| Currently, high-arsenic groundwater has been found successively in more than 70 countries and regions and the security of the drinking water of 150 million people has been threatened.Arsenic (As) is a variable-price element (most common valence is+3 and+5) and highly poisonous metallic element. In most groundwater environments (pH is between 6.5-8.5)As exists in its many forms.Under the oxidizing condition, when pH< 6.9, H2AsO4- is a dominant form; when the pH value is higher, HAsO42- is a dominant form. Under the reducing environment, H3ASO30 is a dominant form when pH< 9.2.The release and migration of As are sensitive to human disturbance.In some areas, as the extraction of groundwater for irrigation, changes in agricultural practices and construction of embankment works will change the groundwater flow field and/or arsenic source, so the content of As will change in groundwater. Hydrogeochemical characteristics of As indicate that its causes are complex.Irrigation as an important human agricultural activity, not only change the groundwater hydrodynamic field and hydrochemical field of high arsenic groundwater areas, but also can carry the geological environment of arsenic in the food chain and thereby threaten the food security of human. Therefore, under the influence of irrigation the study in migration and enrichment of arsenic in groundwater become one of the hot fields in academic research. Hetao Plain, Inner Mongolia is a one of the most widely distributed high-arsenic groundwater areas and serious arseniasis regions.Content of As in groundwater is up to 1.74mg/L, which exceeds maximum contaminant level (MCL) based on the health risk associated with arsenic in drinking water (10μg/L) 174 times. Distribution of high-arsenic groundwater is in 19 villages and towns, including Linhe, Wuyuan, Hangjinhouqi, Tumoteyouqi, Azuoqi, et. Population at risk is more than 300,000 and arsenic poisoning patients are more than 2000 (data of 2002).Hetao Plain as one of the important commodity grain base in China, it has a history of thousands years of Yellow River irrigation and it is largest single-head irrigated area of Asia. Huge net of irrigation and drainage breed more than 500,000 hectares of arable land and million of people.Since the high-arsenic groundwaters were found in Hetao Plain, many researchers have done the effective research around the cause of high-arsenic groundwater, but they neglected one important factor—irrigation. Irrigation water from Yellow River is the main recharge source of groundwater in Hetao Plain. Through irrigation, N,P,As and other elements can enter into groundwater and these elements maybe pollute groundwater. Study on high-arsenic is not comprehensive without regard to influence of irrigation.In this paper, we take Hangjinhouqi as the typical high-arsenic groundwater study area. On the base of systematic hydrogeochemical investigation on groundwater, irrigation water, soil and sediments, using the method of hydrochemical and hydrogeochemical modeling we carry out research on migration and enrichment of arsenic in groundwater under the influence of irrigation for the first time and exposit the hydrogeochemical process between irrigation water groundwater—aquifer, and then establish a conceptual model of arsenic release under the influence of irrigation. This study enriches and develops the research content and method of high-arsenic groundwater, and also has important theoretical and practical value for improving awareness of the causes of high-arsenic groundwater and the scientific development concept to guide rational development and utilization of water resources in high-arsenic areas.Our progresses achieved include: 1.Irrigation water from Yellow River is not the source of elevated arsenic concentration in groundwater.2. Irrigationreturnflow, precipitate water and wastewater run into drainage ditch. Hydrochemical characteristics of drain water:except for source of large drainage ditch, arsenic contents in other drainage ditches are higher than which in irrigation water; TOC and salinity in drainage ditches are higher than which in irrigation water; Total N, total P and NH4-N had no significant variation.3.In order to study the source of arsenic in drain water, we collected 6 surface soil samples at different depths along drainage ditch and determined arsenic content. The result shows that arsenic contents in surface soils are higher than arsenic background value of Inner Mongolia (6.12 mg/kg). This shows that arsenic of surface soil can transport to drainage ditch with irrigation water leaching. Surface soil with high arsenic content is one source of elevated arsenic content in drainage ditches.4. In order to study the source of arsenic in groundwater, we drilled 3 boreholes with depth about 50m. Chemical composition of sediment analysis shows that arsenic contents of sediments in high-arsenic area are higher than low-arsenic area. Clay and loam are main lithological characters in high arsenic sediments. Point of view from the vertical distribution, there is a similar feature between As, Fe, Mn, Sb, B and V content variation. In sediment if P content is high, arsenic content is relatively low. This may be the result of competition adsorption between phosphate and