHO guideline value, As groundwater naturally occurs. One is the arid–semiarid inland the guideline is still set to the 50 lg/L in rural areas because of lack basins. The other is the river deltas. The former mainly includes of adequate testing facilities for lower concentrations and high cost the Yinchuan basin, the Hetao basin, the Huhhot basin, the Da- of water treatment for As removal. tong basin, the Yuncheng basin, the Songnen basin, the Guide South and Southeast Asia is a typical high-As region, where the basin and the Dzungaria basin. The Yinchuan basin (7300 km2), occurrence of As in the alluvial/lacustrine aquifers of many inland the Hetao basin (10,000 km2), the Huhhot basin (4800 km2), basins and deltaic systems has become a well-known tragedy the Datong basin (7440 km2), and the Yuncheng basin (Winkel et al., 2008). Hundreds of millions of people are suffering (4950 km2) lies along the Yellow river from the west to the east. from chronic As poisoning in Bangladesh, India, China, Pakistan, The Songnen basin is located in the west of Jilin province, and Nepal, Cambodia, and Vietnam (Bissen and Frimmel, 2003; the Dzungaria basin in the north of Xinjiang province. The delta Smedley and Kinniburgh, 2002). The groundwater As is of geogenic areas mainly include the Yangtze river delta, the Yellow river origin and considerably patchy on a local scale or a regional scale delta, and the . 198 H. Guo et al. / Applied Geochemistry 41 (2014) 196–217

Fig. 1. Distribution of high As groundwater in China (a) groundwater As concentration >50 lg/L; (b) groundwater As concentration >10 lg/L). The numbers show the locations of As-affected areas. Xijiang: 1. The Dzungaria basin; 2. Luntai, Bohu, and Weili counties; 3. Awati and Bachu counties; 4. Shule county; 5. county; 6. county. Qinghai: 7. Nangqian county; 8. Gande, Maqin and Dari counties; 9. Guide, Zeku, Tonren, Xunhua, Hualong, Jianzha, Gonghe, Huangyuan, Haiyan, Pingan, Huzhu, Datong, Mengyuan, and Tianzhu counties. Inner Mongolia: 10. Alashanzuoqi; 11. The Hetao basin, Wulatehouqi, Wulatezhongqi; 12. The Huhhot basin; 13. Shuniteyouqi; 14. Zhenglanqi, Weichang county, Naimanqi; 15. Xinbaerhuyouqi and Ewenkezuzizhiqi; Jilin: 16. The Songnen basin. : 17. Fuyu, Lindian, Anda, , and Beian counties, 18. Jidong, Linkou, and Ningan counties. Beijing: 19. Tongzhou and Shunyi. Shandong: 20. The alluvial plain of the Yellow river of Shandong. Henan: 21. The alluvial plain of the Yellow river, Henan. Anhui and Jiangsu: 22. The alluvial plain. Jiangsu and Shanghai: 23. The Yangtze river delta. : 24. The delta. Hubei: 25. The Jianghan alluvial plain of the Yangtze river. Shanxi: 26. The Datong basin; 27. The Taiyuan basin; 28. The Yuncheng basin. Shaanxi: 29. The Weihe alluvial plain; 30. Yan’an and Yulin counties. Ningxia: 31. The Yinchuan basin. Gansu: 32. Huanxian county; 33. Weixian, Zhouqu, and counties. Sichuan: 34. Shongpan, Xiaojin, Jinchuan, and Maerkang counties; 35. Luding county. Yunnan: 36. , Lianghe, Yunlong, Eryuan, Binchuan, Mouding, Chuxiong, Nanjian, Changning counties; 37. Gengma and Zhenyuan counties; 38. Mengla, , Menghai, and Simao counties. Guandong: 39. Zhijin, Wuhua, Xingning counties; 40. The Pearl river delta. Taiwan: 41. The Choushui alluvial plain and Chianan alluvial plain; 42. The Lanyang alluvial plain.

2.1. High As groundwater in arid–semiarid inland basins et al., 1993). Saline groundwaters have been found in the shallow aquifers in parts of the region as a result of evaporative concentra- 2.1.1. The Hetao basin and the Huhhot basin of Inner Mongolia tion and many have high F concentrations, although the F does In Inner Mongolia, concentrations of As in excess of 50 lg/L not generally correlate with As (Smedley et al., 2003). Aquifer sed- have been found in groundwaters from aquifers in Keshenketeng iments hosting high As groundwaters are usually deep grey in col- County, the Hetao Basin, and the Huhhot Basin (Sun, 1994; Luo or, while sediments with low As groundwaters are yellow (Zhang et al., 1997). The As-affected areas cover about 3000 km2 with et al., 2000). more than 1 million people being at risk. More than 400,000 people The Hetao basin is located to the west of the Huhholt basin, were drinking high As groundwater (>50 lg/L) with 3,000 con- which is a typical sedimentary basin hosting high As groundwater. firmed arsenicosis patients in 776 villages. Arsenic-related diseases Although the Yellow river lies to the south of the basin and its have been identified by Ma et al. (1995), including lung, skin and water is used for farmland irrigation, groundwater is the major bladder cancer, as well as prevalent keratosis and skin-pigmenta- source of drinking water. Before 1980s, every village had 2–4 dug tion problems. Groundwater As in the Keshenketeng County was wells (3–5 m in depth) for producing drinking water. Since the introduced by arsenopyrite mining, while in the basins of Hetao water usually has high total dissolved solid (TDS)>1000 mg/L, local and Huhhot it has been generally believed to occur naturally in la- residents have changed their drinking water resources from dug ter Pleistocene–Holocene alluvial–lacustrine aquifers (Ma et al., wells to tube wells (usually 15–30 m in depth) with hand pump 1995; Tang et al., 1996; Smedley et al., 2003; Guo et al., 2008a). since the early of 1980s (Tang et al., 1996). These groundwaters The Huhhot Basin is located in the center of Inner Mongolia, generally have lower TDS relative to dug well waters, while contain where arsenicosis was firstly recognized in 1990 (Fan et al., high As concentration (>50 lg/L). The transformation of drinking 1993). High As concentrations up to 1860 lg/L have been found water resources has led to endemic As poisoning since 1990 in groundwaters from alluvial–lacustrine plains of the Yellow river (Gao, 1990). High As groundwater is widespread in the basin, and the Black river (Table 1). Arsenic(III) accounts for 60–90% of to- which poses a significant health risk to more than one million res- tal As, due to highly reducing conditions (Smedley et al., 2003; idents (Guo et al., 2008a). It was reported that the prevalence of Wang et al., 2003a). The problem was the worst in the lowest-lying endemic arsenism is up to 24.79% (Jin et al., 2003). Arsenic concen- parts of the basin. Groundwaters in some shallow dug wells trations range between 1.1 and 969 lg/L in shallow groundwaters (<10.0 m) also have relatively high As concentrations (up to (Table 1), with a significant proportion (up to 90%) of the As occur- 556 lg/L). In some deep wells with depths between 90 and ring as As(III) (Tang et al., 1996). No organic As was detected (Guo 400 m, groundwaters have As concentration between <10 and et al., 2008a). Li and Li (1994), Tang et al. (1996), Guo et al. (2008a),

360 lg/L with relatively high H2S and CH4 concentrations (Fan and Guo et al. (2011a) proposed that groundwater As occurs Table 1 Overview of high As groundwater in China.

Region Area Population Concentration Climate Aquifer characteristics Water chemistry Depth of References Map (km 2) exposed a range (lg/L) aquifers (m) ID c Inland basins The Huhhot basin 1000 345,000 <1–1860 Arid– Mesozoic–Cenozoic Collapsed basin with thick Reducing condition, neutral–alkaline 4–400 Dou et al., 2001; Jin 12 semiarid fluvial–lacustrine aquifers around the pH, high F , Mn, Fe, humic acid, and TDS et al., 2003; Fan conjunction of Yellow River and Hei River et al., 1993; Smedley et al., 2003 The Hetao basin 6100 595,000 <1–969 Arid– Mesozoic–Cenozoic Collapsed basin with thick Reducing condition, neutral–alkaline 10–30 Tang et al., 1996; 11 semiarid alluvial–lacustrine aquifers neighboring As- pH, high F, Mn, Fe, humic acid, As(III) Gao, 1990; Guo containing deposit in the mountains and TDS, low NO3 et al., 2008a ; Zhang et al., 2002 The Datong basin 1350 31,000 <1.0–1300 Arid– Cenozoic basin of the Shanxi rift system, alluvial– Reducing condition, neutral–alkaline 20–50 Guo et al., 2003; 26 2 Wang et al., 1998; semiarid lacustrine aquifers in the right side of Sanggan pH, high F , HPO4 , Mn, Fe, humic acid, river and along the Huangshui River 2 Wang et al., 2004a As(III) and TDS, low SO4 and NO 3 The Taiyuan basin 800 23,600 0.1–116 Arid– Cenozoic basin of the Shanxi rift system, Reducing, high pH, HCO3 , and low 50–200 Zhang and Guo, 27 2 2007; Wang et al., semiarid Holocene and upper–middle Pleistocene aquifer, SO4 =Cl mostly fine-grained aeolian loess interlayered 2007b; Guo et al.,

with fluvial and lacustrine deposits 2007 196–217 (2014) 41 Geochemistry Applied / al. et Guo H. b 156,000 0.1–500 Arid– Loess, fluvial–lacustrine aquifers, in Sushui river 2 20–300 Currell et al., 2011; 28 The Yuncheng basin – High pH, F ,NO3 and SO4 , oxidizing semiarid catchment condition Gao, 2005 The Songnen basin 1800 3800 360 Semiarid Mesozoic–Cenozoic Collapsed basin with thick Weakly reducing condition, with high 50–80 Liu et al., 2002; Lu 16 þ fluvial–lacustrine aquifers Fe, Mn, NH4 , and pH, high F in et al., 2004; Tang unconfined aquifers, and high As in et al., 2010 confined/semi-confined aquifers þ 10–40 Zhang et al., 2010a; 31 The Yinchuan basin, 7900 36,500 200 Arid– Alluvial–lacustrine aquifers in the left side of the Reducing condition, with high, NH4 , Ningxia semiarid Yellow river 3 Han et al., 2010; Tan and PO4 and pH et al., 2006 The Dzungaria basin, >2300 34,000 70–850 Arid Artesian aquifers with fine grain size sediments Reducing condition, high TDS, F and >100 Wang et al., 1 Xinjiang and intercalated lacustrine sediment in 2 2002;Wang and SO4 Quaternary alluvial plain Huang, 1994; Hong, 1983; Luo et al., 2006a Beijing – 3700 143 Arid– Flood alluvial plain of the Chaobei river High pH and Fe 100–150 Pang et al., 2003 19 semiarid River deltas (floodplains) The Qiantang river delta, – 311,820 125 Monsoon Deep groundwater in bay (possibly in – 100–150 Zhou et al., 2003, 24 Zhejiang marine sediments) 2004; Shen et al., 2004 The Yangtze river delta, – >1800 324 Monsoon Deep groundwater in silty sand aquifers (marine High Fe, Mn, and TOC in reducing 100–300 Gu and Zhen, 1995; 23 Jiangsu/ Shanghai sediments) conditions Yu, 1999; Chen, 1998 The Jianghan alluvial Plain – 3500 175 Monsoon The Jianghan alluvial plain of the Yangtze river Shallow groundwater in reducing 20 Wang and Zhao, 25 of the Yangtze river, conditions 2007; Chen et al., Hubei 2007 and TDS 15–40 Yuan et al., 2009; Yu 21 The alluvial plain of the – 6400 258 Arid– Alluvial plain of the Yellow river in low-lying High F et al., 2009; Wei Yellow river, Henan semiarid areas et al., 2005 The alluvial plain of the – – 0.22–440 Monsoon Alluvial plain in the downstream of the Yellow ––Pang et al., 2007; Li 20 Yellow river, Shandong river et al., 2012; Zhang et al., 2010b The Huai river alluvial – 1000 70–1150 Monsoon The alluvial plain of the Huai river Reducing 6.5–50. Li et al., 2006 22 plain, Anhui The Pearl river delta, –– 161 Monsoon The Pearl river delta High TDS, reducing conditions 40 Huang et al., 2010; 40 Wang et al., 2012 199 (continued on next page) 200 H. Guo et al. / Applied Geochemistry 41 (2014) 196–217

naturally under reducing conditions. In contrast, Zhang et al.

c (2002) suggested that the As in groundwater of the Hetao Basin 36, 37, 38 34, 35 32, 33 ID 7, 8, 9 41, 42 should be released from high elevations, where mining had been carried out for a long time, and then transported from the mining district down-gradient.

