Environmental geochemistry of high arsenic groundwater at western Hetao plain, Inner Mongolia

Abstract

Environmental geochemistry of high arsenic groundwater at Hetao plain was studied on the basis of geochemical survey of the groundwater and a core sediment. Arsenic concentration in groundwater samples varies from 76 to 1093 μg/L. The high arsenic groundwater mostly appears to be weakly alkaline. The concentrations of \mathrm{N}{\mathrm{O}}_{\mathrm{3}}^{-} and \mathrm{S}{\mathrm{O}}_{\mathrm{4}}^{\mathrm{2}-} are relatively low, while the concentrations of DOC, \mathrm{N}{\mathrm{H}}_{\mathrm{4}}^{+}, dissolved Fe and sulfide are relatively great. Analysis of arsenic speciation in 21 samples shows that arsenic is present in the solution predominantly as As(III), while particulate arsenic constitutes about 10% of the total arsenic. Methane is detected in five samples with the greatest content being 5107 μg/L. The shallow aquifer in Hangjinhouqi of western Hetao plain is of strongly reducing condition. The arsenic content in 23 core sediment samples varies from 7.7 to 34.6 mg/kg, with great value in clay and mild clay layer. The obvious positive relationship in content between Fe₂O₃, Mn, Sb, B, V and As indicates that the distribution of arsenic in the sediments may be related to Fe and Mn oxides, and the mobilization of Sb, B and V may be affected by similar geochemical processes as that of As.

Keywords

arsenic groundwater / environmental geochemistry / Hetao plain

Introduction

Endemic arseniasis has caused worldwide attention because of its great threat to human health in recent decades. The environmental chemical behavior of arsenic and the controlling strategy of the resultant endemic arseniasis have become priority studies for environmental science. The most serious endemic arseniasis over the world has been reported in Bangladesh (Chakraborti et al., 2002; Kinniburgh and Kosmus, 2002). Endemic arseniasis was found in eleven counties in Hetao Plain of Inner Mongolia, where there were about 300000 victims in an area of 13000 km² (Jin et al., 2003), with the most serious district being at Hangjinhouqi of the western part of the Hetao plain. Numerous efforts have been made on the investigation of the hydrogeochemistry of endemic arseniasis at the Hetao plain (Tang et al., 1996; Gao, 1999; Smedley et al., 2003; Kwok et al., 2007).

Investigations on environmental geology of the Hetao Plain indicated that the clay soil and humus soil were beneficial to the reductive enrichment of arsenic, and As (V) can be transformed into As (III) in the humus-rich reducing environment (Tang et al., 1996). Gao (1999) found that the genesis of the groundwater with high arsenic levels is related to the background of the special geology, topography, paleogeography and geochemical environment in the area. The results of Smedley et al. (2003) indicated that As mobilization occurs under strong reducing conditions in fluviolacustrine aquifers of the Huhhot basin, Inner Mongolia, characterized by moderate abundance of dissolved Fe in association with abundant Mn, \mathrm{N}{\mathrm{H}}_{\mathrm{4}}^{+}, dissolved organic carbon (DOC), \mathrm{H}\mathrm{C}{\mathrm{O}}_{\mathrm{3}}^{-}, P and low \mathrm{S}{\mathrm{O}}_{\mathrm{4}}^{\mathrm{2}-} concentration. Acid-leachable particulate arsenic constitutes about 39±38% of the total arsenic in Inner Mongolia (Gong et al., 2006). While most previous work laid emphasis on the geochemical behavior of arsenic in groundwater, we present herein the environmental geochemistry of arsenic aquifers and sediments at the Hetao plain.

Geology and hydrogeology

The Hetao plain of Inner Mongolia lies on the northern margin of the Yellow River, covered by Linhe City, and Hangjinhouqi and Wuyuan counties affiliated to Bayannaoer City. It belongs to the continental temperate zone in climate, with an average annual precipitation ranging from 130 to 220 mm and evaporation from 2000 to 2500 mm. The geomorphology is featured by high lands in the southwest and low in the northeast (Fig. 1). The Hetao plain is located in a fault basin formed at the end of the Jurassic period with fine clastic sediments mainly deposited in an inland lake. The overlying Tertiary red sandstone and shale with gypsum and rock salt have a thickness up to 2000 m. The Quaternary sediments are 200 m to 1500 m in thickness around the deposition center in the northwest and only 20 to 50 m at the front of the mountain. The Pleistocene lacustrine sediments are mainly composed of muddy clay, silt and fine sand in the lower part, and clay and sand layers in the upper part that are rich in organics, and partly associated with peat deposit (Fig. 2). With the development of the Yellow River at the end of Pleistocene and at the beginning of the Holocene, frequent channel changes left a lot of residual lakes and oxbow lakes where humus and organic mud were deposited. At the margin of Yinshan Mountains, the north Hangjinhouqi County, there are two large-scale polymetallic sulfide ore deposits, which have been mined since the 1970s.

