Characteristics of high arsenic groundwater in Hetao Basin,Inner Mongolia,northern China

YangChun Zhu ,XueYong Zhao,Min Chen,YongQing Luo,Xin Zhou

Naiman Desertification Research Station,Cold and Arid Regions Environmental and Engineering Research Institute,Chinese Academy of Sciences,Lanzhou,Gansu 730000,China

1 Introduction

High arsenic groundwater is considered as a serious environmental problem and is widely found all over the world,including India(Sankaret al.,2014),Bangladesh(Chakrabortiet al.,2010),Pakistan(Fatmiet al.,2009),Argentina(Nicolliet al.,2010),and China(Mukherjeeet al.,2009).Many studies have been done on the geological,hydrological,and chemical aspects of high arsenic in groundwater(Mukherjeeet al.,2009).High arsenic groundwater is mostly found in Southeast Asian countries,especially in the Ganges Delta and in China;Bangladesh is considered to be the worst of all.Chakrabortiet al.(2010)had analyzed 52,202 groundwater samples for arsenic in Bangladesh since 1996.Their results showed that 7.5% of the samples contained arsenic above 300 μg/L,and most exceeded the World Health Organization(WHO)recommended value of 10 μg/L for drinking water(50 μg/L for many developing countries).Compared with Bangladesh,India,and China,Pakistan has relatively low levels of arsenic in its groundwater.A Pakistan national survey(Fatmiet al.,2009)reported that arsenic in groundwater was in the range of 0–500μg/L;9% of the drinking water samples had arsenic >10 μg/L and 0.7% of the samples had >50 μg/L.Symptoms of chronic exposure to arsenic in drinking water are certain skin diseases such as skin keratosis,abnormal pigmentation,dermal hyperkeratosis,and skin cancer;several hematological,cardiovascular,renal,neurological,and respiratory diseases;and severe lung,bladder,liver,kidney,and prostate cancers(Guoet al.,2008).Arsenic has also been detected in plant tissues and roots,with corn exhibiting the highest contents(Neidhardtet al.,2012).

Since 2007,China has lowered the drinking water standard for arsenic from 50 to 10 μg/L.Groundwater with arsenic enrichment has been reported in some areas of northwestern China,especially in Xinjiang(Liuet al.,2013),Shanxi(Xieet al.,2012),and Inner Mongolia(Mukherjeeet al.,2009).Since 1990,about 30,000 people have been exposed to arsenic poisoning in Inner Mongolia.Smedleyet al.(2003)indicated that the arsenic concentration in groundwater from boreholes and wells in the Hohhot Basin of Inner Mongolia,varied from <1 μg/L to 1,480 μg/L.One of the most severely arsenic-polluted areas in Inner Mongolia is the Hetao Basin(the Great Bend of the Yellow River)(Hagiwaraet al.,2011).

The Hetao Basin,with an area around 13,000 km2(Figure 1),lies in the western part of Inner Mongolia,bounded by the Yellow River to the south and the Langshan and Yinshan mountains to the north.The climate is semi-arid to arid,with a low annual average precipitation of 130–220 mm and high evaporation of 2,000–2,500 mm.The annual average air temperature is 5.6–7.8 °C and the major soil type in this area is anthropogenic-alluvial.Due to strong evapotranspiration,about half of the soils are saline.The groundwater is mainly of Na-Cl-HCO3type and the average depth is only 1–1.5 m due to agriculture irrigation.The groundwater is mostly recharged by precipitation,surface runoff,and water from lateral fractures in bedrock in the Yinshan Mountains in the north;in the south,the recharge is from irrigation return flow from farmland and leakage from the Yellow River.Groundwater is discharged via evapotranspiration and artificial abstraction(Denget al.,2009).