2.1.2. The Datong basin, the Taiyuan and the Yuncheng basins of Shanxi province Shanxin province is severely affected by high As groundwater. Wang and Kong, 2006; Luo et al., 2006b Qin and Li, 2008; Deng et al., 2004; Li and Ni, 2005 Bai et al., 2007; Xu et al., 2007 References Map Jin et al., 2003; An et al., 2006; Zhang et al., 2009 Wang et al., 2007b; Liu et al., 2003; Yang et al., 2003 Endemic arsenism was firstly confirmed in 1994 in the Datong ba- sin (Cheng et al., 1994; Jin et al., 2003). High As groundwater was firstly reported in 1997, with As concentrations between 1 and 660 lg/L (Wang, 1997). Recently, groundwater As data obtained 250 aquifers (m) show that tube well waters in the Taiyuan basin and the Yuncheng basin also contain As concentration >50 lg/L (Wang et al., 2007a; Currell et al., 2011). The Datong basin is located in the northern part of the Shanxi Province. In this basin, every village had 3–5 dug wells for drinking water supply before the middle of 1980s. Those wells had depths between 8 and 10 m. Since 1985, groundwater level has declined, which caused no water in those dug wells during dry seasons. Local residents changed their drinking water resources from dug wells to tube wells (between 20 and 40 m in depth) with hand pump. Wang

in shallow groundwater 30–40 et al. (1998) found that As concentrations range between 2.0 and 1300 lg/L in groundwaters (Table 1). A recent investigation shows that 54.4% of the tested 3083 wells have As concentration exceed- –– –– High F – Mostly > 100 High DOC, strong reducing conditions ing 50 lg/L (Wang et al., 2003b). High As groundwater is character- 2 ized by high pH between 7.1 and 8.7 (mean 8.2, n = 15), high HPO4 2 contents up to 12.7 mg/L (mean 1.45 mg/L, n = 15), low SO4 con- centrations generally less than 2.0 mg/L (Wang et al., 2004a). The affected groundwater has been found in alluvial–lacustrine aqui- fers containing relatively high organic matter (up to 1.0% organic carbon). Arsenic is mainly present as inorganic As(III), accounting for 55–66% of total As, in the reducing aquifers (Guo et al., 2003). The Taiyuan basin, lying in the center of the Shanxi province, hosts high As groundwater in the sedimentary aquifers. High As groundwaters are mainly distributed along the Ciyao river and the Wenyu river in the flat plain with an area around 800 km2. Pop- ulation exposed to high As groundwater (>50 lg/L) was about 23,600 (Wang et al., 2004b). Depths of wells producing high As groundwater mainly range between 50 and 200 m. Arsenic concen- Hot spring waters in faultrocks systems in of Tengchong volcanic areas,catchment Springs of in the Lantsang river, andbearing in aquifers the arsenic- valleys along the Springs from arsenic-bearing aquifersand in shallow , groundwater neighboring Maowusu desert in Huanxian. Spring water, shallow groundwater, andwater in thermal the valleys along the upper Yellow River Choushui alluvial plain and Chianan alluvialin plain the southwest, Lanyang alluvialnortheast plain in the tration has the range from 0.1 to 116 lg/L (Zhang and Guo, 2007).

g/L. High As groundwater areas are generally consistent to the regions l with groundwater levels less than 4.0 m below land surface (Guo tropical semiarid Climate Aquifer characteristics Water chemistry Depth of semiarid Monsoon et al., 2007c; Zhang and Guo, 2007). Being located in the southern part of the Shanxi province, the Yuncheng basin has also been found to host high As groundwater. g/L)

l Population exposed to high As groundwater (>50 lg/L) was around 156,000. High As groundwaters were mainly observed in Yanhu 887 Sub- 287 Monsoon Spring water from arsenic-bearing aquifers in the 2160 Arid– 1070 Arid– Concentration range ( district and Yongji city (Jin et al., 2003), neighboring Yanhu lake and Wuxing lake, respectively. Groundwater chemistry is generally 2 a characterized by high F ,NO3 and SO4 (Currell et al., 2011). Ar- senic concentration ranges between 0.1 and 500 lg/L (Table 1).

43,846 High As groundwater mainly occurs in the northern Sushui River Population exposed basin. . ) 2 (km Fig. 1 2.1.3. The Songnen basin of Jilin province High As groundwater has been found in the southwest of the Songnen basin of Jilin province in the late 19th century (Liu et al., 2002). In this area, arsenicosis was first reported in 1995

) (Wang et al., 2006). Around 1964, residents changed their drinking water resources from shallow wells (4–10 m) (F concentrations >1.0 mg/L) to deep wells (20–40 m) to avoid F endemic poisoning. continued ( However, the deep wells mostly have high As concentration

The population in the villagesNo drinking related water data. with AsThe concentrations numbers >50 are shown in >10 g/L (Lu et al., 2000), which is the major cause of endemic Yunnan – Sichuan – 12,000 Gansu – 22,500 Others (mountain areas) Qinghai – 12,219 RegionTaiwan Area – 240,000 3590 Tropical l c a b

Table 1 As diseases (Bian et al., 2012). In Tongyu county and city, H. Guo et al. / Applied Geochemistry 41 (2014) 196–217 201 residents have severely been affected by high As groundwater (Lu (Chen, 1998; Gu and Zhen, 1995). Arsenic concentration is posi- et al., 2004). Around 3800 residents were exposed to high As tively correlated with Fe(II) concentration. Near Nanjing city, As groundwater (>50 lg/L) (Table 1). The basin is a Mesozoic–Ceno- concentration of groundwater near the Yangtze river at the dis- zoic Collapsed basin with thick fluvial–lacustrine aquifers. High tance <5 km is generally greater than that far away from the river As groundwater mainly occurs in confined aquifers at depths be- (Yu, 1999). Arsenic concentration in shallow groundwater is gener- tween 50 and 80 m below land surface (Tang et al., 2010). Arsenic ally low (<4.0 lg/L). concentrations range between <1.0 and 360 lg/L (Tong et al., 2004). 2.2.2. The Yellow river delta of Shandong province The Yellow river is considered as a non-perennial river in the 2.1.4. The Yinchuan basin of Ningxia province downstream of Shandong province, where high As groundwater The Yinchuan basin is located to the southwest of the Hetao ba- has been found. Shallow groundwater has As concentrations be- sin, where the Yellow river passes through. Arsenicosis was firstly tween 0.22 and 235 lg/L (Pang et al., 2007). High As groundwater recognized in 1995 in the basin (Hu et al., 1999). Arsenic-affected has mainly been hosted in the Weibei group of shallow aquifers, areas generally exist as two strips in the flat plain in the north of which is distributed in Xiawa, , Shengtuo, Chenguan, Wopu, the basin, being parallel to the Yellow river (including Pingluo and Houzhen, near the coastal line. In city, shallow groundwa- county, Huimin county, and ), with length of ter generally has As concentrations between 0.2 and 82 lg/L (Li 350 km and width between 10 and 60 km. One is located at the et al., 2012). In city around 80 km downstream of Zibo city front of alluvial fans in the north, and the other near the Yellow riv- along the Yellow river, shallow groundwater has higher As concen- er in the southeast (Zhang et al., 2010a). Arsenic concentration is trations (20 lg/L) than surface water (10 lg/L) (Zhang et al., up to 105 lg/L in the southeastern strip (Guo and Guo, 2013), 2010b). An investigation in the north of Shandong plain in the while in the northwestern strip between <0.1 and 200 lg/L (Tan downstream of the Yellow river shows that shallow groundwater et al., 2006). Vertically, high As groundwater mainly occurs in allu- has As concentration between <5 and 440 lg/L, and high-As vial–lacustrine aquifers at depths between 10 and 40 m (Han et al., (>10 lg/L) groundwater samples account for 37% of 482 samples 2010). Around 36,000 residents were at risk (Tan et al., 2006), taken at depths between 20 and 50 m (Liu et al., 2013). However, being exposed to high As groundwater. Prevalence of arsenicosis no data are available for As concentrations in deep groundwater in the north of the basin was up to 11.7% (Hu et al., 1999). in the Yellow river delta.

2.1.5. The Dzungaria basin of Xinjiang province 2.2.3. The Pearl river delta of Guangdong province The Dzungaria basin is located in the north part of Xinjiang The Pearl river delta is mainly distributed in the Guangdong province. Arsenic-affected areas mainly lie in the southwestern province. Arsenic concentrations between 2.8 and 161 lg/L have part of the basin, with an area of 1200 km2 (Wang et al., 1995). Be- recently been observed in shallow groundwaters of the Pearl river fore 1960s, surface water (including river water and melting snow) delta (Table 1)(Huang et al., 2010; Wang et al., 2012). Groundwa- was used for drinking. Due to water pollution of the rivers, local ter generally occurs in reducing conditions, with neutral-weak þ residents changed their drinking water resources from surface alkaline pH. It is characterized by high concentrations of NH4 water to groundwater after 1960s. Those groundwaters were usu- (390 mg/L) and dissolved organic carbon (DOC) (36 mg/L) (Jiao ally pumped from confined aquifers (Hong, 1983), which normally et al., 2010), and low concentrations of NO3 and NO2 (Wang have high As concentrations. Wang (1984) found As concentrations et al., 2012). Arsenic concentration is independent of salinity. up to 1200 lg/L in groundwaters. Luo et al. (2006a) reported As Huang et al. (2010) believed that groundwater As originates from concentrations between 70 and 830 lg/L in deep artesian ground- sediment As and anthropogenic As penetrating from sewage water waters (>200 m in depth) in the south of the Dzungaria basin to the irrigation. However, Wang et al. (2012) suggested that mineraliza- north of the Tianshan Mountains. It was observed that As concen- tion of organic matter and reductive dissolution of Fe oxyhydrox- tration increases with increasing well depth (Wang, 1994). In the ides should be the major processes for As mobilization. north of the basin, groundwater concentration has the range be- tween 100 and 140 lg/L at depth around 200 m, which increases 2.2.4. The Chianan alluvial plain of Taiwan to 300–400 lg/L at the depth around 300 m (Hong, 1983). Ar- The Chianan alluvial plain is located in the southwest of Taiwan, senic-affected areas are mainly distributed between Aibi Lake (in which is bounded by the Peikang river to the north, the Erjen river the southwest of the Dzungaria basin) and Manasi River, parallel- to the south, the Taiwan Strait to the west, and the Central Range ing the Tianshan Mountain (Wang et al., 2002). Shallow groundwa- to the east, with an area of 2400 km2. Two major rivers, the Pa- ters usually have As concentrations between <10 and 68 lg/L. chang and Tsengwen, flow from the northeast to the west through Ingestion of those high As groundwaters for more than 10 years the northern part and the southern part of the plain, respectively. A had resulted in chronic As poisoning in the local people (Wang, peripheral vascular gangrene disease, known as the Blackfoot dis- 1994). In the early 1980s, the cases of As poisoning were firstly rec- ease (BFD), was first reported in the Chianan plain (Tseng et al., ognized in the Dzungaria basin of Xinjiang province. The preva- 1961), where many people consumed well waters with high As lence of endemic arsenism was up to 40% in 1991 (Liu et al., concentrations in the 1960s (Tseng, 1977). 1991), and decreased to 9% in 2007 due to the transformation of Around 46–61% of shallow wells in the plain had As concentra- drinking water resources (Yang et al., 2009). tions exceeding WHO recommended guideline of 10 lg/L. High As groundwater generally occurs in reducing conditions (Liu et al., 2.2. High As groundwater in river deltas 2003; Wang et al., 2011; Nath et al., 2011). Approximately 86– 93% of high As water had the redox potential (Eh) between 2.2.1. The Yangtze river delta of Shanghai and Nanjing 75.5 and 139.7 mV (Agricultural Engineering Research Center, High As groundwater also occurs in the Yangtze river delta 2008). Arsenic concentrations show a large variation, with the (Table 1). Since 1970s, groundwater with As concentrations greater range between 1.0 and 575 lg/L and the mean of 208 lg/L (Nath than 50 lg/L has been found in the first confined aquifers along et al., 2011). Arsenic(III) is the dominant As species (67% of total –Shanghai in the south of the Yangtze river delta (Chen, As). Iron and Mn concentrations are also variable, with the means 1998). Confined groundwater is hosted in reducing aquifers, gener- of 0.72 ± 1.6 mg/L and 107 ± 187 lg/L, respectively. Besides, ally containing high Fe(II) concentration (mostly >10 mg/L) organic metallic complexes, which have been considered as the 202 H. Guo et al. / Applied Geochemistry 41 (2014) 196–217