Hangjinhouqi County at the western Hetao plain was selected for our case study, where endemic arseniasis is proposed to be the most serious and representative in Inner Mongolia. Most of the local residents are still using the groundwater with great contents of arsenic, TDS, or fluoride derived mainly from the shallow aquifer with a burial depth of mostly less than 30 m. Arseniasis is found to occur in seven villages, and the victims have been counted up to 1169 persons.

Sampling and methodology

Groundwater samples

Thirty-eight groundwater samples were collected from seven villages throughout Hangjinhouqi (Fig. 3) in November 2006. Samples for major and trace element analysis were filtered through a membrane (0.45 μm) in situ and collected in polyethylene (PET) bottles. Those for analysis of cations were acidified by 1% v/v HNO₃. For dissolved organic carbon (DOC) analysis, separate samples were collected in glass bottles. Unfiltered samples for total Fe analysis were collected and acidified to pH=1 using H₂SO₄. Zinc acetate and sodium hydroxide were added into the groundwater samples for total dissolved sulfide analysis. 2.5 L unfiltered samples were collected for the analysis of soluble CH₄ gas in groundwater. Arsenic speciation separation of 21 samples was done in situ; particulate arsenic and soluble arsenite, arsenate, monomethylarsonate (MMA) and dimethylarsinate (DMA) species were separated from total arsenic, following the method reported by Le et al. (2000). Disposable syringes and 0.45 μm membrane filters used herein were from Whatman, UK. Resin-based strong cation-exchange cartridges and silica-based strong anion exchange cartridges were from Alltech and Supelco, USA, respectively.

Water temperature, pH, dissolved O₂ (DO), and specific electrical conductance (SEC) were measured in situ using Hach sension2 portable pH/ISE meter, and Hach LDO^TM HQ10 portable apparatus, respectively. Alkalinity was determined on the same day using Gran titration. Total Fe, total dissolved sulfide and ammoniac nitrogen were analyzed by spectrophotometer. Anions and cations were determined by ion chromatography (DIX-120, Dionex, USA) and ICP-AES (IRIS Intrepid II XSP, USA), respectively. Total arsenic and arsenic species were analyzed by hydride generation atomic fluorescence spectrometry (Titan AFS-830, Beijing) at Key Laboratory of Biogeology and Environmental Geology of Ministry of Education, China University of Geosciences (Wuhan). Besides, 11 samples were determined using HPLC-ICPMS to double check the accuracy and reliability at the Analytic Center of Tsinghua University. DOC was measured using total organic carbon analyzer (Elementar, Germany). Soluble CH₄ was determined by gas chromatography at State Key Laboratory of Gas Geochemistry in Lanzhou Institute of Geology, Chinese Academy of Sciences.

Sediment samples

Sediments spanning a variety of depth intervals were collected from a core in Shahai Village in August 2007, a site witnessed the most serious endemic arseniasis in Hangjinhouqi. The lithology was shown in Fig. 4. The core samples were immediately packed in a box sealed with wax after being wrapped by fresh film over five layers and put into a fresh bag for transportation to the laboratory to be kept in a refrigerator. The laboratory analysis was done within two weeks.

X-ray diffraction (XRD) of the sediments was determined by X'Pert DY2198 (Netherlands) at State Key Laboratory of Geological Processes and Mineral Resources in China University of Geosciences in Wuhan. Chemical composition was determined at Key Laboratory of Biogeology and Environmental Geology of Ministry of Education in China University of Geosciences in Wuhan by ICP-AES (IRIS Intrepid II XSP, USA) after being digested by 3050B of USEPA method. Arsenic and antimony were determined using hydride generation atomic fluorescence spectrometry (Titan AFS-830, Beijing) after being digested by aqua regia (Zhao et al., 2007).