Figure 1 Map of the Hetao Basin,Inner Mongolia,China

The Hetao Basin is a fault basin and comprises a sequence of Cenozoic sediments with thickness ranging from 500 to 1,500 m in the southeast and from 7,000 to 8,000 m in the northwest(Zhanget al.,2013).Yinshan Mountain is mainly composed of a metamorphic complex(slate,gneiss,and marble),and its alluvial-lacustrine sediments are composed of silty sand,fine sand,and muddy clay(Guoet al.,2010).The Hetao Basin is a representative area with arsenicosis in China,and its high arsenic generally occurs in shallow groundwater.The most common forms of inorganic arsenic in the basin are As(V)and As(Ш)(Gonget al.,2006).The arsenic concentrations in the shallow groundwater varied greatly,from 0.58 to 572 μg/L(average of 99.73 μg/L),and the As(Ш)ranged from 0.35 to 456 μg/L(average of 79.33 μg/L)(Guoet al.,2008).Notably,inorganic As(Ш)was the predominant species,accounting for 80% of the total As(Guoet al.,2008).Those results were similar to those of Denget al.(2009),who analyzed various arsenic species in the Hetao Basin groundwater.They proposed that methylated arsenic species was relatively minor(less than 2 μg/L),compared to inorganic arsenic(84%–99% of the total soluble arsenic).Additionally,particulate matter comprises a significant fraction of arsenic in the groundwater,and Gonget al.(2006)indicated that acid-leachable particulate arsenic represented approximately 10%–22% of total arsenic in the Hetao Basin groundwater.

High arsenic groundwater poses significant health risks to local villagers,and arsenic-affected aquifers have special characteristics.The primary objective of the present study was to summarize the spatial distribution of high arsenic groundwater in the Hetao Basin and to analyze its sources and the major influencing factors,with the goal of raising awareness of the arsenicosis problem and seeking possible methods to remove arsenic from the groundwater.

2 Spatial distributions

The Hetao Basin is in the Bayannur Banner of Inner Mongolia,and includes seven counties:Dengkou,Hangjinhouqi,Linhe,Wuyuan,Wulateqianqi,Wulatezhongqi,and Wulatehouqi(Figure 1).High arsenic contents in groundwater primarily appeared in the northern and northwestern parts of the Basin(Neidhardtet al.,2012).Ninget al.(2007)analyzed a large amount of data from well water samples all over the Bayannur Banner,and proposed that the arsenic concentration ranged from non-detectable to 1,200 μg/L.The mean value was 13.2,65.6,34.9,24.9,15.3,37.4 and 22.2 μg/L for Dengkou,Hangjinhouqi,Linhe,Wuyuan,Wulateqianqi,Wulatezhongqi and Wulatehouqi counties,respectively.The highest arsenic concentration was found in Hangjinhouqi County in the western part of the Hetao Basin.Results of isotope and minor element geochemical surveys(Denget al.,2009)in Hangjinhouqi County indicated that the arsenic concentration in more than 70% of the samples exceeded 50 μg/L,and more than 25% of the samples exceeded 400 μg/L,with the maximum up to 1,000 μg/L.Thus,the spatial arsenic concentration in the groundwater in the Hetao Basin increased from southwest to northeast,with the aerobic groundwater decreasing from south to north.Also,Fujinoet al.(2004)measured a high mean arsenic concentration of 158.3 μg/L in groundwater(maximum was up to 197.3 μg/L)in the Wuyuan Prefecture in the northern part of the Hetao Basin.Ninget al.(2007)discovered that the arsenic concentration had a relation with well depth due to different contents of oxygen in aquifers;generally,the arsenic concentration was<50 μg/L in shallow wells(3–10 m),>50 μg/L in deeper wells(10–30 m),and even >300 μg/L in wells deeper than 50 m.