fluorescent substances (or humic substances), coexist with high As As-bearing minerals (Qin and Li, 2008). Arsenic concentration be- concentrations in the artesian well waters (Liu, 1986). Those humic tween 0.1 and 75 lg/L was observed in spring water and stream substances have been suspected to be associated with develop- water in Luding county (Li and Ni, 2005). There are 59% samples ment of Blackfoot disease, as well as high As concentrations (Lu, having As concentration greater than 10 lg/L. 1990). In Yunnan province, high As concentration has mainly been found in geothermal water (Table 1). Arsenic in the hot-spring 2.2.5. The Lanyang alluvial plain of Taiwan waters is in the ranges between 43.6 and 687 lg/L in Tengchong The Lanyang plain is located in the northeast of Taiwan, which area (Liu et al., 2009), where As(III) is the major As species with is the alluvial fan formed by the Lanyang river. The area is triangu- the fraction up to 91% of total As. The highest As concentration lar, with an area of approximately 400 km2, bound to the Pacific (887 lg/L) was found in the Guming spring in Rehai geothermal Ocean in the east and to the Central Mountain in the west (Selim field with temperature of 96.0 °C and pH of 8.93 (Guo and Wang, Reza et al., 2011). The main river, the Lanyang River, flows from 2012). Besides, shallow groundwater and surface water contain west to east through the middle of the area (Chen, 2000). Residents high As concentration in Gengma county and Simao county. Ar- in the Lanyang basin had used shallow well water (<40 m deep) for senic concentration is up to 200 lg/L in Gengma county, leading more than 50 years, which is the main source of exposure to inor- to endemic As poisoning (Wang and Kong, 2006). Spring water is ganic As among local residents (Chiou et al., 2001). Arsenic concen- the major media hosting high As concentration in Simao county, tration in the well water ranged from undetectable levels which was proposed to be related the weathering of bedrock (<0.15 lg/L) to 3590 lg/L, with a wide variation in median values (Luo et al., 2006b). from undetectable (<0.15 lg/L) to 140 lg/L (Yang et al., 2003). High As groundwater has resulted in negative effects on human Although As distribution in the shallow aquifers is mainly con- health. Yu et al. (2007) reported that high As concentration trolled by surface activities, high As groundwater has generally (>50 lg/L), being observed in about 5% of tested 445,638 wells in been found in the reducing zone, where As(III) is the major As spe- 20,517 villages of 16 provinces from the Mainland China, affected cies in groundwater (Lee et al., 2008). Chen et al. (1985) indicated an estimated 0.58 million residences, between 2001 and 2005. that As(III) and As(V) represented 87% and 5.8% of total As, respec- The population being at risk by the consumption of As-contami- tively. High As groundwater (>50 lg/L) is mainly distributed in six nated groundwater is estimated to be 19.6 million according to a townships, including JiaoSi, YiLan, JhungWei, WuJie, DonShan, and statistical risk model (Rodríguez-Lado et al., 2013). However, this LouDon (Lee et al., 2007). 19.6 million may be underestimated since the model mainly in- cludes As groundwater in alluvial sediment aquifers, excluding 2.3. Others (mountain areas) the bedrock aquifers in mountain areas. The model showed that the , the , the basin, the Ejina In Qinghai province, high As concentrations have been found in basin, the Tarim basin, and the Northeastern plain are the potential spring waters and deep groundwaters at depths between 100 and high As basins, although this estimation should be validated by ac- 400 m (Table 1). An et al. (2006) reported that groundwater As tual geochemical investigation. ranges between 2.0 and 308 lg/L, with 22.67% samples having As concentrations >10 lg/L, while As concentrations in spring water 3. Hydrogeology of the aquifers lie between 0.1 and 1070 lg/L, with 6.20% samples having As con- centrations >10 lg/L. High As groundwater mainly occurs in Guide 3.1. Sedimentary aquifers region and Qilian ranges (Zhang et al., 2009). In Guide region, arte- sian groundwater in Neogene siltstone usually has high As concen- 3.1.1. Inland basins tration (Zhang et al., 2010a; Shi et al., 2010). It was suggested that High As groundwater mainly occurred in Quaternary sedimen- those high As groundwater is affected by geothermal water, due to tary aquifers in arid–semiarid inland basins. These inland basins the good correlation between As concentration and water temper- usually have thick sedimentary sequences, up to several kilome- ature (Shi et al., 2010). Groundwater is mainly controlled by the ters. Several aquifer groups are normally recognized, including NNW and the EW faults (Zhang et al., 2010a). Therefore, ground- shallow aquifers, semi-confined aquifers, and confined aquifers. water As is believed to have geothermal sources. Shallow groundwaters in alluvial lacustrine or lacustrine sedi- High As groundwater (>50 lg/L) has also been found in Gansu ments usually have high As concentrations. Although affected by province, which is mainly distributed in Hezuo, Tianzhu, Huanxian, the surface water, groundwater usually flows from the piedmont Zhouqu, Weixuan, and Chengxian (Bai et al., 2007). Arsenic concen- front to the flat plain. Low-lying and flat topography attributes to tration is up to 2160 lg/L (Table 1). Among 210 water samples low hydraulic gradients in the central of the basins. Low perme- with As concentration >50 lg/L, 50.48% samples are shallow ability of aquifer sediments and low hydraulic gradients cause groundwater, and 36.19% spring water. High As waters mainly oc- low flow rates and limited flushing of the aquifers. cur around the fault-induced ravines (Bai et al., 2007). In the Hezuo city, it was found that river water, spring water and shallow groundwater contain high As concentration, which leads to the 3.1.1.1. The Hetao basin and the Huhhot basin. The Hetao basin and occurrence of chronic arsenicosis (Xu et al., 2007). It was confirmed the Huhhot basin in Inner Mongolia are formed by extension in- that 126 residents had symptoms of endemic As poisoning in duced by the Neogene faults (Zhao et al., 1984). The Hetao basin 2007–2008. is located between the Yellow river to the south and the Langshan In Sichuan province, high As groundwater is mainly distributed Mountains to the south. There are sediment sequences of 500– in Jinchuan county, Aba county, and Luding county (Table 1). In 1500 m in the southeast of the basin, and 7000–8000 m in the Jinchuan county, high As concentration with the range between northwest of the basin (Guo et al., 2008a). Affected by palaeo-cli- 56 and 287 lg/L was found in spring water (Deng et al., 2004). mate and tectonic movement, the Tertiary sediments occur in oxic Although no other chemical data were available, they suggested conditions and accumulate great amounts of salinity, while the that weathering of As-containing minerals would be the cause Quaternary sediments have both alluvial and lacustrine sources, for As enrichment in groundwater. An investigation on water As with fine clast. The Quaternary sediments are mainly derived from in Aba county showed that spring water mostly has high As con- the Langshan Mountains and partly from fluvial deposits of the centration, which was believed to be related to weathering of Yellow river. The Huhhot basin lies between the Mountains H. Guo et al. / Applied Geochemistry 41 (2014) 196–217 203 and the Yellow river, which has a gentle WSW slope with elevation fans, and discharged in the low-lying areas via evaporation (Liu between 1100 and 980 m. Quaternary sediment sequences of et al., 1996). sands, silt and clay occur in the basin, with the thickness around 1500 m (Smedley et al., 2003). The sediments are heterogeneous 3.1.1.2. The Yinchuan basin. Three geomorphologically distinct both spatially and with depth. However, Holocene sediments out- units are recognized in this basin, including the piedmont proluvial cropping at surface are typically coarse-grained in the basin mar- plain with a gravel aquifer, the alluvial–diluvial plain with a sandy gins and fine-grained in the central parts of the basin. Due to gravel aquifer and the alluvial–lacustrine plain with a sand aquifer tectonic movement, the rapid uplift of the mountains and the rapid (Han et al., 2013). High As groundwater has mainly been found in subsidence of the basin result in high sediment accumulation rate shallow groundwaters in the north of the alluvial–lacustrine plain in both basins, especially in Middle- and Late-Pleistocene (Han et al., 2010), where faulting with resulting paleo-lakes or ero- (0.6 mm/yr) and Holocene (1.2 mm/yr) (Li et al., 2007). sion (paleo-river channel) result in low-lying topographic areas. In In the Hetao basin, unconfined or semi-confined aquifers occur the piedmont region of the Helan Mountain and the alluvial–lacus- in the shallow deposits at depths <100 m with upper Pleistocene– trine plain in the southernmost of the basin, unconfined aquifers Holocene alluvial–pluvial and alluvial lacustrine sands, while con- mainly occur. In the central and eastern parts of the basin, fined aquifers in the deep deposits with middle Pleistocene lacus- multi-layers of alluvial and lacustrine deposits form the uncon- trine sands occur at depths greater than 100 m (Guo et al., 2008a). fined shallow aquifer with depth between 10 and 40 m, a first High As groundwater is mainly present in the semi-confined shal- confined aquifer with depths between 25–60 m and 140–160 m, low aquifers, showing the strip-type spatial pattern. The high As and a second deep confined aquifer with depth between 140 and zones correspond to the subsidence centers of the basin (Yang 240–260 m (Xue, 2011). Regional groundwater flow is generally et al., 2008). Shallow groundwater may be heavily affected by from the borders of the plain towards the interior of the basin, drainage channels and irrigation channels (Guo et al., 2011a). The and from the southwest to the northeast under low hydraulic gra- main drainage channel, being located in the piedmont depression dients because of the flat topography (Han et al., 2010; Wang et al., of the northern Hetao basin, discharges shallow groundwater from 2013a). The unconfined shallow groundwater is recharged by pre- the piedmont area in the north and from the recharge area near the cipitation and infiltration of irrigation water diverted from the Yel- Yellow river in the south (Zhang et al., 2013). Shallow aquifers in low river, and is discharged by evaporation, drainage canal, the piedmont alluvial–pluvial plain are mostly coarse sands and pumping, and infiltration to deeper aquifer (Wang and Yu, 2001). gravels with hydraulic conductivity up to 20 m/d, and in the allu- The hydraulic gradient of the shallow groundwater in the southern vial lacustrine flat plain primarily being composed of silt and fine part of the alluvial plain is generally greater than 0.4‰, and less sands with hydraulic conductivity less than 2.2 m/d (Guo et al., than 0.4‰ in the northern part of the alluvial–lacustrine plain, 2010). Regions in the piedmont area and near the Yellow river which is related to the occurrence of high As groundwater (Han are the high flow rate area with hydraulic gradients >0.8‰, while et al., 2013). The groundwater renewal rates range from 11% to near the drainage channels the low flow rate area with hydraulic 15% in the south, while between 0.1% and 6% in the north (Wang gradients <0.8‰ (Zhang et al., 2013). Therefore, groundwater flow et al., 2013a). rate in the piedmont with coarse sands and gravels with a hydrau- lic gradient of 0.8‰ is around 1.6 cm/d, and in the flat plain with 3.1.1.3. The Datong basin. The Datong basin is one of the Cenozoic silt and fine sands with a hydraulic gradient of 0.8‰ around basins of the Shanxi rift system, which produces a series of en-ech- 0.18 cm/d. On the one hand, the low flow rate leads to long resi- elon half grabens, as a result of northeast displacement of the Or- dence time of groundwater in the aquifer, and therefore near equi- dos block. Gneiss and basalt of the Hengshan Segment of Wutai librium of water–rock interaction. On the other hand, these flow Group of Archean occur to the east of the basin, and Cambrian rates are too slow to flush As out of the aquifer, because it takes and Ordovician limestone and Carboniferous and Permian sand- thousands of years to flush As over a distance of 1 km in the Gan- stone–shale to the west. Thick Quaternary sediments have depos- ges–Brahmaputra delta with a flow rate of 5.0 cm/d (van Geen ited in the basin, ranging between 200 m in the margin and 2700 m et al., 2008). Groundwater is recharged by vertically infiltrating in the center, with a sedimentation rate around 0.24 mm/yr (Wen meteoric water in the basin and laterally penetrating fracture et al., 2013). The sediments from the basin margin are mostly allu- water from marble, slate and gneiss along the mountain front, as vial–pluvial gravel and sand, and those at the central part of the ba- well as a little leaked water from river and ditches, and irrigation sin sandy loam and silt, lacustrine and alluvial–lacustrine sandy return flow from farmland. Shallow groundwater generally flows loam, silty clay and clay with high organic matter contents (Guo from north to south in the northern part and from southwest to and Wang, 2005). The shallow Quaternary aquifer in the center northeast in the southern part (Guo et al., 2008a). During irrigation of the basin usually consists of 60 m deposits of gray to blackish seasons, groundwater table is high in the flat plain irrigated with lacustrine and alluvial–lacustrine medium–fine sand, silty clay diverted-Yellow river water and low in the piedmont area irrigated and clay (Wang et al., 2009), which hosts shallow groundwater. Ar- with groundwater, which would alter groundwater flow conditions senic concentration is high (up to 1930 lg/L) in the reducing (Guo et al., 2013c). groundwater in the central low-lying area (Guo et al., 2003; Guo In the Huhhot basin, sequences of lacustrine sediments fre- and Wang, 2005). High As groundwater has mainly been found in quently occur in the low-lying central parts of the basin, and allu- the eastern side of the Sanggan river, which suggests that ground- vial deposits along the mountain foothills. Sandy layers in the water As would ultimately originate from local source of gneiss upper 40 m form shallow aquifers (Smedley et al., 2003). Deep and basalt weathering since the higher As contents are found in aquifers exists at depths >100 m. High As groundwater mainly oc- gneiss and basalt to the east of the basin than in limestone and curs in the transition area between the alluvial fans and the Yellow sandstone–shale to the west of the basin (Guo, 2002). river fluvial plain, where clayey sands and fine sands are present in Shallow groundwater is mainly recharged by vertically infiltrat- the aquifers near palaeo-channels of the Yellow river and lagoons ing meteoric water in the basin and laterally flowing fracture water (Li and Li, 1994). Although exact values of water flow rates were in bedrock along the basin margins, as well as irrigation return not reported, groundwater has higher flow rates in the mountain flow. The general direction of groundwater flow is from the margin foothills than those in the low-lying central plain, due to the higher to the center of the basin (Guo and Wang, 2005). The velocity of topography and good permeability (Zhang et al., 2000). Groundwa- groundwater flow ranges between 0.20 and 0.58 m/d near the ter is mainly recharged from the mountain foothill and alluvial mountains (Xie et al., 2009a), but groundwater flow in the flat 204 H. Guo et al. / Applied Geochemistry 41 (2014) 196–217 and low-lying area in the basin center is very low. Although the sediments, including Holocene unconfined aquifers, Middle-Upper hydraulic gradients and the flow rates are not specifically reported, Pleistocene confined aquifers, and Lower Pleistocene confined high As concentration is expected to be associated with the low aquifers (Song et al., 2000; Xu, 2002). The Holocene unconfined flow rate of groundwater in reducing conditions (Guo and Wang, aquifers are widely distributed, with the interbeds of silty clay, silt 2005; Guo et al., 2003; Xie et al., 2012). and fine sand. Continuous aquitards underlie the unconfined aqui- fers. There are two confined aquifers in Middle-Upper Pleistocene 3.1.1.4. The Songnen basin. The Songnen basin is a part of Meso- sediment sequences, including confined aquifers I and II (Chen, Cenozoic Songliao graben basin, with thick sequences (5000 m) 1998; Song et al., 2000; Xu, 2002). Confined aquifer I is character- of inland pluvial–lacustrine sediments (Bian et al., 2012). Vertical ized by silt and silty sand, which originate from both marine and tectonic movement leads to depression in the east and uplift in fluvial deposits, while confined aquifer II by sand, pebbly sand, the west, which forms a gentle SE slope with an altitude around and middle-fine sand being mainly from the Yangtze river estua- 300 m above see level (asl) in the northwest and an altitude around rine deposits (Song et al., 2000; Xu, 2002). Normally, the former 130 m asl in the southeast. The Daxing’anling Mountains lie to the Yangtze river eroded the upper aquitards of confined aquifers I northwest of the Songnen basin. Upper Neogene strata are charac- and II near the palaeo-channels, which makes the hydraulic con- terized by glutenite, sandstone and siltstone with mudstone and nection between confined aquifers I and II. Confined aquifer III is silty mudstone of Da’an group in the lower part, and semi-consol- located in Lower Pleistocene sequences. High As groundwater idated fluvial–lacustrine deposits (including glutenite, sandstone, mainly occurs in the confined aquifers I and II. Arsenic concentra- siltstone, and silty mudstone) of Taikang group in the upper part. tion ranges between 27.7 and 324 lg/L in the confined aquifer I in Lower Quaternary sediments are mainly composed of gravel sands, the section of Nantong–Shanghai (Chen, 1998). In Shanghai city, As interbeded with fine sands and clay. Middle Quaternary sediments concentration is the highest in the confined aquifer II, which is are characterized by clayey silt and silt, while Upper Quaternary reaching up to 600 lg/L (Hu, 1980). sediments by loess, clayey silt, fine sand and middle sand, which are widely distributed in the basin. In Holocene sediments, se- 3.1.2.2. The Yellow river delta. The Yellow river delta is located in quences vary from alluvial sands in the bottom, through alluvial– the southern edge of the Bohai seabasin with an area of lacustrine fine sand, silt and silty clay and lacustrine clay and silty 7893 km3 and natural slope gradients between 1/8000 and clay in the middle, to aeolian silt and silty sand in the uppermost 1/12,000 (Zhang et al., 2005). It has been affected by layer (Feng, 2007; Bian et al., 2007). depression, Jiyang depression, and Bozhong uplift (Yuan et al., Groundwater is mainly hosted in Cenozoic sediments. Aquifer 2006). Since the Cenozoic era, subsidence movement has been pre- systems are divided into the Quaternary porous unconfined and dominated. Continuous subsidence leads to the thick Quaternary confined aquifers (shallow aquifer and mid-deep aquifer), sequences, which is up to 400 m. Topographically, it is divided into and the pore-fracture confined aquifers of Upper Neogene Da’an piedmont plain and alluvial–marine plain (Li et al., 2008). The and Taikang groups (deep aquifer I), the Cretaceous fracture-pore piedmont plain is located to the south of the Xiaoqinghe river, confined aquifers (deep aquifer II) (Tang et al., 2010). Shallow which is mainly deposited by alluvial and fluvial sediments, while groundwater occurs in Holocene alluvial sand aquifer, and Upper the alluvial–marine plain to the north of the Xiaoqinghe river (Li Quaternary fine-sand and middle-sand aquifer, with conductivities et al., 2008; Yao et al., 2002). The alluvial sediments have inter- of 30–50, and 5–15 m/d, respectively. Confined groundwater is laced the marine sediments in the alluvial–marine plain. Based mainly hosted in Lower Quaternary gravel sands with the thick- on buried depths of aquifers and hydraulic connections, three aqui- ness between 10 and 40 m, and in Upper Neogene Da’an and Taik- fer groups are divided in sedimentation sequences of Quaternary ang sandstone and glutenite with the thickness between 20 and and Tertiary Minghuazhen group, including shallow unconfined 90 m (Bian et al., 2009). and semi-confined aquifers (Aquifer I), middle confined porous High As groundwater mainly occurs in shallow aquifers at aquifers (Aquifer II), and deep confined porous-fissure aquifers depths <20 m and in Upper Quaternary confined aquifer at depths (Aquifer III) (Yao et al., 2002). Aquifer I is mainly composed of flu- between 20 and 100 m (Bian et al., 2012). Spatially, As concentra- vial and marine silt and clay silt, with the depth of burial less than tion greater than 10 lg/L has been found in the shallow aquifers in 60 m (An et al., 2012). During the deposition of Aquifer I, two the alluvial–lacustrine plain between the distal edge of the alluvial transgressions had occurred, which leads to the presence of saline fans and the Huolin river, and confined aquifer in the alluvial– water in the alluvial–marine plain. The lower aquitard of Aquifer II lacustrine plain near Tongyu county. High As concentration may lies at depths between 180 and 370 m, which is the upper aquitard be related to glacial deposits in Upper Quaternary and lacustrine of the Aquifer III (Yao et al., 2002). The Yellow river is the natural deposits in Holocene. divide of groundwater systems. In the river delta to the north of the Yellow river, groundwater flows from the south to the north or the 3.1.2. River deltas northeast, while from the north to the southeast in the river delta In the river deltas, thick sequences of Quaternary sediments to the south of the Yellow river (Li et al., 2008). High As groundwa- have usually been deposited due to tectonic subsidence. In those ter has mainly been found in the shallow aquifer (Aquifer I) in the areas, marine sediments are interlaced with fluvial deposits, which alluvial–marine plain, which may be related to the low slope gra- have normally been affected by transgression. The sediments are dients and interbedded marine deposits. relatively young. The flat topography leads to low hydraulic gradi- ents, and therefore low groundwater flow rates. The interbeds of 3.1.2.3. The Pearl river delta. The Pearl river delta is located in the marine and fluvial deposits are mainly composed of silty sand, silt, south-central of Guangdong province, which is surrounded by clayey silt and clay, with high content of natural organic matter. mountains to the north, the west and the east, and by the South This readily leads to reducing conditions in aquifers. China Sea to the south. It is formed as a result of uplift of the Tibe- tan Plateau during the Tertiary and Quaternary Periods (Zong et al., 3.1.2.1. The Yangtze river delta. The Yangtze river delta is located in 2009). During the Late Quaternary, active fault systems have led to the south of the Subei depression. During Quaternary, interbeds of land subsidence of the Pearl river basin at a rate of 0.2 mm/a marine and fluvial deposits have developed due to tectonic depres- (Huang et al., 1986), and therefore sedimentation of terrestrial sion, with the average thickness around 200–300 m (Song et al., and marine deposits on Cretaceous–Tertiary sandstones and Meso- 2000). Aquifer systems were observed in the Quaternary zoic granites (Huang et al., 1982). The Quaternary deposits mainly H. Guo et al. / Applied Geochemistry 41 (2014) 196–217 205 consist of four stratigraphic units: two terrestrial units (T1 and T2) geothermal surface manifestations (e.g. hot springs) have been and two marine units (M1 and M2) (Wang et al., 2012). The old ter- found on the margins of volcanic rock areas, in correspondence restrial unit (T2), mainly composed of sand and gravel, had depos- to fault systems (Zhang et al., 1987). In this region, 68 different vol- ited in the last transgression in the late Pleistocene (Wang et al., canoes, of which three are Holocene volcanoes (namely Daying- 2013b). The old silt and clay marine unit (M2), overlying on T2, shan, Maanshan and Heikongshan), have been identified, and was formed during the interglacial period (about 130 ka before about 140 sites of hot spring activity have been found (Du et al., present) (Zong et al., 2009). A younger terrestrial unit (T1), mainly 2005). Hot spring waters in the Rehai have been classified into four composed of alluvial sand and clayey silt, was deposited along the chemical groups, including Na–Cl–HCO3 or Na–HCO3–Cl waters palaeo-river channels. During Holocene, a large-scale transgression discharged with high temperatures in granite and granitic clastic has resulted in a layer of younger marine sediments (M1) with a sedimentary rocks (Group I), Na–HCO3 waters in areas of meta- thickness of 5–20 m (Wang et al., 2013b). In many places, the old morphic rocks and granite rocks (Group II), Ca(Mg)-HCO3 or Ca/ marine unit (M2) and the younger terrestrial unit (T1) are missing. Mg–Na–HCO3 waters with lower temperatures (<40 °C) in areas The fine-grained silts and clays of M1 and M2 comprise the aqui- with Ordovician–Silurian slates and basalt and/or andesite (Group tards. Accordingly, the old terrestrial unit (T2) is the basal aquifer, III), and Na–HCO3–SO4 or Ca–Na–HCO3–SO4 waters (Group IV) and the younger unit (T1), being fluvial sands and clayey silt, is a (Zhang et al., 1987). Spring waters in Groups I and II usually con- local intermediate aquifer (Jiao et al., 2010). The general direction tain high As concentration, which is up to 887 lg/L (Guo and of regional groundwater flow in the sand and gravel aquifer follows Wang, 2012). roughly the major river flow, which is from northwest to southeast (Wang et al., 2012). High As groundwater has mainly been found in 4. Aqueous chemistry of high As groundwater the basal aquifers (Wang et al., 2012).

