Arsenic in groundwaters of the Lower Mekong

Increasing incidence and awareness of arsenic in many alluvial aquifers of South-east Asia has raised concern over possible arsenic in the Lower Mekong Basin. Here, we have undertaken new research and reviewed many previous small-scale studies to provide a comprehensive overview of the status of arsenic in aquifers of Cambodia and the Cuu Long Delta of Vietnam. In general natural arsenic originates from the Upper Mekong basin, rather than from the local geology, and is widespread in soils at typical concentrations of between 8 and 16 ppm; (dry weight). Industrial and agricultural arsenic is localised and relatively unimportant compared to the natural alluvial arsenic. Aquifers most typically contain groundwaters of no more than 10 μg L⁻¹, although scattered anomalous areas of 10 to 30 μg L⁻¹ are also quite common. The most serious, but possibly ephemeral arsenic anomalies, of up to 600 μg L⁻¹, are associated with iron and organic-rich flood-plain sediments subject to very large flood-related fluctuations in water level, resulting in transient arsenopyrite dissolution under oxidizing conditions. In general, however, high-arsenic groundwaters result from the competing interaction between sorption and dissolution processes, in which arsenic is only released under reducing and slightly alkaline conditions. High arsenic groundwaters are found both in shallow water-tables, and in deeper aquifers of between 100 and 120 m depth. There is no evidence of widespread arsenicosis, but there are serious localised health-hazards, and some risk of low-level arsenic ingestion through indirect pathways, such as through contaminated rice and aquaculture. An almost ubiquitous presence of arsenic in soils, together with the likelihood of greatly increased groundwater extraction in the future, will require continuing caution in water resources development throughout the region.     

X射线微断层扫描技术(XDMT)在多孔介质中含砷聚集物的三维成像应用

利用X射线微断层差异扫描技术,扫描富砷河床沉积物及人工含砷样品,对多孔介质中的含砷聚集物进行成像分析.利用Avizo 6和Blob3D等工具对获取的数据进行处理,从而实现了含砷聚集物的三维可视化成像.结果显示,大量的含砷聚集物沉积在多孔介质颗粒表面,且不同尺寸的聚集物呈现出明显的形态多样化特征.小颗粒聚集物趋向于球形,大颗粒聚集物则趋向于扁平状.研究表明,该技术可在不破坏样品的前提下,直接观测多孔介质因沉积作用导致其内部结构的变化.     

Sediment geochemistry and arsenic mobilization in shallow aquifers of the Datong basin,northern China

Understanding the mechanism of arsenic (As) mobilization from sediments to groundwater is important for water quality management in areas of endemic arsenic poisoning, such as the Datong basin in northern China. The bulk geochemistry analysis of sediment samples from three 50-m boreholes drilled specifically for this study at As-contaminated aquifers, the groundwaters of which have an As concentration up to 1060 μg/l, revealed that the average bulk concentrations of major and trace elements of the samples are similar to those of the average upper continental crust. The average As content of the sediment samples (18.7 mg/kg) is higher than that of modern unconsolidated sediments (5–10 mg/kg). Moreover, the abundance of elements varied with grain size, with higher concentrations in finer fractions of the sediments, such as silt and clay. The concentration of NH₂OH–HCl-extracted iron (Fe) strongly correlated with that of extracted As, suggesting that Fe oxyhydroxides may be the major sink of As in the aquifer. The results of microcosm experiments showed that As mobilization from sediments to groundwater is probably mainly related to changes in the redox conditions, with moderately reducing conditions being favorable for As release from sediments into groundwater.     

Anomalous arsenic concentrations in groundwaters of an island community, Bowen Island, British Columbia

