S C I E N C E O F T H E T O T A L E N V I R O N M E N T 3 9 3 ( 2 0 0 8 ) 1 3 1 – 1 4 4

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

Huaming Guo⁎, Suzhen Yang, Xiaohui Tang, Yuan Li, Zhaoli ShenSchool of Water Resources and Environment, China University of Geosciences, Beijing 100083, PR China

A R T I C L E I N F O

⁎ Corresponding author. Tel.: +86 10 8232 067E-mail address: hmguo@cugb.edu.cn (H. Gu

0048-9697/$ – see front matter © 2007 Elsevidoi:10.1016/j.scitotenv.2007.12.025

A B S T R A C T

Article history:Received 17 October 2007Received in revised form14 December 2007Accepted 17 December 2007Available online 30 January 2008

Arsenic concentrations in shallow groundwaters from the Hetao Basin of Inner Mongoliarange between 0.6 and 572 μg/L. High As groundwaters generally occur in the shallowalluvial–lacustrine aquifers, which aremainly composed of black (or dark grey) fine sands ina reducing environment. They are characterized by high concentrations of dissolved Fe, Mn,HCO3

−, P and S2−, and low concentrations of NO3− and SO4

2−. Low SO42− coupled with high S2−

suggests that SO42− reduction has been an active process. In the reducing groundwaters,

inorganic As(III) accounts for around 75% of total dissolved As. Total As contents in thesediments from three representative boreholes are observed to be 7.3–73.3mg/kg (average of18.9 mg/kg). The total As is mildly-strongly correlated with total Fe and total Mn, while aquite weak correlation exists between total As and total S, suggesting that the As isassociated with Fe–Mn oxides, rather than sulfides in the sediments. It is found in thesequential extraction that chemically active As is mainly bound to Fe–Mn oxides, up to3500 μg/kg. The mobilization of As under reducing conditions is believed to includereductive dissolution of Fe–Mn oxides and reduction of adsorbed As. Although exchangeableAs is labile and very vulnerable to hydrogeochemical condition, the contribution is relativelylimited due to the low concentrations. The competition between As and other anions (suchas HPO4

2−) for binding sites on Fe–Mn oxides may also give rise to the release of As intogroundwater. Slow groundwater movement helps accumulation of the released As in thegroundwaters.

© 2007 Elsevier B.V. All rights reserved.

Keywords:Reductive environmentReductive dissolutionReleaseSequential extraction

1. Introduction

Arsenic, which is ubiquitous in the environment, has beenrecognized as one of the most widespread and problematicwater contaminants (Nickson et al., 1998; Smedley andKinniburgh, 2002; Berg et al., 2007). Health problems asso-ciated with groundwater As have been documented in manyparts of the world, including Bangladesh, India, China, Mexico,Hungary, Vietnam, Argentina and Chile (Smedley and Kinni-burgh, 2002; Guo et al., 2003). Symptoms of chronic exposure toAs in drinking water at concentrations significantly above

9; fax: +86 10 8232 1081.o).

er B.V. All rights reserved

50 μg/L are numerous and often severe and commonly includeskin diseases (pigmentation, dermal hyperkeratosis, skincancer), many other cardiovascular, neurological, hematolo-gical, renal and respiratory diseases, as well as lung, bladder,liver, kidney and prostate cancers (Morton and Dunette, 1994;Das et al., 1995; Smith et al., 1998). The World HealthOrganization has set a provisional guideline limit of 10 µg/Lfor As in drinking water (World Health Organization, 1996)which was subsequently adopted by the European Union(European Commission, 1998) and the United States (EPAOffice of Groundwater and Drinking Water, 2002), while many

.

132 S C I E N C E O F T H E T O T A L E N V I R O N M E N T 3 9 3 ( 2 0 0 8 ) 1 3 1 – 1 4 4

developing countries still adopt the value of 50 μg/L as theirnational standard. Since 2007, China has lowered the drinkingwater standard for As to 10 µg/L. Some areas in NorthwestChina, especially in Xinjiang, Inner Mongolia, and Shanxi,which have many wells with As concentrations above thislimit, have been facing big challenges in finding alternativewater resources and/or remediating high As water in a cost-effective way.

In the early 1980s, the cases of As poisoning were firstlyrecognised in Xinjiang Province. Wang (1984) found Asconcentrations up to 1200 μg/L in groundwaters. Luo et al.(2006) reported As concentrations of between 70 and 750 μg/Lin deep artesian groundwaters (N200 m in depth) in the southpart of the Zhunge'er Basin to the north of the TianshanMountains. Arsenic-affected areas mainly distribute betweenAibi Lake (in the southwest of the Zhunge'er Basin) andManasi River, paralleling the Tianshan Mountain (Wang et al.,2002). Shallow groundwaters had As concentrations betweenb10 and 68 μg/L. Artesian As groundwater has been used fordrinking in the region since the 1960s, and subsequent chronichealth problems have been identified (Wang andHuang, 1994).

Endemic arsenism was firstly confirmed in the DatongBasin, Shanxi Province, in the early 1990s. It happened afterlocal residents changed their drinking water resources fromdug wells (b10 m in depth) to tube wells (between 20 and 40 min depth) with hand pump in the middle of 1980s. Wang et al.(1998) found that As concentrations ranged between 2.0 and1300 μg/L in groundwaters. A recent investigation showed that54.4% of the tested 3083 wells exceeded 50 μg/L (Wang et al.,2003b). High As groundwater was characterized by high pHbetween 7.09 and 8.71 (mean 8.19, n=15), high HPO4

2− contentsup to 12.70 mg/L (mean 1.45 mg/L, n=15), low SO4

2− concentra-tions generally less than 2.0 mg/L (Wang et al., 2004). Theaffected groundwater was found in alluvial–lacustrine aqui-fers containing relatively high organic matter (up to 1.0%organic carbon). Arsenic was mainly present as inorganic As(III), accounting for 55–66% of total As, in the reductiveaquifers (Guo et al., 2003).

Fig. 1 –Location of the Hetao

In Inner Mongolia, concentrations of As in excess of 50 μg/Lhave been found in groundwaters from aquifers in Keshenke-teng County, the Hetao Basin, and the Tumote Basin (Sun,1994; Luo et al., 1997). The As-affected areas covered about3000 km2 with more than 1 million people being at risk. Morethan 400,000 people were drinking high As groundwater(N50 µg/L) with 3000 confirmed arsenicosis patients in 776villages. Arsenic-related diseases have been identified by Maet al. (1995), including lung, skin and bladder cancer as well asprevalent keratosis and skin-pigmentation problems. Ground-water As in the Keshenketeng County was introduced byarsenopyrite mining, while in the basins of Hetao and Tumoteit was generally believed to occur naturally in Holocenealluvial–lacustrine aquifers (Ma et al., 1995; Tang et al., 1996;Smedley et al., 2003). In the Huhhot Basin, high As concentra-tions up to 1500 μg/L were found in the groundwaters, with 60–90% present as As(III), due to highly reducing conditions(Smedley et al., 2003; Wang et al., 2003a). The problem wasworst in the lowest-lying parts of the basin. Groundwaters insome shallow dug wells also had relatively high As concen-trations (up to 556 μg/L). Saline groundwaters were found inthe shallow aquifers in parts of the region as a result ofevaporative concentration and many had high F− concentra-tions, although the F− did not generally correlate with As(Smedley et al., 2003).

In theHetaoBasin,As concentrations rangedbetween1.1and969 μg/L in shallow groundwaters, with a significant proportion(up to 90%) of the As occurring as As(III) (Tang et al., 1996). Li andLi (1994) and Tang et al. (1996) proposed that groundwater Asoccurred naturally under reducing conditions. Contrastingly,Zhang et al. (2002) suggested that the As in groundwater of theHetao Basinwas released from higher elevations, wheremininghad been carried out for a long time, and was then transportedfrom the mining district downgradient.