In this section, we take the Datong basin, the Hetao basin, the 3.2. Geothermal related aquifers Huhhot basin, and Yinchuan basin as representatives of inland arid–semiarid basins, and the Pearl river delta as representative In the Guide basin and the Tengchong area, high As groundwa- of the river delta for delineating aqueous chemistry of high As ters are closely associated with deep geothermal water. Usually, groundwater in China. The chemical data of the Datong basin are deep geothermal water recharges the shallow groundwater via cited from Guo et al. (2003), Xie et al. (2009a), and Luo et al. faults due to recent tectonic movement. Spatial distribution of As (2012), the Hetao basin from Guo et al. (2011a) and Luo et al. concentration is generally consistent with the occurrence of ten- (2012), the Huhhot basin from Smedley et al. (2003) and Mukher- sion faults. jee et al. (2009). The data of the Yinchuan basin are obtained from The Guide basin is a Mesozoic–Cenozoic sedimentary basin, our investigation in 2012. The data reported in Wang et al. (2012) which is controlled by the giant faults occurring in the front of are used for high As groundwater in the Pearl river delta. the mountains. The basin belongs to the Qilian–Helan stratigraphic zone and the Zhamashan Mountains sub-region (Gu et al., 1992). Since the Cenozoic era, the basin has continuously subsided, lead- 4.1. Major ions ing to deposition of clast sediments with a thickness around 2000 m (Chen et al., 2010). The Neogene system with a thickness Concentrations of major ions significantly varied in high As around 1330 m consists of a set of red piedmont fluvio–lacustrine groundwaters. Sodium and Cl concentrations increased from the sediments, which unconformably contact with the underlying Pro- Datong basin, through the Yinchuan basin, the Huhhot basin and terozoic or Triassic basement. The Quaternary deposits overlie the the Hetao basin, to the Pearl river delta (Fig. 2a). In the Datong ba- Neogene system (Gu et al., 1992). Confined aquifer and unconfined sin, the Yinchuan basin, and the Hetao basin (the inland basins), Na+ aquifer were observed in Neogene and Quaternary deposits, concentrations are mostly greater than Cl concentrations, while respectively. Quaternary unconfined aquifers are mainly composed groundwater in the Pearl river delta has higher Cl concentration + of alluvial silt and fine sand, which are mainly distributed in pied- than Na concentration. Most groundwater has HCO3 concentra- mont plains and river valley plains. The Guide group of Neogene tions between 200 and 2000 mg/L, showing less variation relative deposits is widely distributed in the basin, which consists of mid- to Cl concentration (mostly between 10 and 20,000 mg/L). dle-grained sandstone, coarse-grained sandstone, and siltstone Groundwater in the inland basins has generally higher HCO3 (Chen et al., 2010). Artesian groundwater normally occurs in the concentration than Cl concentration, while in the river delta Guide group of Neogene deposits. Groundwater from unconfined Cl concentration is higher than HCO3 concentration (Fig. 2b). Total aquifer usually has undetectable As (<10 lg/L). However, As con- dissolved solid (TDS) has the values between 200 and 20,000 mg/L. centration of artesian groundwaters from confined aquifer is gen- Generally, TDS values in groundwaters of the river delta are greater erally high (between 112 and 319 lg/L) (Shi et al., 2010). Both than those in the inland basins (Fig. 2c). There is a good correlation well water temperature and As increase with increasing the well between Na+ concentration and TDS value (Fig. 2c), showing the depths (Zhang et al., 2010a), suggesting that As should be hydro- major contribution of Na+ to TDS. There is no significant correlation 2+ 2 2+ thermally leached from the organic-rich fine lacustrine deposits between Ca and SO4 (Fig. 2e). In the Pearl river delta, Ca con- 2 of the Neogene Guide group. centration is higher than SO4 concentration, while groundwaters 2 2+ The Tengchong area belongs to the Yunnan–Tibet geothermal mostly have higher SO4 concentration than Ca in the Yinchuan belt of China, a major part of the Mediterranean–Himalayas geo- basin, the Hetao basin and the Huhhot basin. The Datong basin gen- 2 2+ thermal belt (Liao and Zhao, 1999). Tectonically, it is situated in erally has the lowest SO4 concentration, with Ca concentration 2 the mini-Tengchong block and the eastern collision boundary be- greater than SO4 . Additionally, HCO3 concentration is usually 2 tween the India and Eurasia plates, and develops from a micro con- greater than SO4 concentration in both the inland basins and the tinent between Gondwanaland and Eurasia (Du et al., 2005). Since river delta (Fig. 2f). + the Late Paleozoic, tectonic activity has become very intense. Qua- In the Huhhot basin, concentrations of HCO3 and Na are usu- ternary volcanoes and fault structures are presently the most con- ally high in the shallow groundwaters, which are mostly located spicuous features of this region (Shangguan et al., 2005; Zhang in the lower part of the piper plots, although groundwaters vary et al., 2008). The active faults with strikes of NNE, NS, NW and in chemical composition both laterally along the flow paths and NE have well developed in the region (Liao and Guo, 1986). The with depth (Smedley et al., 2001). Along the flow paths, shallow 206 H. Guo et al. / Applied Geochemistry 41 (2014) 196–217