Recently, occurrences of elevated arsenic concentrations in bedrock groundwaters used for individual and municipal water supplies have been recognised along the mainland coast of southern British Columbia, Canada. An area on Bowen Island (Queen Charlotte Heights community) was chosen to investigate the source(s) of arsenic, geochemical processes controlling its aqueous uptake, the role of geology and structure in the formation of these waters, and the use of hydrogeochemical survey methods for detecting arsenious groundwater regions. Pyrite- arsenopyrite-Cu mineralisation in the Queen Charlotte Heights area is present as veins associated with NE-trending fault zones and as disseminations in marginal breccia and stockwork zones of a ryhodacite porphyry stock. Near surface mineralisation was later altered by supergene events to form minor pyrite- arsenopyrite in a matrix of limonite and clay. Geochemical factors that control the concentration and mobility of arsenic in these groundwaters include: (a) anion exchange of arsenic from clay minerals during cation divalent/univalent exchange processes (water softening) which leads to waters of high pH and high anion exchange capacities; (b) desorption of arsenic from Fe and Al oxyhydroxide minerals during pronounced shifts in pH from acid to alkaline conditions brought on by the water softening process; and (c) stepwise oxidation of arsenopyrite mineralisation to form arsenious sulphate-bearing groundwaters. The inconsistent behaviour of the As3+ and As5+ ions during the complex waterrock interaction processes described above, precludes the use of detailed chemical analyses to predict relative abundances of the more toxic As3+ species. For a full appreciation of the toxic impact of groundwaters containing total arsenic concentrations that are above regulatory guidelines an arsenic speciation analysis is required.     

Mobilization of arsenic from technosols in a short-term dynamic column experiment

The release of arsenic from technosols was monitored using short-term dynamic leaching of homogeneous soil columns with native solution. Large amounts of readily available arsenic (16 mg kg^?1) were extracted from arsenic-rich ashy samples while representing less than 4 wt % of their total contents. In the first hour of leaching, the observed concentrations of water-soluble arsenic ranged from 650 to 830 μg L^?1, further increasing in the following leachates. The results showed that the concentrations of water-soluble arsenic were several times higher than the recommended limits for drinking water. Yet, most of arsenic is strongly bound to amorphous aluminosilicate phases. The contents of arsenic in the studied plant samples, including calculated transfer factors, confirmed that increased concentrations of arsenic in the soil of geological environment affected by dam failure of a coal ash pond may have a negative effect on crops since arsenic becomes part of the food chain.     

Mobilization of arsenic in groundwater of Bangladesh: evidence from an incubation study

The extensive extraction of arsenic (As)-contaminated groundwaters for drinking, household and agricultural purposes represents a serious health concern in many districts of Bangladesh. This laboratory-based incubation study investigated the sources and mechanisms of As mobilization in these groundwaters. Several incubation studies were carried out using sediments collected from the Bangladesh aquifer that were supplemented, or not, with different nutrients, followed by an analysis of the sediment suspensions for pH, ORP (oxidation-reduction potential), EC (electrical conductivity) and As and Fe(ΙΙ) concentrations. In the substrate-amended sediment suspensions incubated under anaerobic environment, there was a mobilization of As (maximum: 50–67 μg/l) and Fe(ΙΙ) (maximum: 182 μg/l), while the ORP value decreased immediately and drastically (as much as −468 mV to −560 mV) within 5–6 days. In the sediment suspensions incubated under control and aerobic conditions, no significant As mobilization occurred. The simultaneous mobilization of As and Fe(ΙΙ) from sediments is a strong indication that their mobilization resulted from the reduction of Fe oxyhydroxide by the enhanced activity of indigenous bacteria present in the sediments; this phenomenon also provides insights on the mobilization mechanism of As in groundwater. The concentrations of As in the sediments used in the incubation studies were strongly linked to the gradients of redox potential development that was stimulated by the quantity of organic nutrient (glucose) used. The penetration of surface-derived organic matter into the shallow aquifer may stimulate the activity of microbial communities, thereby leading to a reduction of iron oxyhydroxide and As release.     

