Cunrong Gao¹, Junji Akai², Xiujun Gao³, Iwao Kobayashi², Yoshihiro Kubota²,

1 China Institute for Geo-Environmental Monnitoring, Beijing 100081;

2 Department of Technology and Science, Niigata University, Japan 950-2181

3 Graduate School of Engineering, Tokyo University, Japan 113-8656)

Abstract:To clarify the mechanism of arsenic contamination of groundwater, the Hetao Plain of Inner Mongolia, China, one of the arsenic severely contaminated areas, was studied as a case. To reveal where the arsenic came from and how it was accumulated in groundwater and sediments, this paper discusses the mechanism of migration, accumulation and remigration of the arsenic from geological, geochemical and hydrogeochemical viewpoint. It was found that there are two main causes giving rise to arsenic contamination of groundwater in the Hetao Plain: 1) in natural geological conditions, arsenic in sediments remigrated by groundwater in a long time, and 2) influence of human activities, such as penetration of irrigation water. In the polluted area, arsenic is mainly scattered in the clay and silty clay layers located at less than ac.40m depths. Accumulation of the arsenic in the sediments is caused by oxidation and adsorption of colloid. The sediments containing arsenic are derived mainly from pyrite deposits, igneous and metamorphic rocks in the RangsanMountains, the deep groundwater with high arsenic concentration, and the area around the upper reaches of Yellow River. Dissolution of arsenic from sediments to groundwater is inferred to be the following reasons: 1) under sealed earth’s surface, ox-reductive environment converted to reductive environment, and 2) penetration of irrigation water causing acid and alkaline condition changed.

Key words: groundwater, arsenic contamination, mechanism, the Hetao Plain, China

Introduction

Arsenic contamination of groundwater is one of the important environmental issues that threaten public healthy and living level. In recent years, arsenic contamination of groundwater has become a major environmental geological issue all over the world. The high concentration of arsenic in groundwater has been reported not only in many countries of Asia (B.K.Mandal, K.T.Suzuki, 2002) but also in western and south-western US (Davis et al. 1994).

The Hetao Plain, covering a total area of 13,040km², is one of the main contaminated areas in Inner Mongolian, China. The population of this area is about 900,000, out of which about 340,000 people are engaging in agriculture (by The Hydrogeological Team of Inner Mongolian, 1982). Asia Arsenic Network (1996) and Endemic Disease Society investigated the drinking water wells in Bayinmaodao of Dengkou town, located in the western part of HETAO Plain. It shows that at severe arsenic contaminated villages, 144 (76.2%) out of 189 wells are above the drinking water quality standard of China of 0.05mg/L. The maximum measured value reaches 1.088mg/L. The health investigation also shows that 414(35.4%) out of 1169 residents examined are suffering from chronic dermal arsenic poisoning in this area.

Since the poisoned patient was first found at Shengfeng village of Wuyuan town, located in the central part of the Hetao Plain, (water quality survey, 1992) arsenic investigation has been carried out in the town by the local Hygiene Epidemic Prevention Station in Wuyuan. The survey results show that arsenic concentrations of 266 wells in 66 communities are above standard level (0.05mg/L) to a maximum of 0.59mg/L, among a total of 3071 drinking water wells in 679 communities. The results of health investigations indicate that 22.6% of people are arseniasis patients of a town holding 3530 people and arseniasis patients reach as high as 61% in the most affected area.

To clarify the conditions of arsenic contamination in groundwater and the distribution of contaminated area in the Hetao Plain, in recent years, the authors performed several times of field investigations and conducted extensive laboratory experiments on water, soil and rock samples related to arsenic. This paper reports the surveying and analysis results about the arsenic contamination mechanism in groundwater in the Hetao Plain, Inner Mongolian.

1 Methodology and research process

1.1 Experimental section

1) Samples collected form western and eastern part of the Hetao Plain.

Investigations on drinking water wells (at intervals of 5 – 20 km), ranging from the western plain, Hang Jin Hou Qi town to the eastern part, Wu La Te Qian Qi town, were carried out, (Fig 1). The information including the locations of wells (measured by GPS), user, date of installation, structure of well, well depth, groundwater level, and water temperature were noted.pH, electrical conductivity (EC) and oxidation-reduction potential (ORP) of water samples were measured in situ by RKC DP-300·TOA，HM-12P·TOA and EC CM-14P·TRX-90 (made in Japan) respectively. Arsenic, chloride and fluoride in the water samples collected within 3 days were analyzed by Silver diethyldithiocarbamate method, silver nitrate titration method and SPADNS (p-sulfophenylazochromotropic acid) method at the local Hydraulic Research Institute. 