Results

Hydrochemistry

Most wells abstracting water from the shallow aquifer are less than 30 m deep, although there are several wells deeper than 70 m. The groundwater with a great abundance of arsenic is from 15 to 25 m in depth. Major ionic concentrations of groundwater samples are shown in Table 1 and plotted onto Piper’s diagram (Fig. 5). The groundwater samples are neutral or weakly alkaline, with pH ranging between 7.00 and 7.89. The range of electric conductivity (EC) of groundwater varies from 438 to 5 550 μS/cm. The chloride and bicarbonate concentration ranges between 41.8 and 2 098 mg/L, and between 243.8 and 1 262 mg/L, respectively. Besides, most water samples have elevated concentrations of Cl^-. The concentration of sulfate in most water samples is quite low, though the maxima could reach as high as 1 551 mg/L. Concentrations of \mathrm{N}{O}_3^{-} and total P were also commonly low, less than 1.58 and 0.46 mg/L, respectively. The variation range of major cations such as K⁺, Na⁺, Ca²⁺ and Mg²⁺ were quite large. Most fresh water is of the HCO₃–Na–Mg type, while most saline water is of the Cl–Na–(Mg) type.

Arsenic

The dissolved arsenic concentration in the groundwater sampled is from 76 to 1093 μg/L, with more than 40% samples exceeding 400 μg/L. The most seriously arsenic-contaminated areas are Shahai and Tuanjie villages, and the dissolved arsenic concentration gradually increased from the south to the north. The concentrations of total arsenic and different arsenic speciation of 21 water samples were shown in Fig. 6. The concentration of on-site separated As species fits very well with the results of HPLC-ICPMS. Concentrations of particulate arsenic, As (III) and As(V) varies from 7.29 to 82.6 μg/L, 65 to 1083.3 μg/L, and 4.6 to 81.6 μg/L, respectively. Particulate arsenic constitutes about 10% of the total arsenic. As (III) is the predominant species, with a ratio of As (III) to the total soluble As from 84% to 99%. Concentrations of MMA and DMA are commonly less than 2 μg/L, being significantly low compared with those of inorganic As. Gong et al. (2006) showed that no methyl arsenicals were detected by LC-HGAFS in the arsenic-poisoning endemic area in Inner Mongolia.

Trace elements and organic components of water samples

The concentrations of other trace elements and organic components of the water samples were given in Table 1. The concentrations of dissolved Fe and Mn range between 155 and 3592 μg/L, and between 20 and 772 μg/L, respectively. The concentration of total Fe varies from 447 to 5033 μg/L, being relatively high levels compared with those in Datong basin, Shanxi (Guo and Wang, 2005; Wang et al., 2008). Fluoride concentrations in the shallow aquifer is relatively great, varying from 0.40 to 3.36 mg/L, with 21% of the samples exceeding WHO standard for F^- concentration (1.5 mg/L) in drinking water, which is proposed to be the cause of endemic fluorosis.

It is interesting that the DOC concentration is relatively large, up to 12.88 mg/L. Besides, dissolved CH₄ was detected in 16 water samples. The greatest concentration was found in the sample from a borehole 70 m deep in Taiyangmiao Village (No. 4), with As and DOC being 221 μg/L and 9.46 mg/L in concentration, respectively.

Chemical composition of sediments

The drilling core used to sample the sediments is in the sixth team of Wulan in Shahai Village, the center of the study area. The top interval from 6 to 10 cm was unable to be sampled due to the drilling technique problems of mud. A 5 cm interval sandwiched with bark in SH-14 wasn’t sampled either. The XRD analysis in representative samples indicates that the major minerals are quartz, feldspar and clay minerals (montmorillonite, chlorite, illite), with 30%-50% clay minerals in mud, and 55%-70% quartz and feldspar in sand. Chemical composition was analyzed in 20 sediment samples (Table 2). The As content ranges between 7.7 and 34.6 mg/kg, with the mean value being 14.67 mg/kg. Enhanced arsenic concentration was found in depth intervals of 10, 20, 25 and 28 m, where the lithology is mainly composed of clay and medium and fine sand, and, most notably, the local aquifer is used for drinking.

Discussion

Groundwater environment

The concentration of As in groundwater might show a strong relationship with the environmental conditions of the groundwater. Arsenic can be adsorbed easily by the minerals in aquifer sediments such as iron and manganese oxides, goethite and gibbsite which are positively charged. The increased pH values will reduce the adsorption of arsenate and arsenite on colloid and clay minerals and enhance the migration of arsenic (Wang et al., 2004). Thus, the weak alkalinity may lead to the release of arsenic from the sediments in Hangjinhouqi.