Although the groundwater As problems and the severehealth effects have been identified in several provinces fromChina, many of the As-affected groundwater areas have beenpoorly documented. Especially in the Hetao Basin, the causes

Basin, Inner Mongolia.

133S C I E N C E O F T H E T O T A L E N V I R O N M E N T 3 9 3 ( 2 0 0 8 ) 1 3 1 – 1 4 4

of the high As groundwater are not well understood. Thispaper delineates the hydrogeochemistry of the shallowaquifers in the Hetao Basin of Inner Mongolia. The mainobjectives are to 1) characterize the hydrogeochemical settingof the area; 2) evaluate the nature and the extent of Asenrichment in groundwaters; 3) assess the main geochemicalfactors controlling As mobilization.

2. Geological andhydrogeological characteristics

2.1. Geology and stratigraphy

The basin is one of the Cenozoic rift basins, lying in thewestern part of Inner Mongolia, on the northern margins ofthe Yellow River, and on the eastern fringe of the WuranbuhDesert (Fig. 1). Kidney-shaped, it lies between the Yellow Riverand the Langshan Mountains to the north, with an area ofabout 10,000 km2. The alluvial basin has a gentle SE slope withelevation varying between around 1060 and 1007 m.

The Langshan Mountains are mainly composed of ametamorphic complex (slate, gneiss and marble), generallyof Jurassic to Cretaceous age, which is folded and fractured (Liand Li, 1994). The basin is also fault-bounded (Zhang et al.,1995). Sedimentary environment, palaeo-geography andlithology were affected by both paleo-climate and tectonicmovement. During the Tertiary period, red (or deep brown)sediments occurred in an oxidative environment whichaccumulated great amounts of salinity (such as gypsum andcalcite). In the Quaternary period, due to the closed geologicstructure and continuous tectonic subsidence, inland lacus-trine sediments with fine clast had locally been deposited andthick Mid-Cenozoic sedimentary formation had developed.The thickness of the sediment in the southeast of the basinranges from 500 to 1500 m, and in the northwest of the basinfrom 7000 to 8000 m (Tang et al., 1996).

Quaternary sediments are mainly derived from the Lang-shan Mountains and partly from fluvial deposits of the YellowRiver (having developed in the middle Pliocene, Li and Yang,1991), which aremostly represented by sand, silt and clay. Thefluvial sediments originating from the old Yellow river weremainly deposited in the southern part of the basin, withalluvial and pluvial sediments in the northern part. Thelacustrine sediments at depths between 10 and 40 m belowland surface (BLS), firstly developing in a former lake at theearly stage of late Pleistocene (around 120 ka BP), usually occurin the northwest and the east of the basin (Li et al., 2005).Locally, the topmost part of the sedimentological sequenceconsists of lacustrine sediments of small lagoons separatedfrom the former lake and alluvial deposits. The sediments areheterogeneous both vertically, with two sedimentologicalsequences from coarse grains to fine grains, and spatiallywith coarse grains in the south and fine grains in the north.

2.2. Hydrogeology

Groundwaters mainly occur in the Quaternary alluvial,alluvial–pluvial and fluvial–lacustrine aquifers. The sedimentsare mainly composed of alluvial–pluvial sand, sandy silt,

lacustrine and fluvial–lacustrine sandy silt, silty clay and clayrich in organic matter in the central part of the basin, fluvialsand and fine sand on the banks of rivers, and alluvial sand inthe fan areas.

The aquifers are generally multi-layered. The aquiferconditions vary from unconfined to leaky-confined in theshallow deposits and confined in the deeper ones. TheQuaternary groundwater systems can be basically dividedinto two classes: the shallow aquifers (upper Pleistocene–Holocene alluvial–pluvial and alluvial–lacustrine sands withina depth of around 100 m) and deeper aquifers (middlePleistocene lacustrine sands occurring at depths greater than100m and separated from the upper water bearing formationsby a thick blanket of clay). The shallow aquifers are furthersubdivided into alluvial–pluvial unconfined aquifers andalluvial–lacustrine leaky-confined aquifers. The alluvial–plu-vial aquifers mainly occur in the belt of alluvial fans and onthe banks of rivers. The groundwater table ranges from 5 to20 m. From the top to the front of the fans, the sedimentsdecrease in grain size, with the specific capacity decreasingfrom 30m3/(hm) to 10m3/(h m). The quality of groundwater isbad at the transition zone between alluvial–pluvial aquifersand alluvial–lacustrine aquifers, with the presence of con-siderable dissolved H2 S and CH4. High As groundwaters aremainly present in this zone. The alluvial–lacustrine aquifersmainly occur at depths between 10 and 100 m, which areusually leaky-confined or semi-confined. The quality of watervaries in different parts of the basin, with total dissolved solid(TDS) between b1 and 10 g/L.

Groundwater is recharged by vertically infiltrating meteo-ric water in the basin and laterally penetrating fracture waterfrom marble, slate and gneiss along the mountain front, aswell as a little leaked water from river and ditches, andirrigation return flow from farmland. It is discharged mainlyvia evapotranspiration and artificial abstraction. Groundwatermovements are controlled by the surface topography which isvery gentle over most of the study area. The general directionof groundwater flow in the northern part is from north tosouth, while that in the southern part is from south to north,but the flow rate is very slow.

The region experiences an arid climate with an averageannual precipitation of 130–220 mm (mainly during July toSeptember) and annual evaporation rates of about 2000–2500 mm. Average annual temperatures range from 5.6 to7.8 °C. There are many ditches used for conducting the riverwater into farmland, some of which were built thousands ofyears ago. The well-irrigated condition has made the regionone of the oldest food-producing areas. Due to the scarcity ofefficient management, over-irrigation has been carried out forseveral decades in the region,which gives rise to 35%of surfacesoils being pickled. Most of lagoons, lakes and wetlands areover-salted.

3. Sampling and analysis

Water sampleswere collected inAugust 2006 from 63 boreholesacross theHetaoBasin. Three tapwater sampleswerealso takenfrom a small central water supply to investigate the drinkingwater quality in the villages where the transformation of water

Table 1 – Chemical compositions of groundwater from shallow aquifers in the Hetao Basin

No. Latitude(°E)

Longitude(°N)

Well depth(m)

Temp(°C)

Eh(mV)

pH Na+

(mg/L)K+

(mg/L)Ca2+

(mg/L)Mg2+

(mg/L)Fe

(mg/L)Mn

(μg/L)AsTot(μg/L)

As(III)(μg/L)

F¯(mg/L)

Cl¯(mg/L)

HCO 3̄

(mg/L)CO3

2¯

(mg/L)NO¯3(mg/L)

SO42¯

(mg/L)S2¯

(mg/L)