3 Sources of groundwater arsenic

Groundwater arsenic in the Hetao Basin mainly comes from natural,rather than anthropogenic processes(Guoet al.,2010).Hagiwaraet al.(2011)indicated that arsenic enrichment in groundwater was due to weathering of some natural solid materials in the sediments.Significant differences of arsenic in sediments were found in the arsenic-free and arsenic-affected areas,implying that sediment type might be a major factor influencing the presence of arsenic in the groundwater in the Hetao Basin(Guoet al.,2008).The direct source of the arsenic in the groundwater is widely believed to be caused by the chemically active arsenic fraction in sediments.For example,Smedleyet al.(2003)observed that the source of the arsenic in the groundwater in Inner Mongolia was very significantly associated with total Fe and with a very low concentration of total S,and furthermore,the presence of arsenic in the oxalate-extractable fraction implied that iron oxides were the main sources of arsenic rather than sulphides in the sediments.The iron oxides adsorbed arsenic through co-precipitation and were subsequently released during weathering of primary minerals under favorable hydrogeochemical conditions(Smedleyet al.,2003).Zhanget al.(2002)reported that arsenic was released from mineral in the Langshan Mountains at higher elevations and transported down gradients into the Basin aquifers.In addition,suspended materials in the Yellow River contain 22.0–24.6 mg/kg arsenic and groundwater receives 14.0–17.0 mg/kg arsenic from irrigation water in Hetao Basin(Hagiwaraet al.,2011).

4 Influencing factors

4.1 Reducing environments

In the Hetao Basin,groundwater arsenic released from sediments is presumed to be caused under reducing conditions(Hagiwaraet al.,2011).In general,redox condition is the main influencing factor in the mobilization of groundwater arsenic.According to Guoet al.(2008),the alluvial sediments in the Basin have extremely low Eh values,indicating a strongly reducing environment.In 63 groundwater samples from the Basin,those researchers indicated that Eh values ranging from-153 to 83 mV,showing moderate oxidizing condition to strong reducing condition.Under inorganic or bacterial processes,such as sulfate-reducing bacteria(SRB),SO42-generates H2S gas and the H2S gas reacts with the Fe2O3to change to FeS.Then arsenic adsorbed to Fe(OH)3dissolves into the groundwater(Hagiwaraet al.,2011).Thus,in the Hetao Basin,the highest arsenic concentration always tends to be observed in groundwater with low SO42-and high dissolved Fe(Denget al.,2009).Therefore,dissolved arsenic occurs as As(Ш)reduced from As(V),and little inorganic arsenic is reduced to methylated arsenic.The main aquifer in the Basin is 14–24 m deep and has fine and medium sand sediment,which is dark gray and black where the groundwater is anaerobic.The color results from high Fe2O3concentrations,and Hagiwaraet al.(2011)found that the arsenic content was high in black sediment under reducing conditions.

After the groundwater is drawn from wells for irrigation and is exposed to the atmosphere,As(Ш)is rapidly oxidized to As(V)and is potentially adsorbed to sediment constituents,such as iron oxides and organic matter,under oxidizing conditions(Neidhardtet al.,2012).

4.2 Groundwater hydraulic gradient

Previous studies showed that there were some relationships between groundwater arsenic concentration and groundwater hydraulic gradient(Zhanget al.,2013).Zhanget al.(2013)found that a low groundwater hydraulic gradient was important in promoting arsenic enrichment in shallow groundwater.In the Hetao Basin,there are two geomorphic units:the piedmont alluvial-proluvial basin in the north and the Yellow River alluvial-lacustrine basin in the south,with a gently decreasing elevation from 1,060 to 1,007 m.The groundwater is recharged by the Yellow River in the south and from the Langshan and Yinshan mountains in the north.According to Zhanget al.(2013),in the piedmont recharge area in front of the Langshan Mountain(the western part of the Hetao Basin),the groundwater level ranges from 1,031.71 m to 1,053 m,the median hydraulic gradient is 4.68‰,and the total arsenic is the lowest with a median concentration of 2.23 μg/L.In the discharge area,the groundwater level is 1,033.5 m to 1,035 m,the hydraulic gradient ranges from 0.71‰ to 2.54‰,and the total arsenic(the majority is As(Ш))varies from 37.2μg/L to 176.6 μg/L.In the runoff area,the lateral groundwater flows from south to north,but the movement of the groundwater is slow due to the weak hydraulic gradient of 0.003‰ to 1.3‰.Most of the arsenic(97%)is As(Ш),and it ranges widely from 3.3μg/L to 745.7 μg/L.