2) Samples collected from the central part of the plain.

Arsenic, iron and organic(C, N) content were tested for the rock core samples. Arsenic release test, X-ray diffraction analyses and EDS analysis were carried out for some of samples in different media. In a2km×2km area, there are 50 survey points, out of which, 36 points at depths of 18-25m and 14 points at depths of 28.5-42m.Arsenic and salinity concentrations were measured at various groundwater depths. 

3) Samples collected from regions around the plain

 Arsenic and iron contents in rock core samples were measured by atomic fluorescent spectrometry (WYD-2) and atomic absorption spectrometry (PE3030) at the Inner Mongolia Rock and Mineral Research Institute.  X-ray diffraction analyses were carried out by using a Rigaku Geigerflex(Red-X system) at the science department of Niigata University, Japan. EDS analyses were carried out by using Voyager IV and TN 2000, Noran an Instrument Co., Ltd. Organic (C、N) content analyses were carried out by CHN CORDER MT-5.

1.2 Release test

Samples for the release tests were collected from survey rock cores, surface soil and minerals of the pyrite ore taken at the toe of the Lang Ya Shan Mountains located in the western part of the Hetao plain. The test procedure is as follows:  

All the specimens were dried at a low temperature, and crushed into particles less than 200 meshes. In the alkaline solution test 1g of specimen was prepared by adding 50ml of NaOH (5%) solution. After the solution was heated for one hour at a low temperature, water was added to the specified volume and cooled. Then the water temperature, pH, ORP and arsenic concentration were measured. With respect to the release test in distilled water, 50ml of distilled water was put into a container with 2g of specimens, heated for 2 hours. Then, water was added to the specified volume. After the solution is cooled, the procedure can be repeated as that in NaOH solution. The release test with HCl solution is to put 50ml of HCl (2:1) into the container prepared 1g of specimen, heated for one hour, then, add water to the specified volume until cooled. Arsenic and iron concentrations were then measured.

2 Results of investigation and tests

2.1 Investigation results of regional water quality

2.1.1 As shown in Fig.1, the investigation results of water quality during October 1998 indicated that the contamination area of the Hetao Plain (arsenic levels exceeding the China drinking water standard of 0.05mg/L) is mainly scattered in the central and western part of the plain. The distribution locations of wells with high arsenic concentrations are basically in line with that of the active fault estimated by China Seismological Bureau (1989) and the China Workgroup of IGCP Project 206. In addition, the distribution zone of arsenic contaminated wells located is in agreement with the location of the flood plain of Yellow River and old rivers (Hydrological Geological Team of Inner Mongolia, 1982; China Remote Sensing Satellite Ground Station, 1997)

Fig.1 Map of sample location and distribution of arsenic content in groundwater (sample collected from wells at village)

2.1.2 Fig.2 shows the vertical distribution of arsenic content of groundwater up to a depth of 35m. Arsenic contents exceeding China standard level of 0.05mg/L are mainly distributed at depths of 8-13m and 19-25m.

The maximum value is up to 0.943 mg/L. Contaminated groundwater is almost freshwater with salt content of 53.4-1500 mg/L, electric conductivity of 1278-5000μS/cm(25℃), nearly negative ORP (belonging to reduction environment) and pH of 7.4-8.5.

Fig.2 Relation between depth of well and arsenic concentration of groundwater.

2.1.3 The analysis results of surface water samples collected in October 1998 were given in Table 1

(sample locations shown in Fig.1).

Table 1 Arsenic contamination of surface water

2.2 Results of borehole survey and release test

The analysis results of groundwater samples collected from 4 bore holes (as shown in Fig.1 (A, B, C, D)), including As and Fe concentration of bore-hole soil collected from A, and release test results are given in table 2 and table 3 respectively.

Table 2 As and Fe concentrations and TDS in groundwater at various depths in location A, B, C and D (Gao,1999)

Table 3. Arsenic levels in samples from borehole soils and minerals and results of elution experiments

It shows that there are relatively close relations between arsenic and iron concentration and soil characteristics. Arsenic contents range from 10.2 to 47.10mg/kg in clay, silty clay and silty sand, but in silty sand, fine sand and medium fine sand with lower values of 6.3-9.5mg/kg. Iron contents in borehole soils have the same characters as that of arsenic in the above various soils. It shows a positive correlation between iron and arsenic in soils of this area, as shown in Fig.3.  