1 41.0650 108.3761 15 11.0 −140 7.56 314 2.64 26.9 32.8 0.42 33.0 91.0 91.0 0.99 192 494 b0.3 0.77 164 0.052 41.0662 108.3756 16 11.8 −127 7.81 329 4.07 46.7 75.2 0.01 68.2 48.0 45.7 2.92 320 468 b0.3 43.8 340 0.053 41.0714 108.4021 15 12.1 −70 7.48 174 4.47 178 95.7 0.10 876 29.4 29.9 0.13 370 403 b0.3 6.6 307 0.044 41.0778 108.3590 13 12.0 25 7.45 289 5.90 142 82.7 0.01 601 1.17 0.35 0.09 398 488 b0.3 6.9 332 0.035 41.0783 108.3577 12 13.8 79 7.08 447 41.0 167 140 0.01 810 0.58 0.47 0.48 571 663 b0.3 108 394 0.046 41.0390 108.3830 9 13.5 −68 7.49 928 1.11 11.6 10.2 0.17 44.8 128 101 1.66 1122 699 b0.3 3.2 76.8 0.037 41.1261 108.8101 18 9.8 −34 7.58 208 3.13 78.8 52.0 0.11 132 9.57 7.70 0.11 270 486 b0.3 4.3 117 b .018 41.1223 107.7822 13 11.2 −134 7.88 1657 3.50 26.4 104 1.51 25.6 299 202 0.46 1963 1385 b0.3 5.6 738 0.069 41.1103 107.7703 20 10.4 −128 8.04 1157 2.10 21.5 62.4 1.08 24.4 417 355 1.09 1538 1193 b0.3 0.01 0.23 0.0610 41.1132 107.7649 17 10.1 −133 8.25 968 5.46 14.5 36.8 0.48 20.6 224 191 2.64 697 1261 16.3 1.7 274 0.0811 41.1289 107.8652 20 11.4 −40 7.36 333 12.7 113 90.5 0.19 112 51.7 42.0 0.13 337 678 b0.3 0.01 427 0.0312 41.1214 107.8702 15 10.8 −126 7.54 226 4.11 134 82.0 0.02 699 50.2 50.2 0.12 332 556 b0.3 0.01 256 0.0213 41.1312 107.8785 12 10.9 63 7.40 265 5.71 135 99.1 0.05 536 1.87 0.70 0.16 347 596 b0.3 0.01 373 b .0114 41.1331 107.8659 24 11.3 −105 7.45 389 4.96 68.7 58.3 0.64 46.6 45.8 45.5 0.17 219 760 b0.3 1.4 330 0.0415 41.1349 107.8438 25 9.9 −119 7.40 719 5.37 176 154 3.87 116 52.0 49.6 0.67 1661 563 b0.3 0.01 224 0.0116 41.1349 107.8438 12 10.5 −80 7.45 553 7.89 111 101 1.85 172 5.48 3.20 0.42 985 525 b0.3 0.01 74.1 b0.0117 41.0654 107.9406 13 12.8 36 7.26 311 4.46 128 97.2 0.66 707 6.42 4.90 0.06 306 668 b0.3 0.01 455 0.0318 41.0732 108.0514 13 10.6 −113 7.54 74.4 2.38 56.0 27.2 1.02 309 7.58 6.42 0.30 72.8 236 b0.3 0.01 108 0.0119 41.1634 108.1793 5 10.8 −13 7.44 1100 4.57 67.1 156 0.39 22.5 0.93 0.35 0.08 1389 811 b0.3 11.4 675 0.0320 41.1855 108.0634 15 11.4 −113 7.36 170 3.89 92.2 56.8 2.55 303 33.4 26.4 0.21 180 455 b0.3 2.5 199 b .0121 41.1797 108.0664 20 12.0 −119 7.31 590 7.55 148 213 5.90 424 39.7 32.0 0.58 1354 617 b0.3 0.01 72.6 0.0122 41.1617 108.1701 28 11.4 −117 7.54 557 4.91 76.9 139 1.11 93.6 25.3 19.8 0.11 1093 392 b0.3 0.01 151 b .0123 41.0985 108.3295 23 14.0 −113 7.39 131 3.35 105 49.2 2.81 534 37.0 31.4 0.18 144 472 b0.3 0.25 213 b .0124 41.0922 108.4413 13 11.3 −31 7.43 107 5.10 93.9 46.7 0.23 301 1.75 1.28 0.13 132 478 b0.3 0.01 59.0 0.0625 41.1221 108.5347 9 12.4 98 7.26 212 5.72 165 84.4 0.01 504 1.05 0.35 0.35 330 467 b0.3 0.01 262 0.0226 41.0477 108.289 11 12.4 −115 7.69 135 2.31 83.0 73.0 0.01 95.5 12.6 12.5 0.48 87.3 348 b0.3 449 80.0 0.0127 40.9946 108.2874 18 11.8 −98 7.72 288 2.96 45.0 66.9 1.42 46.7 145 144 0.55 186 601 b0.3 1.9 254 0.0228 41.0337 108.1447 18 11.1 −147 7.49 80.5 2.86 84.0 60.4 1.62 158 40.8 29.2 0.21 114 390 b0.3 2.5 131 0.0429 40.9970 108.0601 20 11.1 −95 7.35 873 6.74 84.8 108 0.15 206 16.5 15.2 0.12 913 877 b0.3 0.01 469 0.06

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30 40.9103 107.8027 13 12.0 52 7.78 195 3.78 38.3 49.8 0.05 16.9 1.63 0.35 0.61 163 527 b0.3 0.02 131 0.0631 40.9786 107.7882 13 10.9 −86 7.33 345 6.22 176 117 2.92 274 29.3 22.6 0.08 489 542 b0.3 0.01 526 0.0532 40.9672 107.0087 20 11.3 5 8.06 314 5.68 19.7 27.3 0.64 39.5 482 428 1.31 233 618 b0.3 0.01 0.03 0.0733 40.9551 106.961 30 11.7 −30 7.73 192 9.81 38.5 72.6 0.14 86.3 4.20 3.62 1.04 328 385 b0.3 0.01 74.0 b .0134 40.9552 106.9608 18 12.3 −123 7.66 901 7.18 46.9 99.4 2.14 66.6 102 77.6 1.08 921 748 b0.3 0.01 598 b .0135 40.9202 106.9507 17 11.3 −153 8.01 189 4.28 18.3 22.6 0.74 116 348 305 1.40 44.6 556 b0.3 0.02 0.01 b .0136 40.9995 107.0313 25 10.9 −113 7.65 368 7.13 58.4 96.2 1.96 98.1 78.2 12.3 0.80 413 689 b0.3 0.01 214 b .0137 41.0488 107.1144 18 12.3 −99 7.30 963 7.19 103 211 3.49 247 278 175 7.87 969 968 b0.3 4.9 1052 0.0438 41.0479 107.1899 18 11.9 −113 7.76 2662 9.85 56.4 240 0.11 30.8 265 120 0.83 3901 1608 b0.3 0.01 395 b .0139 41.0032 107.1761 35 17.5 72 7.47 113 7.75 83.6 33.3 0.07 1.30 10.6 0.70 1.58 163 203 b0.3 22.5 161 b .0140 41.0035 107.1758 20 12.1 −55 7.01 1240 53.5 256 297 1.71 1274 3.27 3.15 1.44 1815 1085 b0.3 6.9 1055 0.0441 40.9623 107.1464 20 11.6 −76 7.03 1754 6.16 132 364 1.76 445 1.98 1.40 5.62 2885 711 b0.3 0.01 1303 b .0142 40.9189 107.1712 17 11.1 −55 7.90 670 5.03 27.4 46.1 0.67 75.2 20.9 19.6 1.43 458 897 b0.3 0.01 279 0.0343 40.9121 107.1129 8 13.2 −45 7.38 785 6.92 143 252 2.56 284 9.45 3.86 0.05 1278 534 b0.3 0.01 1008 0.0544 40.8235 106.8112 33 10.4 −54 7.85 564 6.70 20.6 93.4 0.88 154 67.7 20.4 2.88 779 640 b0.3 5.8 52.6 0.0445 40.8294 106.7985 31 11.0 −64 7.72 1560 10.6 78.8 424 2.05 202 22.2 19.3 2.98 2743 571 b0.3 0.01 1196 0.0446 40.8265 106.6687 30 11.9 −30 7.93 306 3.82 28.1 32.3 0.95 84.4 483 404 1.54 216 660 b0.3 0.01 0.19 0.0547 40.8518 106.7473 38 12.4 −101 7.77 240 6.97 46.1 113 1.79 252 46.9 46.0 1.22 361 350 b0.3 0.01 296 0.0448 40.8604 106.8032 14 11.2 −54 8.17 936 2.90 16.0 41.0 0.42 36.6 125 119 3.55 768 1277 b0.3 0.01 407 0.0649 40.8609 106.8037 80 11.4 −48 7.96 593 3.41 11.5 19.2 0.55 37.9 183 166 2.72 504 854 b0.3 0.01 41.3 0.0550 40.8770 106.832 32 11.3 −53 7.78 691 8.28 34.0 87.2 0.48 81.7 18.2 15.9 1.65 934 642 b0.3 0.01 167 0.0451 40.9111 106.8955 30 14.7 −27 7.76 211 7.95 105 128 1.22 101 363 291 0.49 748 292 b0.3 0.01 0.01 0.0452 40.8764 106.9333 22 12.2 −55 7.59 821 9.07 56.9 126 0.96 141 16.8 13.4 0.08 1155 744 b0.3 0.01 285 0.0253 40.8506 106.9033 13 12.2 −76 7.43 632 2.98 66.0 97.4 0.05 231 1.17 0.58 0.51 482 736 b0.3 2.2 650 0.0454 41.1201 107.2014 12 11.0 −64 7.53 341 8.76 86.4 130 1.99 209 31.2 26.4 0.57 470 642 b0.3 0.01 319 b .0155 41.1528 107.3534 15 10.6 −30 7.86 66.8 3.01 50.7 57.7 0.95 179 45.7 32.1 0.82 136 235 b0.3 0.41 77.2 0.0356 41.0613 107.3013 26 13.1 −101 8.43 311 2.96 8.92 11.3 0.19 6.80 167 121 0.46 148 565 26.4 0.01 7.9 0.1257 41.0274 107.3592 22 11.9 −55 8.12 481 4.32 17.7 65.7 0.84 21.4 81.7 58.3 1.38 352 658 b0.3 0.01 208 0.0558 41.0363 107.3993 26 10.5 −45 8.13 195 2.80 4.26 18.0 0.18 2.40 572 447 2.15 40.5 443 4.65 2.2 14.0 0.0659 41.0369 107.3981 20 10.1 −54 7.87 304 2.74 5.74 19.3 0.25 17.0 537 456 0.86 165 546 b0.3 0.01 48.0 0.0460 41.0373 107.3983 35 17.7 −54 7.45 129 3.54 67.7 60.9 0.01 151 3.73 1.52 0.42 96.0 436 b0.3 2.2 172 0.0461 41.0535 107.4723 12 12.7 −48 7.68 85.0 1.95 50.8 61.3 1.29 94.6 39.7 33.6 0.65 98.7 359 b0.3 0.01 147 0.0562 40.9780 107.2817 30 10.1 −53 7.77 195 4.34 72.6 93.7 1.56 139 17.7 11.8 0.40 264 481 b0.3 5.5 246 0.0463 40.9876 107.2584 30 9.8 −27 7.88 640 2.49 20.9 38.6 0.17 89.2 2.80 1.98 0.40 571 697 b0.3 0.01 336 0.06