In summary,high arsenic groundwater in the Hetao Basin is widely distributed in a region with slow groundwater flow with low hydraulic gradients.Denget al.(2009)gave out an empirical relationship between groundwater arsenic and hydraulic gradient,described by a power function:

where,yis the median value of total arsenic(μg/L)andxis the median hydraulic gradient(‰).

But this does not apply to the discharge area,possibly because of reducing conditions.Correspondingly,Eh values in the Basin groundwater decrease from the recharge zone to the down-gradient along the flow path(Guoet al.,2010).Similar results were found in other basins,and a previous study(Guoet al.,2014)reported that arsenic showed an increasing trend from the recharge area to the discharge area in the Songnen Basin in Northeast China.Guoet al.(2010)also showed that up-gradient of the drainage canal in the Hetao Basin,the arsenic concentration drastically decreased from 395 to 2.5 μg/L,and down-gradient of the drainage canal,the arsenic concentration increased and reached 781 μg/L.

4.3 Organic matter

In some high arsenic groundwater,organic matter plays an important role in the development of reducing conditions.In the Hetao Basin,high arsenic concentration was found to be coincident with high contents of organic matter(Guoet al.,2011b).In that study,high arsenic groundwater contained 2.65–163 mg/L dissolved organic C in the Hetao Basin,and it could be the main carbon source for microbial metabolism,which was higher than that in high arsenic groundwater from Bangladesh,Cambodia,and West Bengal(Guoet al.,2011b).Organic matter,especially in young,unconsolidated sediments,is fresh and reactive and might be a predominant source of electrons for reduction reactions(Smedleyet al.,2003).Therefore,oxidation of organic matter is widely assumed to be an important contributor in generating anoxic conditions for arsenic in aquifers(Guoet al.,2008).Otherwise,dissolved organic matter would compete with arsenic for adsorption sites on mineral phases(Guoet al.,2011b).Bauer and Blodau(2006)suggested that negatively charged dissolved organic matter enhances desorption of arsenic binding to humic acids through electrostatic effects.

Also,oxidation of dissolved organic matter is considered as one of major factors in the initiation of reducing conditions(Guoet al.,2008).First,sulfate-reducing bacteria(SRB)act on SO42-and organic matter to generate H2S gas(Hagiwaraet al.,2011).Most importantly,dissolved organic matter increases the quantity of arsenic to bound to Fe(OH)3colloids(Ritteret al.,2006).Thus,high concentrations of arsenic in the Hetao Basin are associated with strong reducing conditions,which are evidenced by high concentrations of dissolved organic matter(Denget al.,2009).

To sum up,the process of reducing reactions is the oxidative degradation of organic matter.This produces high HCO3-and then generates biogenic CH4through fermentation of microbial processes,creating a favorable reducing environment and facilitating the mobilization of arsenic in aquifers(Guoet al.,2008).

4.4 pH value

Many studies have focused on the effect of pH on sorption and desorption for different forms of arsenic and controlling the solubility of arsenic in groundwater(Bordoloiet al.,2013b).Regarding the function of the equilibrium solution pH,Taoet al.(2011)suggested that pH has an essential effect on As(V)sorption,and the optimal As(V)uptake demands a pH range of 6.0–8.0.Guoet al.(2011a)proposed that samples from high arsenic groundwater(<200 μg/L)across the Hetao Basin generally had pH >8.3,indicating that high pH could affect the arsenic mobilization.In contrast,elevated levels of groundwater arsenic in geologically young sedimentary basins like in Bangladesh and West Bengal,India,occur at near-neutral pH(Nakayaet al.,2011).