Arsenic in ground-water under oxidizing conditions,south-west United States

Concentrations of dissolved arsenic in ground-water in alluvial basins of Arizona commonly exceed 50 g L^–1 and reach values as large as 1,300 g L^–1. Arsenic speciation analyses show that arsenic occurs in the fully oxidized state of plus 5 (As+5), most likely in the form of HAsO4², under existing oxidizing and pH conditions. Arsenic in source areas presumably is oxidized to soluble As before transport into the basin or, if after transport, before burial. Probable sources of arsenic are the sulphide and arsenide deposits in the mineralized areas of the mountains surrounding the basins. Arsenic content of alluvial material ranged from 2 to 88 ppm. Occurrence and removal of arsenic in ground-water are related to the pH and the redox condition of the ground-water, the oxidation state of arsenic, and sorption or exchange. Within basins, dissolved arsenic correlates (P<0.01) with dissolved molybdenum, selenium, vanadium, and fluoride and with pH, suggesting sorption of negative ions. The sorption hypothesis is further supported by enrichment of teachable arsenic in the basin-fill sediments by about tenfold relative to the crustal abundance and by as much as a thousandfold relative to concentrations found in ground-water. Silicate hydrolysis reactions, as defined within the alluvial basins, under closed conditions cause increases in pH basinward and would promote desorption. Within the region, large concentrations of arsenic are commonly associated with the central parts of basins whose chemistries evolve under closed conditions. Arsenic does not correlate with dissolved iron (r = 0.09) but may be partly controlled by iron in the solid phase. High solid-phase arsenic contents were found in red clay beds. Large concentrations of arsenic also were found in water associated with red clay beds. Basins that contain the larger concentrations are bounded primarily by basalt and andesite, suggesting that the iron content as well as the arsenic content of the basin fill may play a role in the occurrence of arsenic in ground-water. Under oxidizing conditions in Arizona, arsenic in ground-water appears to be controlled in part by sorption or desorption of HAsO4² on active ferric oxyhydroxide surfaces.     

Geochemical occurrences of arsenic and fluoride in bedrock groundwater: a case study in Geumsan County, Korea

Bedrock groundwaters in Geumsan County, Korea, were surveyed to investigate the distribution and geochemical behaviors of arsenic and fluoride, mobilized through geogenic processes. The concentrations were enriched up to 113 μg/L for arsenic and 7.54 mg/L for fluoride, and 16% of 150 samples exceeded World Health Organization drinking water guidelines for each element. Simple Ca-HCO(3) groundwater types and positive correlations with pH, Ca, SO(4), and HCO(3) were characteristics of high (>10 μg/L) As groundwaters. The oxidation reaction of sulfide minerals in metasedimentary rocks and locally mineralized zones seems to be ultimately responsible for the existence of arsenic in groundwater. Desorption process under high pH conditions may also control the arsenic mobility in the study area. High (>1.5 mg/L) F groundwaters were found in the Na-HCO(3) type and with greater depth. Fluoride seemed to be enriched by deep groundwater interaction with granitic rocks, and continuous supply to shallow Ca-HCO(3)-type groundwater kept the concentration high. In the study area, drinking water management should include periodic As and F monitoring in groundwater.     

Geochemical occurrences of arsenic and fluoride in bedrock groundwater: a case study in Geumsan County,Korea

Bedrock groundwaters in Geumsan County, Korea, were surveyed to investigate the distribution and geochemical behaviors of arsenic and fluoride, mobilized through geogenic processes. The concentrations were enriched up to 113 μg/L for arsenic and 7.54 mg/L for fluoride, and 16% of 150 samples exceeded World Health Organization drinking water guidelines for each element. Simple Ca-HCO₃ groundwater types and positive correlations with pH, Ca, SO₄, and HCO₃ were characteristics of high (>10 μg/L) As groundwaters. The oxidation reaction of sulfide minerals in metasedimentary rocks and locally mineralized zones seems to be ultimately responsible for the existence of arsenic in groundwater. Desorption process under high pH conditions may also control the arsenic mobility in the study area. High (>1.5 mg/L) F groundwaters were found in the Na-HCO₃ type and with greater depth. Fluoride seemed to be enriched by deep groundwater interaction with granitic rocks, and continuous supply to shallow Ca-HCO₃-type groundwater kept the concentration high. In the study area, drinking water management should include periodic As and F monitoring in groundwater.

     

Lung retention and bioavailability of arsenic after single intratracheal administration of sodium arsenite,sodium arsenate,fly ash and copper smelter dust in the hamster

Arsenic is present in airborne particulate material released by coal-fired power plants and non-ferrous metal smelters. We have assessed whether the physico-chemical properties of arsenic in such particles play a role in its lung retention and uptake by the body. Female hamsters were given a single intratracheal instillation of fly ash or copper smelter dust suspensions (at doses of 50 or 100 g As kg^–1) or identical amounts of soluble tri- and pentavalent arsenic, in the presence or absence of an inert dust material (tungsten carbide). The concentration of the element was measured in a 24 hour urine sample collected on the 1st, 2nd and 6th day after treatment and arsenic remaining in lung tissue was determined at the end of the same time periods. Both lung retention and urinary As excretion indicate a prolonged contact of the lung tissue with particulate As in contrast to soluble As salts. In addition to the effect of solubility described here, more research is needed to determine the effect of particle size and lung loading on retention, as well as the potential differences in the lung inflammatory response using arsenic-rich particulates from various sources.     