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Fig. 2 –Variation of Na+ with Cl- in shallow groundwatersfrom the Hetao Basin, Inner Mongolia.

136 S C I E N C E O F T H E T O T A L E N V I R O N M E N T 3 9 3 ( 2 0 0 8 ) 1 3 1 – 1 4 4

resource has been finished. Parameters, including watertemperature, pH, Eh, and sulfide, were measured at the timeof groundwater sampling. Alkalinity was performed on thesampling day and measured by the Gran titration method.

Water samples were also collected for subsequent labora-tory analysis. Water samples for major and trace elementanalysis were filtered through 0.45 µm Millipore filters on siteinto 100 mL HNO3-washed polyethylene bottles. Those foranalysis of trace elements were acidified to pH 1 using 6 MHNO3. Arsenic(III) in the water samples was separated in thefield immediately following filtration with As-speciationcartridges (Metalsoft Center, Highland Park, NJ) (Meng andWang, 1998; Meng et al., 2002). The cartridge packed with 2.5 gof aluminosilicate adsorbent selectively removed As(V) but didnot adsorb As(III) in a pH range 4 to 9. Arsenic (III) wasseparated from As(V) by passing approximately 50 mL ofsample through the cartridge at a flow rate of 60±30 mL/minusing a syringe. The concentration of As(V) was determinedindirectly as the difference between total As (AsTot) and As(III)in the filtered sample. Samples for anion analysis were leftunacidified.

Concentrations of major cations and trace elements weredetermined by ICP-AES and ICP-MS, respectively. Unacidifiedaliquots were analyzed for F−, Cl−, NO3

−, NO2−, NH4

+, HPO32−, SO4

2−

by Ion Chromatography with an instrument model DX-120(Dionex). Concentrations of As and Se were determined byatomic fluorescence spectrometry (AFS) with hydride genera-tion. In most cases, analytical charge imbalances were lessthan 5%.

In addition, three representative boreholes, including twoin As-affected areas (BH1 in Langshan and BH3 in Shahai) andone in an As-free area (BH2 in Shanba) (Fig. 1), were drilled totake sediment samples from different depths up to 23 m BLS.These were dried, disaggregated, and milled for XRD analysis.An aliquot was subjected to a mixed acid (HNO3–HClO4–HF)digestion before analysis of a range of trace elements by ICP-MS. The same procedure was followed to measure chemicalcompositions of China National Geochemical Standard Mate-rials (GSS1, GSS8, GSD 9 and GSD 10). Differences betweenconcentrations determined by the above procedure and thecertified values were less than 5%.

Analysis of total element concentration can determine thedegree of trace element enrichment in sediments, whileelement speciation provides an insight into transformationandmobility of trace element species. Speciation analysis is ofmajor importance in environmental research since it canprovide crucial information about the ecotoxicological char-acteristics (reactivity, bioavailability and toxicity) of contami-nants (Van Herreweghe et al., 2003). Sequential chemicalextraction was carried out to determine As-speciation insediments using a procedure developed by Tessier et al. (1979),with few modifications. We used the original wet sediments;drying of the sediment samples was avoided since this wouldcause erroneous results (Buykx et al., 2000). Sediment As isgenerally considered to be present in the following forms:exchangeable, carbonate-bound, Fe–Mn oxide-bound, organicmatter and sulfide-bound, and residual. Exchangeable phase(F1) was extracted with 0.5 M Mg(NO3)2 at room temperature,carbonate-bound phase (F2) with 1 M NaOAc at roomtemperature, Fe–Mn oxide-bound phase (F3) with 0.08 M

NH2OH HCl at 96 °C, organic matter and sulfide-bound phase(F4) with 0.02 M HNO3 and 300 mL/L H2O2 at 85 °C and then3.2 M NH4OAc in 20 mL/L HNO3 at room temperature, andmineral matrix phase (F5) with concentrated HNO3 at 105 °C.

4. Results and discussion

4.1. Groundwater chemistry

The results for chemical composition of 63 groundwaterssampled from selected wells in the study area are shown inTable 1. All boreholes abstracted groundwater from theshallow aquifer with depths between 5 and 80 m. Wells forgroundwater sampling also include one dug well (No. 19 inTable 1).

4.1.1. Characteristics of groundwater chemistryThe groundwaters are generally of Na–Cl–HCO3 type with highsalinity in the study area mainly located in the centre of thebasin,which is a result of evaporation exacerbated by theeffectsof irrigation losses. This is demonstrated by the plot of Na+

versus Cl− (Fig. 2), where the ratio of [Na+]/[Cl−] is approximately1:1. Concentrations of Cl− reach up to 3900 mg/L, and Na+ up to2660 mg/L.