Favorable pH can also be utilized to remove arsenic in groundwater by precipitation-coagulation,generally using NaHCO3,KMnO4,and FeCl3as the pH-conditioner(Bordoloiet al.,2013b).The removal mechanisms might form through three major steps:(1)pH above 3.0,As(V)exists in anionic forms of H2AsO4-,HAsO42-or AsO43-;(2)increasing pH around 7.3,Fe(OH)3particles occur and have a net positive charge,and at the same time soluble arsenic species converses to insoluble reaction products;and(3)arsenic is an anion and adsorbs onto positively charged Fe(OH)3particles.But when pH was above 8.0,arsenic removal would decrease.Thus,the optimal pH for arsenic removal is 7.3 or less(Bordoloiet al.,2013a).

4.5 Evapotranspiration and soil texture

Hydrochemical and hydrogen and oxygen isotope geochemical studies have shown that evapotranspiration is also an important process in controlling the enrichment of Na,Cl,and trace elements such as arsenic in groundwater(Denget al.,2009).Especially in arid and semi-arid climates,evapotranspiration contributes to further increase the arsenic concentration in waters(Nicolliet al.,2010).Some studies showed that high Na+water had high Cl-but did not contain high arsenic,but high TDS(total dissolved solids)groundwater contained high arsenic(Guoet al.,2011a).This indicated that arsenic enrichment was independent on evaporation,and evaporation would elevate arsenic concentration in groundwater.

The textures of sediments in the Hetao Basin range from fine sand to clay,and their color varies from brown to grey to black in boreholes(Guoet al.,2008).The results of X-ray diffraction analyzed by Guoet al.(2008)showed that sediments in arsenic-affected areas exhibited various contents of calcite and dolomite,while the fine sand was mainly composed of quartz and the clay sediment contained considerable amounts of clay minerals.The black color,full of siderite and pyrite,was found in silty clay sediments,indicating that it developed in a strongly reductive environment;higher percentages of arsenic concentration also occurred in the black clay sediment.The possible reason was that argillaceous materials might have restricted diffusion of atmospheric oxygen into the aquifers.On the other hand,clay soil was responsible for more organic carbon and required higher cation exchange capacity in soil compared to coarse soil.The direct result would be the marked differences in soil microbial activity for combination with heavy metals(Patel and Patra,2014).

5 Conclusions

1)The spatial arsenic concentration in the groundwater in the Hetao Basin increases from southwest to northeast,and the high arsenic concentrations were found in Hangjinhouqi and Wuyuan counties in the western and northern part of the Hetao Basin.This is believed to be caused by different contents of oxygen in aquifers in south and north.

2)The direct source of the arsenic in the groundwater is originated from the chemically active arsenic fraction in sediments such as iron oxides.

3)The redox condition is the main influencing factor in the mobilization of groundwater arsenic in the Hetao Basin.Dissolved arsenic occurs as As(Ш)reduced from As(V),and little inorganic arsenic is reduced to methylated arsenic under reducing conditions.

4)High arsenic groundwater in the Hetao Basin is widely distributed in a region with slow groundwater flow with low hydraulic gradients.

5)In the Hetao Basin,high arsenic concentration was found to be coincident with high contents of organic matter and the process of reducing reactions is the oxidative degradation of organic matter.

6)pH has an essential effect on As(V)sorption and favorable pH can also be utilized to remove arsenic in groundwater by precipitation-coagulation.

7)The higher evaporation and the texture of sediments in the Hetao Basin contribute to further increase the arsenic concentration in the groundwater.

The authors wish to thank all the members of Naiman Desertification Research Station and Water Conservancy Bureau of Bayannur League(Inner Mongolia,China).This study was financially supported by the research projects 2011BAC07B02,XDA05050201-04-01,and 31170413.

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