+ + 2+ 2 2 Fig. 2. Na vs. Cl (a), HCO3 vs. Cl (b), Na vs. TDS (c), HCO3 vs. TDS (d), Ca vs. SO4 (e), and HCO3 vs. SO4 in groundwaters of the Datong basin, the Hetao basin, the Huhhot basin, the Yinchuan basin, and the Pearl river delta.

groundwaters evolve from Ca–HCO3 type in the pediment areas between 1000 and 3000 mg/L and water types of Na–Mg–HCO3– along the basin, to Na–HCO3/Cl or mixed-ion types in the central Cl, Na–Cl–HCO3, Na–Cl–SO4, and Na–Mg–SO4–Cl. Spatially, high parts of the basin (Smedley et al., 2003). Mukherjee et al. (2009) As groundwater mainly occurs in the alluvial–lacustrine plains showed that the shallow groundwater is generally of a mixed and near the Hasu Lake (Smedley et al., 2003).

Ca–Na–HCO3–Cl type. They observed dilute Ca–Mg–HCO3 and In the Hetao basin, groundwater chemistry evolves from the Na–HCO3 groundwaters with TDS <500 mg/L in the alluvial plains alluvial fans to the alluvial–lacustrine plain. Shallow groundwater in the north of the basin and in the alluvial fans on the SE side of is generally of Ca–HCO3–SO4 type with low salinity in the regional the Manhan Mountains, and slight saline groundwaters in the allu- recharge area of the alluvial fans, Mg–Ca–HCO3–Cl in the distal vial and lacustrine plains downstream the Daheihe river and lacus- edge of the fans (Guo et al., 2010), and Na–Cl–HCO3 with high trine deposits in the foothills of the Manhan Mountains with TDS salinity in the central part of the basin (Guo et al., 2008a; Luo H. Guo et al. / Applied Geochemistry 41 (2014) 196–217 207 et al., 2012). High salinity in the center of basin is related to evap- mostly <0 mV (Fig. 3a). The groundwater in the Hetao basin has oration and irrigation losses. The widely-distributed irrigation the lowest ORP values, followed by the Huhhot basin, the Datong channels and drainage channels have posed an impact on ground- basin, and the Yinchuan basin. As an effective candidate for elec- water chemistry. Near the drainage channels, groundwater is gen- tron donor in groundwater systems, dissolved organic carbon erally of Na–Cl–HCO3 type with electric conductivity between (DOC) is normally high, mostly ranging between 5 and 20 mg/L 1930 and 9500 lS/cm, while of Ca–HCO3 type near the irrigation (Fig. 3a). In the Hetao basin, DOC concentration is the highest, with channels with electric conductivity between 500 and 900 lS/cm the average of 12.0 mg/L, followed by the Huhhot basin (average (Guo et al., 2011a). An investigation carried out in Hangjinhouqi 8.3 mg/L), the Yinchuan basin (average 6.0 mg/L), and the Datong in the northwest of the Hetao basin showed that shallow ground- basin (average 5.0 mg/L). Generally, there are negative correlations waters are generally of Na–HCO3 or Na–HCO3–Cl type along the between DOC and ORP of the high As groundwater. It may indicate Langshan Mountains front and in the south recharged from the that DOC is the trigger in developing the reducing conditions in the