Water temperatures vary between 9.8 and 14.7 °C, whichhas no distinct correlation with well depths (r2=32.4%). Thislikely indicates that the geothermal gradient is not the onlyfactor controlling the temperature of groundwater fromshallow aquifers (b80 m). Solar radiation may also affect thetemperature of shallow groundwaters.

The groundwaters generally reflect the moderate reducingenvironment of the aquifers, which are lithologically char-acterized by the finer lacustrine sediments. Redox potentialsrange from −153 to 83 mV. Nitrate concentrations are mostlybelow detection limit (b0.01 mg/L) with a median value ofb0.01mg/L (Table 1). There are two groundwater samples withhigh NO3

− concentrations, which are possibly due to theinfluences of pollution, mainly from agricultural sources. Afew groundwaters contain high NO2

−, reaching up to 26.1 mg/L,with 5% of analyzed samples exceeding the WHO guidelinevalue for acute exposure to NO2

− in drinking water (910 μg/L). In

137S C I E N C E O F T H E T O T A L E N V I R O N M E N T 3 9 3 ( 2 0 0 8 ) 1 3 1 – 1 4 4

comparison with NO3−, the groundwaters contain more SO4

2−,with a median value of 224 mg/L. The lower concentrations ofNO3

− suggest that denitrification has been an active process inthe aquifers.

Under anaerobic conditions, concentrations of total Fe(FeTot) and Mn are high, reaching up to 5.90 mg/L and 1270 μg/L, respectively. Of the samples collected, 64% exceed the WHOguideline value of 0.3 mg/L Fe, and 14.3% exceed the value of500 μg/L Mn. 78% of the collected samples contain detectableS2− (detection limit of 0.01mg/L), with amaximumof 0.12mg/Lin the As-affected aquifers. The occurrence of S2−, togetherwith relatively low SO4

2− concentration in the reducingenvironment, suggests that SO4

2− reduction has also takenplace in the aquifers. This indicates that moderate reducingconditions exist in the As-affected aquifers, which is sup-ported by the negative Eh values observed (Table 1).

Groundwater pH is near-neutral to weak alkaline (7.01–8.43). Concentrations of HCO3

− are very high, reaching up to1600 mg/L. Carbonate (CO3

2−) is also observed in three ground-water samples, with concentrations up to 26.4mg/L. Dissolvedorganic matter may be an active source for HCO3

− in ground-water. Bicarbonate (HCO3

−) presenting in the reducing aquifersfrom the neighbouring Huhhot Basin is believed to substan-tially derive from the oxidation of organic matter in thesediments and in solutions (Smedley et al., 2003). One featureof particular interest is the relatively high concentration ofdissolved organic matter in the groundwater. A few unfilteredgroundwater samples analyzed for total organic carbon (TOC)show that the average concentration is around 650 mg/L.Groundwaters are mostly discoloured and frothy, possiblyindicating that humic acid is present. The occurrence of humicacid in As drinking water has been speculatively linked to thedevelopment of some chronic health problems, includingblackfoot disease in Taiwan (Yu et al., 2002).

Concentrations of P are also high in the As-affectedgroundwaters, up to 3.54 mg/L. Despite these high concentra-tions, the groundwaters are undersaturated with respect tovivianite and hydroxyapatite. The sources of P in the ground-water are possibly solid organic matter and Fe oxides, whichwould release phosphate in the reducing conditions (Holdrenand Amstrong, 1980).

Saturation indices (SI) for selected minerals, calculated bythe hydrogeochemical code PHREEQC (Parkhurst and Appelo,1999) using groundwater chemical data, show that most of the

Fig. 3 –Distribution of total As in the groundwaters from th

groundwaters are supersaturated with respect to calcite anddolomite. This suggests that the elevatedHCO3

− concentrationsare not only controlled by the dissolution of the carbonates inthe aquifer. It may partly originate from the oxidation oforganic matter in the sediment and in solution. In addition,positive SI values are observed for most of the groundwaterswith respect to siderite (average 0.24) and barite (average 0.51).The precipitation of siderite and barite is an importanthydrogeochemical process in the As-affected aquifers. All ofthe groundwaters are undersaturated with respect to pyriteand gypsum.

4.1.2. Spatial distribution of arsenicArsenic concentrations in the groundwater range between0.58 and 572 μg/L, with 71% of the selected samples exceedingthe Chinese Drinking Water Standard of 10 μg/L (Ministry ofHealth of PR China, 2006). The regional distribution of As in thegroundwaters is shown in Fig. 3. High As groundwaters aremainly localized in five hotspots, including Shuangmiao-Sandaoqiao, Shahai-Manhui, Bainaobao-Langshan, Taerhuand Shengfeng. Dark grey fine sand layers are universallyfound in these As-affected aquifers, where groundwaters areanaerobic. The sediments were possibly deposited in theestuary of lakes during the later Pleistocene, where water flowbecame stagnant. It could be proved by the presence of organicmatter-enriched clay in the sand layers. In comparison, theAs-free aquifers are mainly composed of light-brown finesands, which were deposited in the fluvial environment. It isfound that the shallow groundwaters close to the Yellow Rivergenerally contain lower As concentration.

Arsenic(III) concentrations are variable in the shallowgroundwater, ranging between 0.35 and 456 μg/L (median29.2 μg/L). As(III) is the major As species. Of the analyzedsamples, 83% have ratios of As(III) to the total concentrationgreater than 60% (Fig. 4). The remaining constituents aredominated by As(V), although methylated As species (MMAA,DMAA) may occur in the organic matter-enriched water.Organic As species were previously detected in groundwatersfrom the Hetao Basin, and found to be minor constituents,accounting for ~7% of the total concentration (Lin et al., 1999).The prevalence of As(III) over As(V) also indicates that theshallow groundwaters are anaerobic.

In theanaerobicgroundwaters, theNO3− concentration isvery

low. Of the analyzed samples, 61% have NO3− concentration of

e shallow aquifers of the Hetao Basin, Inner Mongolia.

Fig. 4 –Histogram of As(III)/AsTot (%) ratios in shallowgroundwaters from Hetao Basin.

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less than 0.01 mg/L, which suggests that the reduction of NO3−

hasperhaps occurred throughout the aquifers. However, Fig. 5 Ashows that a relatively low correlation exists between As andNO3

− (r2=−10.5%). It implies that, besides NO3−, other electron

acceptors are available in the system to allow oxidation oforganic matter, which is widely believed to be an importantcontributor to the generation of anoxic conditions in the As-affected anaerobic aquifers (Stüben et al., 2003; Ahmed et al.,2004; Agusa et al., 2006; Varsányi and Kovács, 2006). Ground-water SO4

2−would readily act as the electron acceptor,which canbe proved by the strong negative correlation (r2=−87%) betweenAsand SO4

2− in the groundwater sampleswith SIbariteb0 andNO3−

concentration b0.01 mg/L (Fig. 5B). Because the precipitation ofbarite controls the SO4

2− concentration in the groundwater aswell, a weak negative correlation ( r2 =−8.1%) exists betweenAsand SO4

2− in the case of SIbariteN0 (Fig. 5B).

Fig. 5 –Variation of total As (AsTot) with NO3−, SO4

2−, HCO3−, an

Arsenic exhibits a good positive correlation (r2=76.7%)with HCO3

− in the case of SIsideriteb0, while a relatively lowcorrelation (r2=16.7%) exists in the case of SIsideriteN0 (Fig. 5C).Dissolved inorganic carbon is mainly derived from organicmatter in the reducing aquifers from the neighbouringHuhhot Basin (Smedley et al., 2003), which may also occurin the reducing Hetao Basin aquifers. Therefore, the strongpositive correlation indicates the oxidation of organic matterhas possibly caused the increase in HCO3

− concentration in thegroundwaters (Ravenscroft et al., 2001; Ahmed et al., 2004),the decrease in redox potential, and consequently themobilization of As in the aquifers. In the case of SIsideriteN0,the precipitation of siderite has decreased HCO3

− concentra-tion and resulted in the poor correlation between As andHCO3

−. The reductive dissolution of Fe(III) oxyhydroxides isbelieved to be a cause of As release and has caused coincidentincreases in dissolved Fe2+, which can be speculated from agood correlation between FeTot and AsTot (r2=66%) in thegroundwaters with SIsideriteb0 (Fig. 5D). However, the pre-cipitation of Fe2+ as siderite (FeCO3) has been an importantfactor, producing a relatively low correlation (r2=−30.5%)between AsTot and FeTot in the reducing groundwaters withSIsideriteN0 (Fig. 5D).