arsenic.5.High-arsenic groundwaters distribute mainly in the areas where large drainage ditch flows through. Most of high-arsenic groundwater is alkaline and the electrical conductivity change significantly. TOC was in negative correlation with the HCO3-, and the same as TN with TOC, As with TOC and total As with TP.6. The sources of elevated As in groundwater near the large drainage ditch contain:(1)the arsenic release by multi metal pyrite deposit extraction along the Lang Mountain in north plain, and leach into the groundwater with precipitation. (2) arsenical pesticide release and leach into the groundwater with irrigation water because of the agricultural irrigation in plain. The high-arsenic groundwater with different sources finally converges near the large drainage ditch. Therefore contents of arsenic in groundwater near the large drainage ditch are higher.7. The reasons of enrichment of arsenic in groundwater in the vicinity of the large drainage ditch are:(1)contents of arsenic are high in substrate sludge in large drainage ditch. So substrate sludge is potential source of elevated arsenic in groundwater. When pH values of drain water, redox conditions, groundwater temperature etc are change, arsenic adsorbed on substrate sludge will release into groundwater; (2) with leaching of irrigation water and rain water phosphate in soils will downwards migrate. During the period of irrigation in summer and autumn, groundwater table variation can cause phosphate migrate to groundwater. As the result of competition adsorption between phosphate and arsenic, arsenic will release into groundwater. With the direction of groundwater flow, arsenic radually accumulated in groundwater in the vicinity of the large drainage ditch; (3) the formation of reducing environment is conducive to enrichment arsenic in groundwater near the large drainage ditch. As irrational irrigation methods, the groundwater table significantly elevate in low-lying areas near the large drainage ditch. In the strong effect of evaporation, soil salinity is serious, therefore, soil permeability reduce, and oxygen in the air difficult to enter the aquifer, so aquifer environment easy to form reducing environment. In addition, due to organic matter riched in substrate sludge in large drainage ditch, it also can contribute to form reducing environment of aquifer. This will facilitate the reductive dissolution of iron and manganese oxides in sediments, and therewith arsenic will release to groundwater.8.Simulation has done for chemical thermodynamic equilibrium between irrigation water—groundwater—aquifer use of hydrogeochemical positive modeling in the study area. Through analyses of geochemical processes to study migration and enrichment process of arsenic. In order to better reflect the difference mixed proportion between irrigation water and groundwater interacted with sediments, we design several groups of mixing ratio. Simulation results show that the amount of irrigation water increase in the aquifer will promote the non-congruent dissolution of the aluminum silicate mineral and then the formation of chlorite, montmorillonite and kaolinite and other secondary minerals will increase, they can adsorb more arsenic. While irrigation water infiltration, P in the soils because of excessive fertilization will leach to groundwater with groundwater. Increase the amount of irrigation water increased the probability of P into groundwater. When more P into the groundwater they will competitive adsorb with arsenic, so more arsenic will desorbs from clay mineral surface and then transfer to the groundwater. In addition, pH values have important influences on arsenic adsorption in clay minerals. When the pH value increases, the number of negative charge of the clay mineral surface will increase and their adsorption capacity in arsenic will fall. Groundwater pH value is 6.74-8.64. So with the increase of pH value, the release of arsenic to the groundwater increases. However, due to different local hydrological conditions and redox environment, and also different arsenic contents in sediments, the release of arsenic in different areas varies widely. 9.Combined with the simulation results, conceptual model of arsenic release under the influence of irrigation has established:the infiltration of a large number of irrigation water contributes to arsenic adsorbed on soil migrate to aqueous phase. Arsenic migrates downward, and clay minerals become potential source or sink. With the increase in the amount of irrigation water, water table rise and more contact with the clay mineral and groundwater. Soil phosphate also with irrigation water infiltrates into groundwater. Elevation of water table makes more phosphate migrate to groundwater. Because of competitive adsorption of phosphate and arsenic, arsenic adsorbed on the surface of clay minerals will release to groundwater. Therefore arsenic contents increase in groundwater.The innovations of this study contain:(1)put forward arsenic release, migration and accumulation mode under the influence of irrigation;(2) quantitatively reflect hydrogeochemical process between irrigation water groundwater—aquifer by means of hydrogeochemical positive modeling.
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