Applying chemical sedimentation process in drinking water treatment plant to address the emergent arsenic spills in water sources

Arsenic (As) spills occurred more frequently and sometimes polluted water sources in recent years in China. It is as urgent need to develop emergency treatment technologies to address the arsenic threat for large-scale water treatment plants. In response, we developed a chemical sedimentation technology to remove arsenic contaminants for water treatment plants. Bench-scale experiments were conducted to investigate the efficiency of arsenic removal and the influencing factors of the chemical sedimentation treatment process. The influencing factors included the choice and dosage of coagulants, the valence of arsenic and pH value of solution. The As(V) contaminants can be almost completely removed by ferric or alum coagulants. The As(III) contaminants are more recalcitrant to chemical sedimentation, 75% for ferric coagulant and 40% for alum coagulant. The quantitative results of arsenic removal load by different ferric or alum coagulants were presented to help determine the parameters for arsenic treatment technology. The dominant mechanism for arsenic removal is static combination, or adsorption of negative arsenic species onto positive ferric hydroxide or alum hydroxide flocs. The efficiency of this treatment technology has also been demonstrated by a real production test in one water treatment plant with arsenic-rich source water and one emergency response. This technology was verified to be quick to set-up, easy to operate and highly efficient even for high concentration of arsenic.     

16S rDNA克隆文库解析江汉平原高砷地下水系统中的细菌多样性

为研究江汉平原高砷地下水系统中细菌多样性和种群结构,以2011年11月采自江汉平原仙桃市沙湖镇高砷含水层沉积物为研究对象,沉积物采自同一50m钻井,分别选取埋深9.9～10.1m和15.7～16.0m样品。提取沉积物中总DNA,采用16SrDNA克隆文库方法,分析高砷含水层沉积物中土壤细菌的多样性。结果发现,伯克氏菌目(Burkholderiales)、假单胞杆菌目(Pseudomonadales)和肠杆菌目(Enterobacteriales)构成了高砷系统中抗砷的优势菌群。结果表明,高砷含水层中生活有大量微生物,这些微生物在砷的地球化学循环中扮演着重要角色。     

An incidence of skeletal fluorosis associated with groundwaters of the maritime carboniferous basin,Gaspé region,Quebec, Canada

Consumption of unusually high concentrations of F^– in groundwaters of the Maria area in the Gaspé peninsula of Quebec have resulted in symptoms of skeletal fluorosis in two members of the population. One of these individuals consumed approximately 50 mg of fluoride per day over a 6 year period before being hospitalized and later diagnosed with skeletal fluorosis. It is estimated that, until this case came to light, approximately 15–20% of the rural population (total approximately 1,600) in the area were consuming groundwaters with F^– levels between 5 and 28 mg L^–1 for at least 6 years. The high concentrations of F^– in well waters of the Maria area occur only in wells completed in Carboniferous sandstone-siltstone-conglomerate sediments that underlie a thick blanket of alluvial-colluvial-glacial overburden. These fluoriferous groundwaters exhibit high Na and HCO₃ ^– contents and low Ca and Mg concentrations compared to those associated with the overburden sediments. The high F levels greatly increase the risk for fluorotic diseases such as skeletal fluorosis and skeletal radiculomyopathy. Wells completed in overburden, although having suboptimal F^– levels are safer for the health of individuals in this region. Effective regulations for well drilling need to be formulated for regions underlain by Carboniferous formations in the Maritime provinces of Canada. In some regions, high F^– levels (10–25 mg L^–1) in groundwaters will seriously affect how, and to what extent, groundwater supplies can be developed for domestic use.To whom correspondence should be addressed.     

Factors affecting arsenic concentration in groundwater in West Bengal

Groundwater is the major source of drinking water for the population in Malda district, West Bengal. Holocene sediments from Himalayas have contaminated those places through the river Ganges and arsenic was found a potential groundwater contaminant. Field and laboratory experiments were attempted to identify the relationship between arsenic versus Eh of groundwater and content specific elements (As, Co, Mn, Fe, C and S) in different types of Holocene sediments on a freshly constructed wells. Arsenic versus Eh of groundwater and Holocene clay over Holocene sand were found performing a major role in the aquifers of arsenic affected areas.     