4.2. Sediment geochemistry

The chemical compositions of the sediments sampled from threerepresentative boreholes (BH2 in theAs-free area, BH1 andBH3 intheAs-affectedareas) aregiven inTable2.Thesediments range intexture from fine sand to clay and in colour from brown throughgrey to black. The SiO2 concentrations range between 48 and76 wt.%, with high concentrations in the sand samples and low

d FeTot in shallow groundwaters from the Hetao Basin.

Table 2 – Chemical compositions of sediment samples collected from three representative boreholes

Bore Depth Lithology SiO2 Al2O3 Fe2O3 MgO CaO Na2O K2O As Ba F Mn Mo P S SrNo (m) % % % % % % % mg/

kgmg/kg

mg/kg

mg/kg

mg/kg

mg/kg

mg/kg

mg/kg

1 0.7 Brown clay 58.52 11.80 4.31 2.61 7.49 2.22 2.42 11.9 501 735 635 0.96 592 636 2392.0 Brown silty clay 69.52 10.03 2.83 1.75 4.62 2.13 2.27 11.2 468 412 546 0.80 459 274 1903.5 Brown clay 56.11 13.03 4.85 2.96 8.07 1.92 2.47 17.0 508 644 722 0.79 660 498 2434.7 Dark grey silty clay 56.80 13.19 4.75 2.82 7.43 2.06 2.44 16.7 463 616 674 0.83 647 566 2407.0 Yellow silty clay 58.90 16.75 5.98 3.31 3.69 1.94 3.22 18.7 594 802 558 0.80 607 292 1927.0 Pale green silty clay 58.28 14.02 5.32 3.30 5.04 2.00 2.72 23.8 530 735 615 0.97 802 327 2319.8 Yellow silty clay 60.09 12.06 4.02 2.68 7.04 2.22 2.30 12.5 495 616 607 0.97 584 348 22610.7 Brown silty clay 51.74 15.16 6.06 3.80 8.03 1.59 2.95 17.5 575 1040 755 1.08 571 333 31812.3 Grey silt 60.26 11.36 3.97 2.32 7.62 1.83 2.17 12.2 448 540 751 0.93 578 268 24413.5 Yellow silty clay 51.61 14.66 6.38 3.21 8.55 1.27 2.92 40.6 626 768 881 0.74 589 191 28414.5 Dark grey fine sand 72.88 9.54 2.62 1.40 4.06 2.05 2.06 13.0 478 344 556 0.92 387 142 18916.0 Black fine sand 72.85 9.36 2.52 1.42 4.06 2.05 2.07 12.2 475 285 531 0.51 363 535 18720.0 Black fine sand 74.28 8.85 2.15 1.21 3.38 2.08 2.04 11.2 492 299 427 0.46 330 235 17321.5 Black fine sand 73.80 8.87 2.56 1.28 3.63 2.02 1.98 16.7 474 285 534 0.97 337 1433 18722.8 Deep green silt 75.35 8.54 2.11 1.18 3.27 2.13 1.96 8.2 462 272 444 0.67 307 874 169

Average 63.40 11.82 4.03 2.35 5.73 1.97 2.40 16.2 506 560 616 0.83 521 463 2212 2.5 Brown silty clay 59.12 11.51 4.15 2.60 7.30 2.01 2.26 12.7 487 673 667 0.88 627 394 231

4.7 Yellow brown silty clay 59.90 11.49 4.23 2.56 7.26 2.05 2.25 13.0 486 673 679 0.99 618 234 2245.8 Yellow clay 48.30 14.65 6.60 3.55 9.42 1.30 3.09 21.6 549 1040 961 1.62 638 375 2746.5 Brown silt 69.11 10.35 3.46 1.72 4.58 2.18 2.21 13.0 465 494 740 0.57 381 102 2067.5 Brown silty clay 53.80 13.13 5.47 3.14 8.40 1.55 2.53 19.1 527 735 841 0.76 635 216 27910.0 Yellow fine sand 65.49 10.71 3.68 2.02 5.82 2.04 2.16 11.7 433 540 722 0.78 535 103 21411.0 Brown silty clay 54.07 13.29 5.30 3.13 8.22 1.44 2.53 15.8 550 768 774 0.92 610 187 23911.5 Yellow silt 66.91 10.33 3.19 1.82 5.46 2.12 2.19 10.1 453 394 625 1.16 502 75 19611.6 Deep yellow sand 72.55 9.53 2.57 1.35 3.91 2.10 2.19 11.2 467 344 561 0.58 360 124 18615.0 Deep yellow silt 75.18 8.66 2.26 1.17 3.29 2.10 2.07 7.7 457 214 558 0.74 307 42 17717.5 Deep yellow silty fine

sand76.36 8.52 2.42 1.14 3.00 2.08 2.03 20.6 484 313 584 0.75 299 37 177

18.5 Deep yellow clayey finesand

73.96 9.14 2.64 1.36 3.48 1.96 2.10 12.5 446 328 625 1.05 340 77 176

21.0 Pale grey fine sand 75.69 8.71 2.44 1.20 3.07 2.09 2.08 7.3 446 272 587 0.51 305 60 177Average 65.42 10.77 3.72 2.06 5.63 1.92 2.28 13.6 481 522 686 0.87 474 156 212

3 1.0 Yellow silty clay 64.24 10.64 3.91 2.18 5.98 2.20 2.16 10.9 474 540 817 1.51 562 505 2133.0 Yellowish brown silt 73.14 8.76 3.47 1.44 4.18 2.18 1.85 13.0 430 360 1024 1.66 447 377 1964.5 Silty clay 50.88 13.66 6.23 3.26 9.79 1.40 2.95 24.2 599 875 936 1.24 672 453 2765.5 Grey silty clay 61.67 10.97 3.84 2.24 6.51 2.12 2.16 9.6 438 564 741 1.05 585 318 2277.0 Grey clay 50.17 14.05 6.07 3.36 9.50 1.49 2.77 23.8 577 875 973 2.22 529 242 3307.5 Yellow silty clay 62.90 10.69 3.56 2.18 6.78 2.17 2.14 12.5 452 516 700 0.79 564 195 2578.3 Yellowish brown clay 53.02 12.86 5.26 3.13 9.34 1.72 2.40 24.8 503 954 836 1.15 600 228 3179.0 Brown clay 52.84 15.59 5.84 3.67 7.18 1.63 3.10 10.1 574 996 695 0.72 560 174 28510.5 Grey silty clay 63.04 10.63 3.52 2.16 6.35 2.28 2.12 9.4 428 540 594 0.90 606 106 21811.8 Brown clay 49.80 14.76 6.45 3.67 9.17 1.50 2.98 22.8 564 1040 924 1.14 570 276 32012.8 Grey silty clay 62.64 11.08 3.61 2.28 6.75 2.23 2.15 27.4 443 494 671 0.87 583 116 22213.2 Brown silty clay 49.84 15.05 6.75 3.52 8.66 1.31 3.04 73.3 554 914 919 0.98 622 553 28414.0 Grey silty clay 58.47 11.80 4.31 2.56 7.94 1.80 2.27 20.9 467 673 734 0.51 622 206 25216.3 Dark grey clay 50.29 14.24 6.34 3.33 9.40 1.24 2.87 71.9 617 802 968 0.69 630 227 34916.8 Dark grey clayey silt 70.25 10.22 2.93 1.68 4.10 2.05 2.20 15.2 517 412 564 0.78 410 109 19117.5 Dark grey clay 52.84 13.83 5.65 2.95 9.53 1.41 2.76 36.3 698 914 904 0.47 579 217 32319.5 Black fine sand 70.06 10.02 3.14 1.55 4.06 1.87 2.28 29.6 511 412 677 0.56 394 486 19621.0 Dark grey fine sand 68.89 9.62 2.87 1.59 4.88 2.14 2.08 13.0 490 394 654 1.21 395 714 20322.3 Black fine sand 72.04 9.36 2.62 1.36 4.07 2.17 2.12 11.9 484 328 706 0.85 342 422 19423.0 Black silty clay 55.59 13.62 5.27 2.81 7.16 1.51 2.86 24.8 663 735 813 2.11 500 2309 245