Plots of the concentrations of arsenic and dissolved sulfide, \mathrm{N}{\mathrm{H}}_{\mathrm{4}}^{+}, dissolved Fe, Mn and DOC in groundwater from Hangjinhouqi were given in Fig. 7. The concentrations of \mathrm{N}{\mathrm{H}}_{\mathrm{4}}^{+}, dissolved Fe and sulfide are relatively high, while the concentrations of \mathrm{N}{\mathrm{O}}_{\mathrm{3}}^{-} and \mathrm{S}{\mathrm{O}}_{\mathrm{4}}^{\mathrm{2}-} are relatively low, indicating that shallow aquifers are of strong reducing conditions. More importantly, positive correlation is observed between the concentrations of \mathrm{N}{\mathrm{H}}_{\mathrm{4}}^{+}, dissolved sulfide, and As in groundwater. Elevated concentrations of As in shallow aquifers are associated with the reducing environment at the Hetao plain. Arsenic is believed to show a relatively low concentration in groundwater under oxidic conditions because arsenides (arsenate and arsenite) can be adsorbed by colloid or Fe and/or Mn oxides and Fe oxyhydroxides. However, the colloid becomes unstable and Fe (Mn) oxyhydroxides will be reduced when the aquifers change into a reducing environment, and adsorbed arsenic would become mobile and dissolved into the groundwater. What is more, the arsenides absorbed on these minerals will be dissolved into the groundwater at the same time (Nickson, 1998). Dissolved Fe and Mn increased in concentration with the elevation of arsenic, correspondingly (Fig. 7).

The dominance of As(III) among arsenic species is additional evidence for the occurrence of strongly reducing conditions. Tang et al. (1996) indicated that the ratio of As (III) and As (V) varied from 2∶8 to 1∶9 under oxidic conditions, and from 8∶2 to 1∶0 under reducing conditions. The ratio of As (III) to the total soluble As in groundwater samples from Hangjinhouqi is about 90%. Our data are comparable with those of Smedley et al. (2003) from the Huhhot basin, Inner Mongolia, both being dominated by As (III) under the strongly reducing conditions.

The increase in organic content would lead to the release of arsenic under weakly alkaline conditions (Redman et al., 2001). Lacustrine sediments are the main Quaternary deposits in the Hetao plain. After a long geological evolution, a large number of aquatic plants and plankton in lakes, swamps and rivers were decomposed and deposited. A large amount of organic matter was accumulated in the aquifer. Such kind of ancient environment is a fundamental condition for the formation of high arsenic groundwater (Yang et al., 2008), supported by the positive correlation in abundance between DOC and As (Fig. 7). Previous work confirmed humic acid as the main component of DOC (Yu et al., 2002). The existence of abundant dissolved CH₄ in the water samples provides further evidence of the strongly reducing environment. Consequently, the highly arsenic groundwater investigated herein is likely to be alkaline and reductive, favoring the release of As from the sediments.

Geochemical characteristics of the sediments

There is a clay and sand interbedded stratum in the area, and the concentration of As in clay is greater than in sandy soil. It is consistent with the result of Gao (1999) that the aquifers with arsenic pollution are mainly in grayish black fine silty sand layer or of interbedded fine silty sand, clay and silty clay. The adsorption capacity of sediments correlates with the size of particles. The greater the sedimentary particles are, the lower the content of organic matter is. The sediment with abundant organic matter and small particles is demonstrated to be the dominant absorptive complex of arsenic (Su, 2006). Junji et al. (2004) also found that the sediment with abundant fine particles or low permeability contains abundant As, which provides further evidence that the clay contains great As content.

Vertical distributions of some trace elements in the core sediment (Fig. 8) reveals an obvious correlation in content between As and Fe₂O₃, Mn, Sb, B, V. As is great in content in the clay or mild clay layer, so are some trace elements. The distribution of As in sediment is shown to have relevance to the oxides of Fe and Mn (Junji et al., 2004). The positive correlation in concentration between As and oxides of Fe and Mn indicates that a large amount of As may be absorbed by the oxides of Fe and Mn. The vertical distribution of Sb, B and V accords with As in content, indicating that those elements may be affected by similar geochemical processes.

Conclusions

Elevated concentrations of As in shallow aquifers in Hangjinhouqi of western Hetao Plain are found in association with great concentrations of DOC, \mathrm{N}{\mathrm{H}}_{\mathrm{4}}^{+}, dissolved Fe and sulfide, the dominance of As (III) among arsenic species, relatively low concentrations of \mathrm{N}{\mathrm{O}}_{\mathrm{3}}^{-} and \mathrm{S}{\mathrm{O}}_{\mathrm{4}}^{\mathrm{2}-}, and occasionally elevated content of dissolved CH₄. The shallow aquifers are demonstrated to be neutral or weakly alkaline and strongly reductive.

The content of arsenic in the core sediment is from 7.7 to 34.6 mg/kg, with great values in clay and mild clay layer, arising from the adsorption of clay minerals. The distribution of arsenic in the sediments may also have relevance to the oxides of Fe and Mn, while the mobilization of Sb, B and V may be affected by similar geochemical processes as that of As.

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