土著细菌对江汉平原浅层含水层沉积物中砷迁移的影响

对江汉平原水文地质调查发现,该地区地下水砷含量已远超国家饮用水标准。以沉积物培养的土著细菌混合液为生物材料,以江汉平原高砷含水层沉积物为研究对象,在实验室内模拟地下水系统,研究厌氧环境条件下,不同生物量土著细菌和pH值对沉积物中砷迁移转化的影响,以及土著细菌活动下砷在不同沉积物中的迁移转化。结果表明,不同生物量菌悬液都能促进沉积物中As的释放,增加总As和As(III)的浓度,但150mL处理组,在研究后期,总As和As(III)的浓度呈现减缓趋势;在初始生物量一定的条件下,沉积物中As含量越高,细菌活动下总As相对释出量就越低,而且As(III)占所释出总As的比值就越高,但两个高砷含量沉积物组的差异较小;在初始pH值为5、7和9的培养条件下,细菌都能加速砷的迁移,但pH值为5的处理组(简称pH5处理组)最弱,在前8天,pH9处理组较pH7处理组的低,随后超过pH7处理组。研究表明,土著细菌悬液能加速As从沉积物中释出,并且释出的As以As(III)为主;在耐受的弱碱性环境条件下,细菌对砷的迁移和转化随环境的pH值增加而增强。     

Contamination levels and state assessment in the lakes of the Oliveri-Tindari Lagoon (North-Eastern Sicily,Italy)

The Oliveri-Tindari Lagoon, located in North-Eastern Sicily (Italy) and composed of six lakes, is subject to continuous environmental changes. An integrated study focusing on sediment features and levels of contamination was carried out for three of the six lakes, which are of ancient origin: Verde, Mergolo della Tonnara and Marinello. A high primary production (26.89 μg l^?1) was detected at Lake Verde; texture classification showed a typical grain size in the sediments of all lakes; the study of macronutrients highlighted 17.08 of total carbon in sediments from Lake Mergolo della Tonnara; toxic elements were detected at higher concentrations in the sediments of Lake Marinello in comparison to the others, while arsenic was found in high concentrations in all the samples tested, especially in Lake Verde, with a mean value of 17.25 mg kg^?1 dry weight (d.w.). All the organic contaminants, except 4, 4′-dichlorodiphenyldichloroethylene, were below the detection limits in the sediments. Minimal microbiological contamination was found in both water and sediment samples. In the latter, we isolated several bacterial strains thriving in the presence of arsenic, which play a role in the biogeochemical cycle of arsenic. These preliminary results, obtained for the first time using a multidisciplinary approach, provide general information about the Oliveri-Tindari Lagoon area.     

A medical geology study of an arsenic-contaminated area in Kouhsorkh, NE Iran

High concentrations of arsenic were determined in sediments from the Kouhsorkh area, Khorasan province, NE Iran. The main rock formations in the area consist of Tertiary volcanic rocks as Tuffaceous sandstone, polymictic conglomerate and andesite. Furthermore, some As–Sb–Au mineralization occurred in this area. Concentrations of arsenic in sediments were determined to range between 4.2 and 268.2 ppm, exceeding US EPA (2004) limits. It seems that young volcanic activity is one of the most important factors for arsenic contamination in this area. The first stage of this medical geology study was done at 2 villages in the Kouhsorkh area in which the arsenic concentration in water is high. People in this residential area suffer from skin diseases including hyperpigmentation, hypopigmentation, keratosis on head, hands, and feet. The 24-h urine specimens were tested for arsenic, the level of total arsenic in urine were determined to range between 13.66 and 75.92 μg/l day, exceeding permissible limits from 5 to 40 μg/day. More systematic studies are needed to determine the link between As exposure and its related diseases.     