Average 59.63 12.07 4.58 2.55 7.07 1.82 2.46 24.3 524 667 793 1.07 539 412 255

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concentrations in the clay samples. The Fe2O3 concentrations liein the range 2.11–6.75 wt.%, which are close to the values in thesediments from As-affected areas in Bangladesh (BGS and DPHE,2002). The high concentrations are found in clay or silty clay,which usually enriches organic C with the origin of lacustrineenvironment (McArthur et al., 2004). Although total organic Cwas

not determined in the selected samples, the contents are likely tobe relatively high as in the sediments from other areas with asimilar sedimentary environment (Guo et al., 2003; Smedley et al.,2003).

The results of XRD analyses show that fine sands containconsiderable amounts of quartz (up to 61.9%), while clay

Fig. 6 –Vertical profiles of total As and As dissolved by the sequence of extractions that target different solid phases. (A) BH1with high As groundwater at depths between 14 and 23.5 m; (B) BH2 with low As groundwater; (C) BH3 with high Asgroundwater between 18.4 and 22.9 m.

140 S C I E N C E O F T H E T O T A L E N V I R O N M E N T 3 9 3 ( 2 0 0 8 ) 1 3 1 – 1 4 4

sediments aremainly composed of clayminerals (up to 59.6%).All sediments exhibit various contents of calcite (3.6%~25.0%)and dolomite (b1.0%–10%). Siderite and pyrite are found inseveral silty clay sediments, indicating that they occur in astrongly reductive environment. Sand sediments from BH1and BH3 have a higher content of clay minerals (average15.0%), in comparison with those from BH2 (average 13.0%).Although a big difference in colour has been observed betweenaquifer fine sands from BH1 (or BH3) (dark grey or black) andthose from BH2 (deep yellow), no significant variations arefound in their mineralogical compositions.

Concentrations of the sediment As range from 7.3–73.3 mg/kg (average of 18.9 mg/kg). Although they are not unusuallyhigh compared with average values found in sediments fromelsewhere, the As distributions in the sediments from theregions with high As groundwater are different from thosewith low As groundwater (Table 2; Fig. 6). The As concentra-tions from the As-affected areas lie in the range 8.2–73.3mg/kg(average of 20.8mg/kg), which are a little higher in comparisonwith those in the As-free area (average of 13.6 mg/kg).Therefore, the As contents of the sediment may be a factoraffecting the As concentrations in the groundwater since thesediment is the major As resource in the aquifer. Gao (1999)observed that As concentrations were of the same order (6.3–47.1mg/kg) in the sediments fromHaiziyan in the Hetao Basin.The As is mildly-strongly correlated with total Fe (r2=60%) andtotal Mn (r2=55%). The sample with the highest As concentra-tion (73.3 mg/kg) is a black clay with the highest measuredFe2O3 concentration (6.75 wt.%) in BH3. A quite weak correla-tion exists between total As and total S (r2=8%). This suggeststhat the As is associated with Fe oxides (or Mn oxides), rather

than sulfides in the sediments. It is well known that Fe oxides(such as goethite: Sun and Doner, 1998; ferrihydrite: Ravenet al., 1998; hematite: Guo et al., 2007) readily scavenge greatamounts of As. The redox condition of the fine sand aquifersin BH1 and BH3 is distinct from that in BH2. The black finesand together with organic matter is found in the aquifersfrom BH1 and BH3, which implies that it occurs in a moderatereductive environment, while the yellow–deep yellow sand isobserved in BH2 of weak oxidative environment. High Asgroundwaters also occur in the reductive alluvial aquifercomprising of fine sand, relatively higher percentage matrix(silt and clay) and organic matter in Bangladesh (Ravenscroftet al., 2001; Ahmed et al., 2004).

Although sequential leaching methods are based on theassumption that selected solvents attack and dissolve parti-cular forms of trace elements, which are specificmineralogicaloccurrences or operationally defined, the sequential leachingof samples by means of chemical extractants is often used foridentification and evaluation of the availability of tracemetalsin sediment samples (Carbonell-Barrachina et al., 1999; Keonet al., 2001; Huggins et al., 2002; Van Herreweghe et al., 2003;Kelderman and Osman, 2007). Chemically active As forms(exchangeable, bound to carbonates, bound to Fe–Mn oxides,bound to insoluble organic and sulfides) were investigated bythemethod of sequential extraction. The vertical profiles of Asdissolved by the sequence of extractions are shown in Fig. 6.The active forms exhibit a good correlation with total As(r2=0.67), and generally increase with the increase in depthsbetween 0 and ~18 m in the sediments from those threeboreholes (Fig. 6). The peak appears at depths of 21.5, 17.5 and16.3 m in BH1, BH2 and BH3, respectively. It is generally

141S C I E N C E O F T H E T O T A L E N V I R O N M E N T 3 9 3 ( 2 0 0 8 ) 1 3 1 – 1 4 4

coincident with the location of the aquifers where high Aswells are located in the As-affected areas. However, high Asgroundwater does not occur at BH2 although 2960 μg/kg ofchemically active As is observed at the depth of 17.5 m in thefine sand aquifer. The possible reason is that the active Asforms keep relatively stable in the weakly oxidative aquifer,which can be proved by the yellow–deep yellow colour of theaquifer sediments. This speculation is also supported by thehigher relative concentrations of As bound to Fe–Mn oxides inthe aquifer sands from BH2 (74%) in comparison with thosefrom BH1 and BH3 (around 67%).

Of the chemically active fractions, 60–86% of As is bound toFe–Mn oxides (F3) in the sediments. Relatively higher percen-tages are observed in the fine sands fromBH2 due to the higherredox potential. There is still a certain amount of Fe–Mnoxidesand oxyhydroxides in the sediments from BH1 and BH3 thatmay adsorb As, althoughmost of the Fe andMn possibly existsin a low valence state under reductive conditions. With regardto BH1, themaximumboundAs of 2500 μg/kg is observed in thefine sand, in which the high As wells with hand pump aremainly located in. The clay and the silt (silty sand) from BH3contain much As bound to Fe–Mn oxides, up to 3500 μg/kg.When the redox potential decreases and/or pH increases, theboundAswill be released into groundwaters (Feijtel et al., 1998;Keon et al., 2001; Wang and Guo, 2004).

Although the F3 fraction is operationally defined for the Asbound to both Fe oxides and Mn oxides, the release of As fromthis fractionmay be sensitive to the concentration of Fe oxidesand the concentration of Mn oxides in the reductive environ-ment. It has long been known that the reduction of Mn oxidesis thermodynamically more favourable than is the reductionof FeOOH (Edmonds, 1986). The As originally adsorbed to Mnoxides in the F3 fraction is firstly released during reduction.The released As would be resorbed to FeOOH rather than betransported into groundwater (Stüben et al., 2003).