Fig.3 Arsenic and iron contents in borehole soils.

2.3 Survey results of water quality in the study area of the central part of the Hetao plain (Fig.1). Arsenic concentrations of 28(74%) out of 38 wells are above the China standard level (CSL) of 0.05mgl^-1 at depths of 18-25m. Of all the 14 bore-holes at depths of 28.5-42m, arsenic concentration of groundwater is above the CSL of 0.05 mg/L to a maximum value of 0.377mg/L. Arsenic level of groundwater shows higher values along the both sides of surface drainage canal (Fig.4). Arsenic concentrations not only increase with increasing depth but also vary rapidly within a small range (Fig.5). Although varying greatly within a small range (as shown in Fig.6), salt concentration does not show close correlations with arsenic concentration. 

Fig.4 Distribution of arsenic concentration of groundwater at different depths in the investigation area (Gao et.al.,1999).

3 Analysis

3.1 Circulation of arsenic at the earth crust surface

Circulation model of arsenic and regional distribution of arsenic contaminated groundwater are shown in Fig. 7 (Gao, 2000). Arsenic contamination in groundwater caused by natural reason is an environmental hydrogeochemical phenomena occurring in the circulation process of arsenic at the crust surface (Gao, 2000). Primary arsenic is mainly existing in two forms, i.e., as solid existing in primary rock and mineral, or as ion or component existing in deep groundwater (primary water, thermal water and fissure water et al.). The above two forms of arsenic affected by various natural factors and human activities are released into groundwater and directly

Fig.5 Contour of arsenic concentration of groundwater at depths of 18-25m (Gao, 1999).

Fig.6 Contour of salt contamination of groundwater at depths of 18-25m.

lead to groundwater pollution in the following two regions: 1) around ore; 2) along fault or around thermal spring. Arsenic scattered to the earth crust surface due to earth crust movement and various weatheringre-accumulated at the bed of old river, delta and old lake with colloid absorption and mineralization. Over a long period of time and the change of environment, arsenic released from sediment into the groundwater results in much severer groundwater contamination. It is discovered that arsenic contamination of groundwater occurring in many countries of Asia is similar to the contamination mechanism mentioned above（Yamazaki et al. 2000;　Research Group for Applied Geology, Sub-group for Arsenic Contamination et al. 2000.

Fig.7 Arsenic circulation model on the crust surface and the distribution of arsenic contaminated groundwater (Gao 2000)

3.2 Mechanism of arsenic contamination in the Hetao Plain

3.2.1 Source of arsenic contamination

Layers of clay, silty clay and silty sand high in arsenic found from the surface to the depth of more than 40m in contaminated area, correlate well with arsenic concentration of groundwater at the same depth (Fig.8). Therefore, it is presumed that these layers are the dominant sources supplying arsenic for groundwater. It is considered that the arsenic in the

layers is from 3 sources. ①Rural region around the plain.

Large pyrite ore rich in arsenic are distributed in the rural region of the western part of the Hetao Plain. The concentration of arsenic measured in mineral is as high as 87.30mg/kg. Arsenic concentration in the surface water from the rural region is 0.011-0.022 mg/L, while in the sediment of surface water reaching 17.6-22.7 mg/L (Gao, 2000), and in rock of this region

Fig.8 Arsenic concentrations in the groundwater and layers

up to 10-60mg/kg (Li & Li, 1995). Arsenic in rock is 5.5-39 times as high as that of earth crust (1.8ppm) (Oki et al. 1989). It is supposed that rocks and minerals containing arsenic over a long period of weathering and erosion, with impaction of surface water, are accumulated in the sediments of old lakes.

②     Upper reaches of the Yellow River

Most of sediments in the Hetao basin are mobilized by the Yellow River, and arsenic concentration ranges from 13.7 to 17.0mg/kg in modern sediment of the Yellow River (Kuboda et al. 1999). The Hetao plain is labeled as a population pathological change area. The isotope ratios of ⁸⁷Sr/⁸⁶Sr(0.7100-0.7164),²⁰⁶Pb/²⁰⁴Pb(18.3817-19.1871), and ²⁰⁷Pb/²⁰⁴Pb (15.7581-15.9578) in groundwater of the population pathological change area are close to the ratios measured in water from mine areas (⁸⁷Sr/⁸⁶Sr=0.7196,²⁰⁶Pb/²⁰⁴Pb=19.1940,²⁰⁷Pb/²⁰⁴Pb=15.9574), and are somewhat close to ratios in Yellow River water, which is used to irrigating in the Hetao Plain(⁸⁷Sr/⁸⁶Sr=0.7168,²⁰⁶Pb/²⁰⁴Pb=18.3495,²⁰⁷Pb/²⁰⁴Pb=15.5969) (Zhang et al., 2002). The average arsenic concentration of soil in China is 11.2mg/kg (Huang 1994). Therefore, it is possible that arsenic at the upper reaches of the Yellow River is carried to lakes or the bed of rivers to be accumulated.     