Yellow river, while mostly Na–Cl–HCO3 or Na–Cl type in the flat aquifers (McArthur et al., 2004; Polizzotto et al., 2008). 2 plain (Deng et al., 2009). In the reducing conditions, concentrations of SO4 and NO3 are In the Yinchuan basin, groundwater chemistry spatially varies, relatively low (Fig. 3b). Sulfate concentration is the lowest in the which is dependent of hydrogeological settings. Shallow ground- Huhhot basin, with the average of 40 mg/L, while the Hetao basin + 2 water displays increasing trends in TDS, Na ,Cl , and SO4 along groundwaters have the lowest NO3 concentration (average 2.3 2 the direction of groundwater flow path from SW to NE (Han, mg/L). The Yinchuan basin groundwaters have the highest SO4 2013). In the south of the basin which is the recharge area, shallow concentration, with an average of 277 mg/L. In the Hetao basin, 2 groundwater is generally of Mg–Na–HCO3–SO4 type. In the center groundwaters have SO4 concentrations between <0.1 and of the basin, groundwater is of a mixed Ca–Na–HCO3–SO4 or 1260 mg/L (average 230 mg/L). In comparison with the Hetao basin Mg–Na–HCO3–Cl type. In the north which is the discharge area, and the Yinchuan basin, the Datong basin and the Huhhot basin the hydrochemical type is mainly of Na–Ca–Cl–SO4 with elevated have higher concentrations of NO3 (average 12.5 and 9.2 mg/L, 2 TDS (Wang et al., 2013a). In the center-north of the basin, the re- respectively), and lower concentration of SO4 (average 61.5 and turn flow of irrigation water with low TDS (around 350 mg/L) leads 65.8 mg/L, respectively). The general low concentrations of NO3 2 to the scattered distribution of fresh shallow groundwater, and SO4 imply that denitrification and sulfate reduction occur in although groundwater mostly has high TDS values (1000– aquifers with high As groundwater (Guo et al., 2008a; Deng 3000 mg/L) (Han et al., 2013). High As groundwater has mainly et al., 2009; Xie et al., 2009a). been observed in the north of alluvial–laustrine plain (Han et al., Additionally, concentrations of Fe and Mn are generally high. In 2013; Guo and Guo, 2013). the Pearl river delta, Fe and Mn concentrations are the highest, with

In the Datong basin, groundwater is generally of Ca–HCO3 type average values of 29 mg/L and 5600 lg/L, respectively, followed by in the pediment recharge areas for regional shallow groundwater the Yinchuan basin (with averages of 2.10 mg/L and 532 lg/L, system, with TDS between 247 and 792 mg/L. In the converging respectively) (Fig. 3c). In comparison, the Hetao basin and the Huh- zone between recharge and discharge areas, the major ions are hot basin have relatively lower concentrations of Fe (with averages 2+ 2+ + HCO3 ,Mg ,Ca , and Na , plotted at the middle-left of Piper trilin- of 0.50 and 0.76 mg/L, respectively) and Mn (with averages of 254 ear diagram (Guo and Wang, 2005). In the enrichment zone, inter- and 201 lg/L, respectively). In the Datong basin, groundwaters mediately downgradient of the converging zone, shallow have the lowest concentrations of Fe and Mn. Generally, it has been groundwater is generally of Na–HCO3–SO4–Cl type with TDS be- found that Fe concentration is positively correlated with Mn con- tween 440 and 8900 mg/L, plotted at the middle right of Piper tri- centration (Fig. 3c). The relatively lower concentrations of Fe and linear diagram. Shallow groundwater, being located in the Mn in the Hetao basin and the Huhhot basin with stronger reducing 2 2 discharge area of the regional groundwater system and at the cen- conditions may be due to the coexistence of SO4 and S . Detect- 2 ter of the basin, has the water type of Na–HCO3. High As concentra- able S has been found in groundwaters of the Hetao basin (Guo tion has mainly been found in the shallow groundwater at the et al., 2010, 2011a), the Huhhot basin (Smedley et al., 2003), and center of the basin (Guo et al., 2003; Xie et al., 2009a). These so- the Datong basin (Xie et al., 2013). In the reducing conditions, the 2 2 dium-rich waters usually have high concentrations of As and F , S produced from SO4 reduction would restrain the accumulation ranging between 7.9 and 1530 lg/L, and between 0.48 and of Fe and Mn in groundwaters (Guo et al., 2013a,b). 7.33 mg/L, respectively (Guo et al., 2003; Wang et al., 2009). Although both Fe and As would preferentially be released in Shallow groundwater has been substantially affected by both reducing conditions, there is no significant correlation between paleo seawater and freshwater in the Pearl river delta (Jiao et al., Fe concentration and As concentration in high As groundwaters 2010). Groundwater is mostly of Na–Cl type in the southwest of (Fig. 3d). In the Hetao basin, groundwaters have high As concentra- the delta, Ca–HCO3 or Na–HCO3 in the northwest, and Na–Cl in tion with an average of 213 lg/L, although Fe and Mn concentra- the northeast (Wang and Jiao, 2012; Huang et al., 2013). Due to tion are relatively low. Arsenic concentration is the highest in the the strong seawater influence, groundwaters close to the sea usu- Datong basin, with an average of 308 lg/L, where the lowest Fe ally have water type of Na–Cl. Along the regional groundwater flow and Mn concentrations are observed. The average As concentra- from the northwest to the southeast, groundwaters evolve from tions are 28.0 and 173 lg/L in the Yinchuan basin and the Huhhot

Ca–HCO3 or Na–HCO3 type to Na–Cl type (Wang and Jiao, 2012). basin, respectively. In the Pearl river delta with the highest Fe and In this area, groundwater is not only affected by seawater intru- Mn concentrations, groundwater As concentration is relatively sion, but also shows a clear path of hydrogeochemical evolution lower, with the average of 33.0 lg/L. Although the reductive disso- from Ca–HCO3, through Ca–Cl, to Na–Cl types (Huang et al., lution of Fe oxides would release As from solids into groundwater, 2013). High As groundwater has been mainly found in the north- Fe(II) may be scavenged by means of resorption on the minerals or west and northeast of the delta, with water types of Ca–HCO3, precipitation of siderite or pyrite (Guo et al., 2013a). Besides, the Na–HCO3 and Na–Cl (Wang et al., 2012). released As may be co-precipitated with pyrite or adsorbed on Fe minerals. Pathways of Fe and As cycling are expected to be differ- 4.2. Redox sensitive species ent, and therefore As is decoupled with Fe in high As groundwater systems (van Geen et al., 2004; Burnol et al., 2007). Either in the inland basins or in the river delta of China, high As Both U and Mo are redox-sensitive elements. Their concentra- groundwater usually occurs in reducing conditions, with ORP tions show big variations in high As groundwaters (Fig. 3e). 208 H. Guo et al. / Applied Geochemistry 41 (2014) 196–217

2 Fig. 3. ORP vs. DOC (a), SO4 vs. NO3 (b), Fe vs. Mn (c), Fe vs. As (d), U vs. As (e), and Mo vs. As in groundwaters of the Datong basin, the Hetao basin, the Huhhot basin, the Yinchuan basin, and the Pearl river delta (The legend is the same as Fig. 2).

Concentration of U ranges between 0.02 and 172 lg/L, with an age values of 5.07 and 6.14 lg/L, respectively. Likewise, Mo average of 4.99 lg/L, in the Hetao basin. The average U concentra- concentrations are generally lower in the higher As groundwater 2 tion is the highest in the Huhhot basin (5.55 lg/L with the range (Fig. 3f), since it is readily immobilized as MoOxS4x in reducing between <0.01 and 38 lg/L). In comparison, U concentration is rel- conditions (Tossell, 2005). The relationships between U and As, atively lower in the Yinchuan basin (between 0.04 and 31.0 lg/L, and Mo and As support naturally occurrence of high As groundwa- average 2.77 lg/L) and in the Datong basin (between <0.01 and ter in reducing conditions (Xie et al., 2009a). 23.0, average 2.79 lg/L). In general, there is a negative correlation between U and As (Fig. 3e). High U concentrations are associated 4.3. Arsenic species with low As waters, and vice versa. Uranium is relatively immobile as insoluble U(OH)4 species in the reducing conditions (Gorby and Arsenic species not only control the toxicity of high As ground- Lovley, 1992), showing low concentration, while mobile as soluble water and its impact on human health, but also play an important 4 2 UO2ðCO3Þ3 or UO2ðCO3Þ2 species in oxic conditions (Moon et al., role in As cycling in groundwater systems. Arsenite (As(III)) and 2009). Similar to U, Mo shows the highest concentration in the arsenate (As(V)) are two major As species predominantly occurring Huhhot basin groundwater, with an average of 7.35 lg/L, followed in natural water (Cullen and Reimer, 1989). The former is about 60 by the Hetao basin groundwater, the Datong basin groundwater, times toxic than the latter (Morton and Dunette, 1994). Further- and the Yinchuan basin groundwater. The Hetao basin groundwa- more, As(V), mainly being present as oxyanionic forms (H2AsO4 , 2 ter has identical Mo concentrations to the Datong basin, with aver- HAsO4 ) at neutral pH (Smedley and Kinniburgh, 2002), is readily H. Guo et al. / Applied Geochemistry 41 (2014) 196–217 209 captured on the aquifer sediments by means of adsorption and co- (10–22% with an average of 16%) by means of on-site filtration precipitation (Dinesh et al., 2007). In comparison, As(III), being pre- and species separation in six well water samples. In order to avoid dominantly as a neutral species (H3AsO3), is quite mobile due to exposing to air and settling the soluble components, on-site filtra- the lack of electrostatic attraction and easily mobilized in the tion in N2 atmosphere is necessary. groundwater systems (Ravenscroft et al., 2009). Therefore, data On-site filtration by progressively decreasing membrane pore on groundwater As species have usually been provided in re- size (10, 5, 3, 1, 0.8, 0.45 lm) under N2 in the Hetao basin shows cently-published literatures. that 30%-60% Fe is associated with the large particles (>0.45 lm), Arsenic(III) is the major As species in those high As groundwa- although As concentrations do not exhibit large variations through ters. In the Datong basin, As(III) concentrations range between <1.0 the different decreasing pore size cut-offs (Guo et al., 2009). They and 1040 lg/L, accounting for 0–92% total As. In average, 73.9% of suggest that Fe complex is the major component of the large-size total As is present as As(III) (<1.0–1430 lg/L) in groundwater of the particles (>0.45 lm), and conclude that a small proportion of As Huhhot basin. The Hetao basin groundwaters have As(III) concen- (<15%) is retained in large-size Fe complexing particles in ground- tration between <1.0 and 755 lg/L, accounting for from 0% to waters saturated with respect to siderite and pyrite. 97% total As. Although total As concentrations are relatively low, Later, Guo et al. (2011b) have carried out successive ultrafiltra- ratios of As(III) to total As range between 0% and 100% with the tion through a progressively decreasing pore size (0.45 lm, 100, average of 73.4% in the Yinchuan basin. 30, 10, 5 kDa) for 8 well waters in the Hetao basin. They have iden- Seventy-seven percent of groundwater samples have ratios of tified two types of colloids, including large-size Fe colloids and As(III) to the total concentration greater than 50% in the Datong ba- small-size organic colloids. A large drop (30%) of Fe concentration sin, 75% samples in the Huhhot basin, 87% samples in the Hetao ba- was observed between 0.45 lm filtrate and 100 kDa ultrafiltrate, sin, and 82% samples in the Yinchuan basin (Fig. 4a). Statistically, showing the presence of Fe as large-size colloids with grain size be- ratios of As(III) to total concentration are the highest in groundwa- tween 100 kDa and 0.45 lm. However, a large proportion of As is ters of the Yinchuan basin, followed by the Hetao basin, the Huh- retained in the 5–30 kDa fractions, which is consistent with organ- hot basin, and the Datong basin (Fig. 4b). The median values of ic carbon (OC) distribution among the ultrafiltrates (Guo et al., As(III)/total As are 61.8% and 89.5% in the Datong basin and the 2011b). It indicates that As would be more likely associated with Yinchuan basin, respectively. Methylated As species (MMAA, organic colloids than Fe colloids, which is confirmed by SEM DMAA) was only reported in the Datong basin by Xie et al. images, EDS analysis and synchrotron XRF analyses. The better cor- (2009a), with concentrations of MMA and DMA between <0.4 relation between As(V) and OC than As(III) and OC in the 5–10 kDa and 4.6 lg/L and <0.4 and 5.4 lg/L, respectively. In the other ba- fraction implies that OC would have a better affinity for As(V) than sins, methylated As species are undetectable (<0.4 lg/L). The prev- As(III) (Guo et al., 2011b). This As-NOM colloidal association has alence of As(III) over As(V) also indicates the reducing natures of also been observed in wetland environments (Langner et al., high groundwaters (Guo et al., 2008a). 2012; Buschmann et al., 2006) and the laboratory experiments (Bauer and Blodau, 2009). Guo et al. (2011b) suggest that the NOM-complexed As would be difficult to be removed from high 4.4. Colloid particles As groundwater in the Hetao basin by means of ultrafiltration due to the presence of 5–10 kDa colloids. However, distributions In China, particulate and colloidal As has been intensively inves- of particulate and colloidal As in other inland basins and river del- tigated in the Hetao basin (Gong et al., 2006; Guo et al., 2009, tas are still unknown and remain to be discovered. 2011b), although no related data are available in other inland ba- sins and the river delta. Investigation of 583 groundwater samples from 120 wells in the Hetao basin of Inner Mongolia shows that 4.5. Dynamic variation in As concentration approximately 35% of total As in well water is present as particu- late As, which does not pass through 0.45 lm filters (Gong et al., In the As-affected areas, groundwater As concentrations are 2006). However, they found substantially less particulate As quite patchy on a local or regional scale both vertically and