Exchangeable As (F1), the As bound to carbonates (F2) andthe As bound to insoluble organic and sulfides (F4) account for4–20% of the chemically active form. The exchangeable As isvulnerable to ionic activity or pH, and could readily bemobilized by an increase in ionic strength of the aqueoussolution (Keon et al., 2001). This fraction is assumed to consistof easily exchangeable, outer-sphere complexes of As (Wenzelet al., 2001). A big difference in the exchangeable As isobserved between the sediments from the As-affected areasand those from the As-free area. The average exchangeable Asin the sediments from BH1 is 120 μg/kg, while from BH2 it isonly 60 μg/kg. Therefore, the exchangeable As may directlyreflect the groundwater As in the aquifer, although a relativelylow contribution of the fraction has been observed.

In comparison with F1, the contribution of F2 is muchgreater in the high pH environment (pH 8.46–10.35). Underthose weakly alkaline conditions, this binding fraction is quitestable. Only when the carbonate minerals are dissolved willthe As be released into groundwaters. For example, a loweringof sediment pH may give rise to mobilisation of As bound tocarbonates (Calmano et al., 1993).

A proportionally higher contribution of the organic matterandsulfide-boundAs (F4) is observed in the reductive sedimentsfrom BH1 and BH3 (~24.5%), in comparison with that in thesediments from BH2 (~13.3%). Moore et al. (1988), Mok andWai

(1994) and Guo et al. (1997) also found that the As has a highaffinity for sulphidic andorganic substances insedimentsunderreducing conditions.

4.3. Implications for As mobilization

The chemical characteristics of high As groundwaters and theregional distribution of dissolved As in the Hetao Basin indicatethat mobilization in groundwater takes place under moderatereducing conditions, where Fe and Mn have been reduced to Fe(II) and Mn(II) forms, and the reductions of NO3

− and SO42− have

occurred. Investigations of the sediment stratigraphy andlithology across the basin show that lacustrine layers mainlycomposed of argillaceous materials widely occur in the upper-most subsurface sediments (b8 m). This geological setting willhave restricted diffusion of atmospheric O2 into the aquifers.Solid organicmatter is also likely to be concentrated in the fine-grained lacustrine sediments. In the young unconsolidatedsediments, the organic matter should be fresh and may be adominant source of electrons, which is thought to be a primecause of the reducing conditions observed in the As-affectedaquifers. Besides, the dissolved organic matter is present withrelatively high concentrations in the groundwater, oxidation ofwhich is another of themain factors in the initiation of reducingconditions (Bauer and Blodau, 2006). High concentrations ofdissolvedorganicmatterhavealsobeen recorded inanumberofother As-affected areas (Varsányi and Kovács, 2006; Yu et al.,2002; Guo et al., 2003). Further impacts of dissolved humicsubstances have been proposed. Azcue and Nriagu (1995)reported that the possibility exists for binding of As to humicacids. The presence of negatively charged DOC also enhancesthe desorption of As from binding sites through electrostaticeffects (Bauer and Blodau, 2006).

The competitive effects of other anions may also beimportant factors. It is well known that phosphate competesstrongly with As (especially arsenate) for available exchangesites on Fe oxides (Roy et al., 1986). Because concentrations of Parehigh inmostof the groundwaters, thismaybeanadditionalfactor in promoting As mobilization. High P concentrationshave also been observed in groundwaters from Shanyin, China(Guo et al., 2003), Bangladesh (BGS and DPHE, 2002), andArgentina (Smedley et al., 2002). Although concentrations ofHCO3

− are very high, it may play a minimal role in affecting theconcentrations of As in groundwater (McArthur et al., 2004).

The direct source of the As in the groundwater is believedto be the chemically active As fraction in the sediments. Theweakly bound As is relatively labile, and may be readilyreleased under favourable hydrogeochemical conditions. Forexample, an increase of ionic strength would lead to mobiliza-tion of exchangeable As. As noted above, the chemically activeAs is mainly bound to Fe–Mn oxides, which suggests that Fe–Mn oxides are a major potential source.

Potential mechanisms for As release include reductivedissolution of Fe–Mn oxides under reducing conditions(Nickson et al., 1998; Islam et al., 2004). The groundwaterdata, indicating that As concentrations increase with theincrease in dissolved Fe(II) concentrations in the ground-waters with SIsideriteb0, strongly support this implication.However, it is observed that a high proportion of dissolved Asexists as As(III) in the high As groundwaters, suggesting that

142 S C I E N C E O F T H E T O T A L E N V I R O N M E N T 3 9 3 ( 2 0 0 8 ) 1 3 1 – 1 4 4

reduction from As(V) has taken place in the Fe–Mn oxide-bound fraction. In this case, reductive desorption would occur,because the adsorption affinity of Fe oxides for As(III) at near-neutral pH is known to be substantially less than that for As(V)(Dzombak and Morel, 1990).

Groundwater flow in the study area is believed to be slowasaresult of low hydraulic gradient, poor aquifer permeability andsmall recharge. This would restrict aquifer flushing, solute (As)transport and removal from the system. The areas with slowgroundwater movement are very sensitive to the release ofsmall amounts of As resulting from the various hydrogeochem-ical processes above.

5. Conclusion

The groundwaters are generally of Na–Cl–HCO3 type with highsalinity, which is a result of evaporation exacerbated by theeffects of irrigation losses. High As groundwaters in the HetaoBasin are characterized by high concentrations of dissolved Fe,Mn, HCO3

−, P and S2−, and low concentrations of NO3− and SO4

2−.Low SO4

2− coupled with high S2− suggests that the ground-waters exist in the reducing environment and that some SO4

2−

reductionwould readily take place. In this environment, As(III)is correspondingly the major As species, an average account-ing for 75% of total As.

In the anaerobic groundwaters, NO3− and SO4

2− have acted aselectron acceptors in the system to allow oxidation of organicmatter, which is widely believed to be an important contributorto the generation of anoxic conditions. The oxidative degrada-tion of organic matter produces high HCO3

−, while microbialfermentation leads to thegenerationof biogenicCH4 as reportedin the sameareas (Tang et al., 1996).Microbial processes in thesesediments rich in organic matter create a favourable reducingenvironment, facilitating mobilization of As in the aquifers.

Concentrations of Asmeasured in the sediments from threerepresentative boreholes have a range 7.3–73.3 mg/kg (averageof18.9mg/kg). TheAsconcentrations fromtheAs-affectedareas(average of 20.8 mg/kg) are a little higher in comparison withthose in the As-free area (average of 13.6 mg/kg). The As ismildly-strongly correlated with total Fe and total Mn, while aquite weak correlation exists between total As and total S. Thissuggests that the As is associatedwith Fe oxides (or Mn oxides),rather than sulfides in the sediments. Sequential extractionstudies also show that chemically active As is mainly bound toFe–Mn oxides, up to 3500 μg/kg. This fraction would be readilymobilized in the reducing condition through reductive dissolu-tion of Fe–Mn oxides and reductive desorption of As. Althoughthe exchangeable As (F1) is very labile and vulnerable tohydrogeochemical conditions, the contribution of this fractionis relatively limited due to the low concentrations.

Acknowledgement

This study is financially supported by the Nation ScienceFoundation of China (No. 40572145), the Program for NewCentury Excellent Talents in University (No. NCET-07-0770),

and the Scientific Research Foundation for the ReturnedOverseas Chinese Scholars, State Education Ministry. TheAuthors would like to thank Dr. Fei Liu and Mr. Minggao Tangfor invaluable laboratory assistance. Constructive commentsby Prof. John McArthur and another anonymous reviewer arealso gratefully acknowledged.

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