③     Deep groundwater

The result of borehole investigation shows that at the same site arsenic concentration of the groundwater increases with increasing depth, and that iron and salt concentrations have the same tendency. Tectonic movement was relatively intensive in the Hetao Plain. Deep groundwater rich in arsenic effused out along the fault and arsenic was easily accumulated in the shallow layers. Confined water is alkalized and microelementshows extreme anomaly along tectonic line at the north of Hangjinhouqi, in western part of the plain, between Linhe and Wulamuqianqi, in the southern part of the plain and Shengfengxiang, where arsenic poisoning wasfirst founded, located in the central and eastern part of the Plain (Inner Mongolia Hydrogeological Team, 1982). 

Concentrations of B, I and Br in the groundwater of this plain are 1.2-12.0，3.0-80.0 and 18.0-80.0mg/L respectively, while salt content up to 73.3g/L  at Xinjiacun village. Concentrations of La(13-28mk/L), Sm(0.74-1.53mk/L) and Yb (1.25-3.86mk/L)in groundwater are tens to hundreds of times as high as these in freshwater of the world standard. Therefore, it is estimated that deep groundwater containing arsenic effusedalong faults into shallow confined water and arsenic was possibly accumulated in the shallow layers.

3.2.2 Sedimentary environment of layer containing arsenic

As mentioned above, the layers rich in arsenic are mainly clay, silty clay and silty sand layer (Fig.8). These layers belong to strata of Holocence Series (Q₄) and Early Pleistocene Series (Q₃) and the sedimentaryenvironment is a transition environment, which is transformed from the reductive environment of deep lake phase to oxidative environment of lake, swamp and river phase (Inner Mongolia Hydrogeological Team, 1981;Sun,1984)

3.2.3 Existing state of arsenic in strata

The results of release tests indicate that arsenic in sediment is prone to be dissolved in alkali solution (pH 13.1-13.29) and the release rate is 87.5-98.75% for sand sediment and 72.73-84.21% for clayey sediment. In dilute hydrochloric acid solution (HCl [2+1]) the release rates for the above two kinds of materials are 41.56% and 42.46% respectively. In alkali solution, the release rate of arsenic for pyrite ore is only 0.02%. The test results by X-ray and EDS installed with electric microscope manifest that arsenic minerals are not found in 8 borehole samples collected from different depth and different rocks in the polluted area, including arsenic content reaches 47.1mg/kg in one of samples. Concentrations of organic matter in 18 borehole core samples show that there are no positive correlations between concentration of arsenic and TN+TOC (content of total organic matter in sample, ranging from 0.0409 to 0.6254%). Therefore, it is estimated that arsenic exists in non-crystal compound form or in colloid form. This result is consistent with the analysis results of the form of arsenic existing in sediment and soil (Sakata (1987), Brannon and Patrick (1987), Mok and Wei (1989, 1990), Han et al. (1991)).

3.2.4 The mechanism of arsenic accumulation in strata

The most important mechanism of arsenic accumulation in sediment is that arsenic is adsorbed by iron hydroxides and deposited in the process of interaction between water and sediment  (Aggett and  O’Brien,1985；Aggett and Roberts, 1986). Concentrations of iron and arsenic show clear correlations in the sediment of the Hetao Plain. As above-mentioned, the Pale-geographic environment in Late Pleistocene is a transition environment from reductive environment to oxidative environment. Acid water rich in organic matter helpful for formation of colloid is easily formed in such environment. Fe(III) in water forms positive colloid of [Fe（OH）₃]_n or [Fe（OH）₃]·Fe³⁺ and arsenic forms negative colloid. Acid swamp water and mineralized water, containing lots of colloid particles, mixing with the neutral Yellow River water cause the stability of colloid damaged. Plenty of colloid particles are coagulated and eventually deposited to the bed of lakes or rivers (Gao, 1999).