Fig. 4. Box–Whisker plots (a) and distribution (b) of As(III)/total As ratios in groundwaters of the Datong basin, the Hetao basin, the Huhhot basin, the Yinchuan basin, and the Pearl river delta. 210 H. Guo et al. / Applied Geochemistry 41 (2014) 196–217 horizontally (Guo et al., 2012; Deng et al., 2009; He et al., 2009; 2003), the Yinchuan basin (Han et al., 2013), and the Pearl river Han et al., 2013; Xie et al., 2009a; Smedley et al., 2003). In local delta (Wang et al., 2012). or regional groundwater flow systems, spatial variations in As con- The lowest As contents were observed in the Yinchuan sedi- centration may lead to temporal dynamics of As concentrations ments with the range between 3.7 and 18.3 mg/kg and an average (Guo et al., 2013b). In the Hetao basin, groundwater As mainly of 6.66 mg/kg, followed by the Huhhot sediments (Fig. 5). The low- ranges between <1.0 and 1000 lg/L over distance from tens of me- est As contents in the aquifer sediments would be associated with ters to kilometers. A small set of samples (n = 23), obtained in July/ the low groundwater As concentration in the Yinchuan basin. It August and November 2006 in the low-lying plain with surface indicates that, in the similar conditions, groundwater As would water irrigation, show that shallow groundwater generally dis- be dependent of sediment As. In the Datong basin, high As ground- plays higher As concentration in November than in July, although water mostly occurs in the eastern side of the Sanggan river (Guo a larger set of groundwater samples (n = 30), annually collected et al., 2003; Xie et al., 2009a), where the granite has high As con- in July/August from 2006 to 2010, have showed no significant tents up to 12.4 mg/kg in the Hengshan Mountain (Wang, 1998), change in As concentration (Guo et al., 2013c). They believe that while low As groundwater in the left side of the Sanggan river, the higher As concentration is associated with the higher water le- where As contents range between 0.8 and 3.4 mg/kg in the lime- vel in November, during which more reducing condition induced stone and the sandstone and shale in the Hongtao Mountian by flooding irrigation leads to more As release via local reductive (Guo, 2002). dissolution of Fe oxides. Further investigation has been carried Although groundwater As concentrations are generally higher out based on monthly monitoring of shallow groundwaters, illus- in the Datong basin than in the Hetao basin, the Datong aquifer trating temporal evolution of groundwater As approximately along sediments normally have lower As contents with the range be- the flow path at depths between 5 and 20 m (Guo et al., 2013b). It tween 4.9 and 26.8 mg/kg and an average of 11.8 mg/kg than the indicates that high As groundwaters have slight increasing trends Hetao basin sediments with the range between 7.26 and at depths around 20 m and 15 m. However, groundwater As keeps 73.3 mg/kg and an average of 18.9 mg/kg. Moreover, the Pearl river relatively constant at depths <10 m, where low As groundwaters are delta has the highest sediment As contents with the range between present. It is suggested that variations in groundwater As are not 5.0 and 39.6 mg/kg and an average of 17.8 mg/kg (Fig. 5), but the only controlled by redox potentials (or groundwater level), but also lowest groundwater As concentration. Therefore, groundwater As by the recharge of neighboring groundwater (Guo et al., 2013b). concentrations do not necessarily depend on As contents in the In the Yinchuan basin, around four-year monitoring data show aquifer sediment (Fendorf et al., 2010; Harvey, 2008). The bias of that low As groundwater in deep aquifer (around 80 m) has no sig- abnormal groundwater As concentration from the sediment As nificant temporal variation in As concentration, while evident vari- content would be the result of redox processes, microbe-related ations have been observed in shallow groundwaters (<30 m) with reduction, and desorption processes occurring in the aquifer sys- high As concentrations (>70 lg/L) (Han et al., 2013). The increase tems, which are discussed later. in water level is not always connected to the increase in ground- Good relationships between As and Fe/Mn have been observed water As concentrations. They have proposed that infiltration of in the aquifer sediments (Fig. 6). It indicates that As would be asso- irrigation water with more labile carbon would induce reducing ciated with Fe/Mn oxide minerals in the aquifers, which usually conditions and increase As concentration during 2008 to 2009, have strong affinity for both As(III) and As(V) (Goldberg and and suggested that longer-term observation should be needed to Johnstonm, 2001; Mohan and Pittman, 2007). This is confirmed confirm variation trends. by sequential leaching experiments (Guo et al., 2008a; Deng In the Datong basin with intensive extraction of groundwater et al., 2011; Xie et al., 2009b). Guo et al. (2008a) show that As is for irrigation over decades, leaching of irrigation return flow, halite mostly bound to Fe/Mn oxides in aquifer sediments of the Hetao dissolution, and evaporation have been recognized using environ- basin, although Fe/Mn oxide-bound As is generally higher in ment isotopes (d18O and d2H) and Cl/Br ratios in groundwaters (Xie et al., 2012). They found that a group of groundwaters with a significant increase in Cl concentrations and less increase in d18O values mostly have high As concentration (>10 lg/L), while those groundwaters showing lateral mixing and/or evaporation generally have low As concentrations (<10 lg/L). It is suggested that irrigation return flow would predominately promote As mobi- lization, although no temporal variation data are available to sup- port this hypothesis.

5. Mobilization of As in aquifers

5.1. Sources of arsenic

Groundwater As is generally geogenic, which mainly comes from aquifer sediments (Guo et al., 2008a; Wang et al., 2009). High As groundwater generally occurs in aquifers, where sediments do not contain high As contents (Ravenscroft et al., 2009; Fendorf et al., 2010). However, the ultimate source of groundwater As may come from the aquifer sediments, which would release solid As into groundwater by means of (bio)geochemical processes. Chemical data of sediments are used for evaluating the difference in sediment chemistry in the Datong basin (Xie et al., 2009a), the Fig. 5. Box–Whisker plots of As contents in aquifer sediments of the Datong basin, Hetao basin (Guo et al., 2008a), the Huhhot basin (Smedley et al., the Huhhot basin, the Hetao basin, the Yinchuan basin, and the Pearl river delta. H. Guo et al. / Applied Geochemistry 41 (2014) 196–217 211

Fig. 6. Arsenic contents vs. Fe2O3 (a) and Mn (b) in aquifer sediments of the Datong basin, the Huhhot basin, the Hetao basin, the Yinchuan basin, and the Pearl river delta.