Variations in heavy metal contamination of stream water and groundwater affected by an abandoned lead–zinc mine in Korea

This study evaluated variations in heavy metal contamination of stream waters and groundwaters affected by an abandoned lead–zinc mine, where a rockfill dam for water storage will be built 11 km downstream. For these purposes, a total of 10 rounds of stream and groundwater samplings and subsequent chemical analyses were performed during 2002–2003. Results of an exploratory investigation of stream waters in 2000 indicated substantial contamination with heavy metals including zinc (Zn), iron (Fe) and arsenic (As) for at least 6 km downstream from the mine. Stream waters near the mine showed metal contamination as high as arsenic (As) 8,923 μg L⁻¹, copper (Cu) 616 μg L⁻¹, cadmium (Cd) 223 μg L⁻¹ and lead (Pb) 10,590 μg L⁻¹, which greatly exceeded the Korean stream water guidelines. Remediation focused on the mine tailing piles largely improved the stream water qualities. However, there have still been quality problems for the waters containing relatively high concentrations of As (6–174 μg L⁻¹), Cd (1–46 μg L⁻¹) and Pb (2–26 μg L⁻¹). Rainfall infiltration into the mine tailing piles resulted in an increase of heavy metals in the stream waters due to direct discharge of waste effluent, while dilution of the contaminated stream waters improved the water quality due to mixing with metal free rain waters. Levels of As, Cu and chromium (Cr) largely decreased after heavy rain but that of Pb was rather elevated. The stream waters were characterized by high concentrations of calcium (Ca) and sulfate (SO₄), which were derived from dissolution and leaching of carbonate and sulfide minerals. It was observed that the proportions of Ca and SO₄ increased while those of bicarbonate (HCO₃) and sodium and potassium (Na+K) decreased after a light rainfall event. Most interestingly, the reverse was generally detected for the groundwaters. The zinc, being the metal mined, was the most dominant heavy metal in the groundwaters (1758–10,550 μg L⁻¹) near the mine, which far exceeded the Korean standard of 1000 μg L⁻¹ for drinking water. The decreases in the heavy metals contents in the groundwaters associated with reduced rainfall were quite different from the increases observed for the stream waters, which is not clearly understood at this time and warrants further investigation.     

Species distribution of arsenic in sediments after an unexpected emergent discharge of high-arsenic wastewater into a river

The unexpected emergent discharge of high-arsenic wastewater into water environments results in significantly increased levels of arsenic in water; however, the species distribution of arsenic in sediments has never been reported before for such cases. This study focuses on an As pollution accident in the Dasha River, and uses sequential extraction procedures with deionized water, 1?mol·L^-1 MgCl₂ at pH= 8, 1?mol·L^-1 NaH₂PO₄ at pH= 5, and 1?mol·L^-1 HCl to investigate four binding phases of arsenic (i.e., water soluble, ion-exchangeable, strongly-bound, and precipitates) in sediments at different layers in different cross-sections along the river. The average ratio of arsenite (As(III)) to arsenate (As(V)) was found to decrease from 0.74:1 in river water to 0.48:1 in sediment, owing to its higher affinity toward As(V) than As(III). The content of arsenic in the sediments was relatively low and the maximum content was observed to be 36.3?mg·kg^-1 for As(III) and 97.5?mg·kg^-1 for As(V). As(III) and As(V) showed different binding phases in sediments, and the average fractions of these four species were determined to be 0.09, 0.11, 0.17, and 0.63 for As(III) and 0.03, 0.14, 0.63, and 0.20 for As(V), respectively. For all the sediment samples, the content of arsenic showed no relationship with the characteristics of the sediments such as the particle diameter, the content of organic carbon, Fe, and Mn, although a negative correlation with particle diameter was observed for the sediments in the uppermost 2-cm layer. The unexpected emergent As incident results in the high content of total arsenic in the surface sediment, which may be potential secondary source to the elevated As levels in surface water.     

沙颍河沈丘段底泥、土壤中砷及重金属污染与潜在生态风险评价

为查明沙颍河沈丘段底泥、土壤中砷(As)和重金属污染状况及其潜在生态风险,对底泥和土壤样品中的砷(As)、铬(Cr)、汞(Hg)、镉(Cd)和铅(Pb)的含量水平进行测定,并进行潜在生态风险指数计算。结果表明,沙颍河沈丘段底泥中As含量为9.206～11.641mg·kg-1,距河5km的白果村农田土壤中As含量为8.52～80.31mg·kg-1,超标率达到57.8%,Cr、Hg、Cd和Pb的超标率分别达到32.8%、59.4%、67.2%和39.1%。沙颍河沈丘段底泥和附近村庄农田土壤中As和4种重金属的总潜在生态风险为中等生态风险,主要来源于Hg和As。