In addition, as described above, distribution of arsenic contaminated area has good relationships with fault in the Hetao Plain. It is estimated that the concave landform formed from the fault eventually turns into the bed of river. Deep groundwater rich in ion of Fe(Ⅱ) effuses from fault. The Fe(Ⅱ) is oxidized into Fe(III)  by surface water and atmospheric oxygen. The Fe(III) adsorbs arsenic of water in the process of mix, coagulation and deposition, finally is accumulated at the bed of river. It is supposed that this is the main reason of contaminated areas distributed in linear.

     The reaction process can be represented as the following equations (Munemiya and Tsuno, 1998)

4Fe²⁺ + 2H₂O → 4Fe³⁺ + 4OH^－

2Fe³⁺ +6OH^－ → 2Fe（OH）₃（=Fe₂O₃·3H₂O）

  And (Mok et al., 1988; Wilson and Hawkins, 1978),

Fe(OH)₃ + H₃AsO₄ = FeAsO₄·2H₂O + H₂O 

3.2.5 The mechanism of arsenic in layers entering into groundwater

Arsenic release and mobilization is affected by pH, Eh and arsenic concentration of fissure water (Xu and Grimvall, 1988；Mok and Wai, 1989，1990；Brannon and Patrick, 1987；Masscheleyn et al., 1991).

①     Oxidation-Reduction condition

With increasing oxidation (decreasing Eh), not only AsO（OH）₃ and As（V） in soil are reduced into AsO（OH）and As（III）respectively, but ferric arsenate and Fe(III) combined with other Arsenate are reduced into dissolvable Fe(II) (Hu·Luo, 1984; Shimada,1997). Namely, concentrations of dissolvable arsenic and iron increase with decreasing Eh.

In many areas, sources of arsenic in groundwater are mostly similar, such as silty medium fine sand and silty clay layers (Hu and Lou, 1984; Shimada, 1997; Tonokai and Mitamura 1997). It is supposed that these layers are aquitards, which are covered with Quaternary alluvial clay. Eh nearly shows negative value in such a closed groundwater basin. The reductive environment is helpful for content of dissolvable arsenic rising andfor arsenic mobilizing to water filled in soil pores. Irrigation of the Yellow River water in this area causes great changes of groundwater level and groundwater pressure. Thus arsenic in pores of soil particles is prone to getting into groundwater. This is the main reason that arsenic concentration of groundwater is high during high groundwater level and low during low groundwater level.

②     Acid and alkali condition

Acid and alkali conditions play important roles on arsenic adsorption in strata. As Eh decreasing and pH increasing, solubility of arsenic increases (Han, 1991). Namely, with Eh decreasing, [Aso₄]^3- reduced to [Aso₃]^3- , adsorption of soil particles is reduced. Because of pH increasing, positive electric charges of colloid decrease. Colloid is weakened by the ability of arsenic adsorption and arsenic concentration in soil increases.This result is proved by the release test in alkali solution. As (V) content eluted from sediment increased rapidly when pH values changed from 6.3 to11.5 (Mok and Wai, 1989). When Fe(OH)₃ adsorbing arsenic was put into solution with different pH values, content of arsenic dissolved from solution increased with pH value increasing (Kaneko, 1979). PH value (7.2-8.6) in the groundwater of the Hetao Plain is beneficial for the elution of arsenic.

Another factor is the effect on colloid forming in acid water. The pH value in this area is 8.1 for irrigation water and 9.7-9.9 for lake water. The water infiltrated into subsurface causes the colloid forming in acid water decomposed. Thus, arsenic adsorbed by colloid is in a free state and eventually gets into groundwater. 

　　 Fig. 4 and 5 show distributions of groundwater containing high concentration arsenic on the both sides of irrigation drainage in Haiziyan.

Conclusion

Arsenic contamination in groundwater in the Hetao Plain of the Inner Mongolia is mainly caused by natural hydro-geochemistry action. Penetration of irrigation water, high in alkali increases the arsenic in groundwater as well.

The dominant sources of arsenic are derived from layers of clay, silty clay and silty sand from the surface to the depth of more than 40m in the contaminated area. Arsenic in the strata is mainly from:

(1)Rural region around the plain, especially with large pyrite ore rich in arsenic in the western part of the plain.

(2)Deep ground water.

(3)Part sediments of the Yellow River. Accumulation of arsenic in the strata results from the coagulation of colloids, microorganism activity and oxidation. Due to the change of redox condition underground, arsenic transforms from a steady state to a free one. Then it enters into groundwater by hydrodynamics of groundwater, i.e. with the irrigation of the Yellow River water.

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