low-As groundwater aquifers than in high-As groundwater Mn oxides is not directly related to high As concentration in the aquifers. Arsenic strongly adsorbed on and co-precipitated with groundwater systems (McArthur et al., 2004). amorphous Fe oxyhydroxides accounts for 35% and 20% the total Under redox conditions, with As(V) reduction predominated, As contents in the Hetao sediments, respectively (Deng et al., both dissolved As(V) and adsorbed As(V) are expected to be re- 2011). Although around 10% sediment As is extracted from pyrite, duced to As(III) (Mukherjee et al., 2012). The reduction of adsorbed it is unknown whether this fraction is the sink of groundwater As As(V) results in the release of solid As into groundwater, due to the in the authigenic pyrite. Extraction of 12 sediment samples in lower affinity of As(III) species to mineral surfaces compared to the the Huhhot basin using 0.2 M acid ammonium oxalate solution As(V) species (Guo et al., 2007b; Manning et al., 1998). The reduc- shows that up to 30% of sediment As is oxalate-extractable and tive desorption of As increases groundwater As(III) concentration considered as amorphous Fe oxyhydroxide-bound As (Smedley (Smedley and Kinniburgh, 2002), and leads to the fact that As(III) et al., 2003). In aquifer sediments of the Datong basin, it has been is the predominant As species in the high As groundwaters of the found that Fe oxyhydroxides may be the major source of As in the inland basins and the river deltas. aquifer, which is based on the fact that concentration of NH2OH– Besides, relatively high concentrations of Fe(II) in high-As HCl extracted Fe is strongly correlated with that of extracted As groundwaters indicate that reductive dissolution of Fe(III) oxyhy- (Xie et al., 2009b). Further investigation has found the good corre- droxides universally occurs in the reducing aquifers. During reduc- lation between As concentrations and two magnetic parameters tive dissolution, As adsorbed onto Fe(III)-oxyhydroxides is released (i.e., saturated isothermal remnant magnetization (SIRM) and iso- (Nickson et al., 1998; Islam et al., 2004). This is the major mecha- therm remnant magnetization (IRM)), showing that the ferrimag- nism for genesis of high As groundwater in reducing aquifers. Iro- netic minerals including maghemite and haematite are the n(II) produced by Fe(III) reduction would like to be adsorbed on the dominant carriers for As (Xie et al., 2009c). residual Fe-oxyhydroxides if being present (Appelo et al., 2002; van Geen et al., 2004; Handler et al., 2009), or combined with 2 5.2. Redox processes S and HCO3 to form pyrite and siderite (Guo et al., 2013b). Iron isotope study in the Hetao groundwater identifies three pathways As mentioned above, dissolved organic carbon (DOC) is the trig- for Fe and As cycling (Guo et al., 2013a). Light d56Fe values and high ger for the formation of reducing conditions. High concentrations As concentrations reflect the occurrence of Fe(III) oxide reduction of DOC provide electrons for electron acceptors, which leads to in anoxic conditions. In strongly reducing conditions, precipitation 2 reduction in the sequence NO3 , Mn(IV), As(V), Fe(III) and SO4 of isotopically light Fe-pyrite and/or siderite following Fe-oxide (Stumm and Morgan, 1996; Mukherjee et al., 2012). Most of these reduction results in enrichment of isotopically heavy Fe in ground- species are reduced within the sedimentary aquifers in both the in- water with both high and low As concentrations. Re-adsorption of þ land basins and the river delta. Coexistence of NO3 and NH4 has Fe(II) following Fe(III) reduction leads to further enrichment of iso- been observed in the Hetao basin (Guo et al., 2011a; Deng et al., topically light Fe and relative low As concentrations in anoxic–sub- 2009), the Huhhot basin (Smedley et al., 2003), the Yinchuan basin oxic conditions (Guo et al., 2013a). The mixed effect of those (Han et al., 2013; Guo and Guo, 2013), the Datong basin (Xie et al., pathways controls As and Fe cycling in groundwater systems, 2009a), and the Pearl river delta (Wang et al., 2012), showing that which is most likely the cause of the fact that dissolved Fe concen- NO3 reduction usually occurs. The fact that groundwaters with trations are poorly correlated with dissolved As concentrations NO3 greater than 10 mg/L generally have low As concentration (Guo et al., 2008a; Deng et al., 2009; Xie et al., 2009a; Han et al., suggests that As should be readily bound to Fe(III) oxides in the 2013; Wang et al., 2012). In the Pearl river delta, Wang et al. sediments in NO3 -stable environment. As the reducing sequence (2012) observed the presence of pyrite in the aquifer sediments, enters Mn(IV) reduction, dissolved Mn concentration spans a large and suggested that coprecipitation of As with authigenic pyrite range between 20 and 72,000 lg/L in the high-As groundwater would significantly scavenge aquatic As in the coastal aquifer– areas, due to roles of Fe-oxyhydroxides in the fate of reduced Mn aquitard system. In addition to authigenic pyrite, siderite precipi- (Stüben et al., 2003). Although reduction of Mn-oxyhydroxides tation would also eliminate As from groundwater in the Hetao would lead to the release of the adsorbed As, As concentrations groundwater (Guo et al., 2013b). 2 are generally less than 30 lg/L in groundwaters with Mn concen- As redox potential further decreases, SO4 reduction would pre- tration >150 lg/L in the Hetao basin (Guo et al., 2012). In the Pearl dominate with the production of S2. Sulfide has been detected in river delta, groundwaters with Mn <700 lg/L have As concentra- most of groundwater samples in the Hetao basin (Guo et al., 2010, tion less than 50 lg/L (Wang et al., 2012). Therefore, reduction of 2011a; Deng et al., 2009), the Datong basin (Xie et al., 2009a), and 212 H. Guo et al. / Applied Geochemistry 41 (2014) 196–217 the Huhhot basin (Smedley et al., 2003). In the Hetao basin, molar As(V)-reducing bacteria do not directly convert the As-hosting 2 34 ratios of SO4 to Cl show a negative correlation with d SSO4 values minerals (such as Fe/Mn oxides/oxyhydroxides), they may catalyt- in groundwaters from moderate flow-reducing zone and low flow- ically reduce adsorbed As(V) to As(III), and consequently lead to 2 reducing zone, indicating that bacterial reduction of SO4 occurs in the reductive desorption of As because the lower adsorption affin- the aquifers (Guo et al., 2011a). Although pyrite precipitation may ity of As(III) to oxide minerals has been known than that of As(V) at occur in the aquifer, high As concentration in groundwaters being near-neutral pH (Dzombak and Morel, 1990; Manning et al., 1998). oversaturated with respect to pyrite is possibly due to the greater Additionally, a SO4-respiring archaea, Thermoprotei, has been amount of As released from Fe(III)-oxide reduction than that sub- identified in high As groundwater of the Hetao basin, although bac- sequently eliminated by pyrite precipitation (Guo et al., 2013a). terial diversity is lower in high-As groundwater than low-As In the Datong basin, Xie et al. (2009a) also found that the high groundwater (Li et al., 2013). Thermoprotei has been known to 34 2 2 d SSO4 values correspond to low SO4 concentration and high As use SO4 as the electron acceptor and H2 the electron donor (Chag- 2 2 concentrations, and suggested that SO4 reduction accompanies anti et al., 2012). Reduction of SO4 may lead to elimination of As reduction and mobilization. In the recent study using multiple solution As by means of As sulfide precipitation or coprecipitation isotope (O, S and C) approach, they observed that higher As con- of As with sulfides, which play a role in As cycling in the aquifers centrations occur in aquifers with reduction of Fe-oxyhydroxides (Guo et al., 2013a). 2 than with reduction of both Fe-oxyhydroxides and SO4 (Xie In the Datong basin, indigenous bacterium Bacillus cereus has et al., 2013). They suggest that, in this reducing condition, more been isolated from high As aquifer sediments, which is capable As would be released from reduction of Fe-oxyhydroxides than of mobilizing sediment As, Fe, Mn, and Al (Xie et al., 2011). what would be removed from groundwater during sulfide Although they do not report its 16S rRNA gene clone data, the bac- precipitation. terium may mediate the reduction of Fe oxides, and facilitate the The above-mentioned redox processes are coupled with the oxi- release of As from sediments into groundwater. In the incubation dation of a large amount of DOC. Guo et al. (2011b) showed that experiments on a sandy loam sample with the addition of Bacillus about 30% of As is associated with organic colloids with molecular cereus and labile carbon (glucose and sodium acetate), up to 5.7% of weights between 5 and 30 kDa. Decomposition of the low-molecu- sediment As has been released (bulk As 11.7 mg/kg) (Xie et al., lar weight organic matter would also release the colloid-bound As 2011). For a grey fine silt sample taken at depth of 12 m, about into groundwater. However, the cycling of colloidal As still needs 10% of sediment As has been released in the microcosms using to be further investigated during redox processes. As-resistant strains as inoculated strains, and glucose and sodium acetate as carbon sources (Duan et al., 2009). Another bacterium, 5.3. Roles of microbes in As mobilization Bacillus sp., has also been isolated from the high As soil in the Da- tong basin, which is an aerobic As(V)-respiring bacterium (Wu Microbes, which are fueled by organic matter in the aquifers et al., 2012). The strain effectively reduces aqueous As(V) to and catalyze the formation of favorable reducing conditions, play As(III) under aerobic conditions, and enhances As mobility in the an important role in As mobilization in both inland basins (Guo system. et al., 2008b; Xie et al., 2011) and river deltas (Islam et al., 2004; van Geen et al., 2004; Sultana et al., 2011; Dhar et al., 2011). 5.4. Competitive desorption Although no microbe-related data are available in other sites, mi- crobes and their roles in groundwaters and/or sediments have Groundwater pH values are generally high in both inland basins been documented in the literatures in the Hetao basin (Guo and the river deltas, with the range between 7.2 and 8.7 (mean 8.0) et al., 2008b; Li et al., 2013; Jiang et al., 2013) and the Datong basin in the Datong basin, between 7.1 and 8.9 (mean 8.1) in the Hetao (Duan et al., 2009; Xie et al., 2011; Wu et al., 2012). basin, between 6.8 and 8.7 (mean 7.6) in the Huhhot basin, be-

Most probable number (MNP) shows that NO3-respiring bacteria tween 7.5 and 8.9 (mean 7.9) in the Yinchuan basin, and between (NRB), iron-respiring bacteria (IRB), and SO4-respiring bacteria (SRB) 5.7 and 9.5 (mean 7.6) in the Pearl river delta, showing that high As are universally present in the As-affected aquifers of the Hetao basin, groundwaters are mostly neutral-weak alkaline. As the major As although the dominant microorganisms are grouped into different carrier in the aquifer sediments, Fe(III) oxyhydroxides are predom- microbial groups in different sediments (Guo et al., 2008b). In the inantly positively charged at pH 3.07.0 and negatively charged at microcosm experiments, a larger amount of As has been released pH 8.010.0 (Dixit and Hering, 2003; Zhang et al., 2004). As pH in- in the batches amended with glucose (0.71–3.81 mg/kg) than in creases, both As(V) species and the surface of Fe oxyhydroxides be- the unamended batches (0.03–0.30 mg/kg), which accounts for come more negatively charged, resulting in the electrostatic 60–70% of As bound to Fe/Mn oxides in the sediments (Guo et al., repulsion. Therefore, the increase in solution pH > 8.2 drastically 2008b). The IRB presence would lead to dissimilatory reductive dis- decreases As adsorption on Fe-oxyhydroxides (Mamindy-Pajany solution of Fe(III)-oxides/oxyhydroxides. Sediment incubation et al., 2011; Qiao et al., 2012), and consequently leads to As desorp- shows that As is released into aqueous solutions along with the in- tion due to the less abundance of positively charged adsorption crease in dissolved Fe concentrations in the batches amended with sites. This can explain the fact that high pH groundwater normally glucose (Guo et al., 2008b). However, Jiang et al. (2013) did not find has high As concentration in the same hydrogeological units of the Fe reducing bacteria strains by means of DGGE and 16S rRNA in the Hetao basin (Guo et al., 2011a) and the Datong basin (Guo et al., Hetao sediments. Although Li et al. (2013) suggest that 2003). Acinetobacter, Brevundimonas, Thermoprotei, Geobacter and Methan- Additionally, high As groundwaters usually have high HCO3 osaeta would be associated with high concentrations of As, methane concentrations with the range between 198 and 1300 mg/L (mean 2 and Fe(II) and low concentrations of SO4 and NO3 in groundwaters, 484 mg/L) in the Datong basin, between 191 and 1450 mg/L (mean they did not observe Fe reducing bacteria strains in groundwaters. 606 mg/L) in the Hetao basin, between 72 and 1150 mg/L (mean Thiobacillus strain has been found to be the predominated bac- 388 mg/L) in the Huhhot basin, between 198 and 927 mg/L (mean terial populations in the Hetao aquifer sediments, which can use 523 mg/L) in the Yinchuan basin, and between 216 and 3590 mg/L As(V) and NO3 as the terminal electron acceptors (Jiang et al., (mean 827 mg/L) in the Pearl river delta. On the one hand, the 2013). In addition, Wu et al. (2012) have isolated an aerobic presence of HCO3 would alter the surface charge properties of Fe As(V)-reducing bacterium, Pantoea, which has effective As oxides via inner-sphere monodentate mononuclear surface species (V)-reducing capacity under aerobic conditions. Although these (Su and Suarez, 1997). On the other hand, HCO3 readily competes H. Guo et al. / Applied Geochemistry 41 (2014) 196–217 213 for adsorption sites on the haematite at concentration >61 mg/L, the National Key Basic Research Development Program (973 Pro- although low concentrations of HCO3 (<6.1 mg/L) enhance As(V) gram, No. 2010CB428804), the Geological Survey Program of China adsorption (Arai et al., 2004). Experiments show that As adsorption Geological Survey (No. 12120113103700), the Fundamental Re- on the activated natural siderite (mostly haematite) decreases by search Funds for the Central Universities (No. 2652013028), and 6.7% when HCO3 concentration increases from 61 to 3050 mg/L the Fok Ying-Tung Education Foundation, China (Grant No. (Zhao and Guo, 2013). Although other coexisting anions, such as 131017). Authors would like to thanks Jimmy J Jiao (Hongkong 2 3 NO3 ,SO4 and PO4 have also been found in high As groundwaters, University) for advising us to initiate this work. they may play a minimal role in affecting As concentrations due to their low concentrations in the inland basins and the river delta investigated. Whatever, the relative contribution of competitive References desorption and reductive dissolution of Fe oxides to high As con- centrations in groundwaters of different areas is still unknown. Agricultural Engineering Research Center, 2008. 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