河套平原农业灌溉影响下地下水中砷迁移富集规律研究
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摘要
目前,高砷地下水已相继在70多个国家和地区被发现,并威胁着1.5亿人口的饮水安全。高砷地下水已成为各国政府、公众和学术界高度关注的全球性问题。
砷(As)是变价元素(常见的是+3和+5),毒性强(As(Ⅲ)>As(ⅤV)),在多数地下水环境中(pH为6.5-8.5之间)As可以多种形式存在。氧化环境中,当pH约小于6.9时,H2AsO4-占优势；当pH值较高时,以HAsO42-为主(H3AsO40和AsO43-可能分别在强酸和强碱环境中存在)。还原环境中,当pH值约小于9.2时主要以电中性的H3AsO30形式存在。As的释放和迁移对人为干扰很敏感。在某些地区,由于抽取地下水用于灌溉、改变农业方式、建造防堤工程等都将改变地下水流场和／或砷的物质来源,从而影响地下水中As的含量。砷的独特的水文地球化学特点决定了其成因是复杂的。
灌溉作为人类的一种重要的农业活动,不仅改变着高砷地下水分布区地下水的水动力场和水化学场,同时可能将地质环境中的砷带入食物链,进而威胁人类的粮食安全。为此,灌溉影响下地下水中砷的迁移富集规律研究成为当今地学界高度关注的热点研究领域。
内蒙古河套平原是我国高砷地下水分布最为广泛和严重的地区之一。地下水中砷的含量最高达1.74mg/L,超过国家饮用水标准(10μg/L)174倍。已发现的高砷地下水分布于巴彦淖尔市临河区、五原县、杭锦后旗、阿拉善盟阿左旗等19个乡镇,近100个自然村,受威胁的人口超过30万，砷中毒患者达2000余人(2002年)。河套平原作为我国重要的商品粮基地之一,有着上千年的引黄灌溉历史,构筑了亚洲最大的一首制自流灌区。巨大的排灌水网孕育着50多万公顷的耕地和100多万河套人民。
自河套平原高砷地下水被发现的十多年间,国内外学者围绕该地区高砷地下水成因开展了卓有成效的研究,但却忽略了灌溉这一重要影响因素。引黄灌溉水是该地区地下水最主要的补给来源,通过灌溉,土壤中的氮、磷、砷等组分将被淋滤而进入地下水环境,有可能对地下水造成污染。该地区任何不考虑灌溉影响的高砷地下水研究是不全面的。
本文以河套平原高砷地下水分布典型区——杭锦后旗为研究区,以地下水系统理论和水岩相互作用理论为指导,在对研究区地下水、灌溉水、土壤、沉积物等系统开展水文地球化学野外调查基础上,综合运用水化学、正向水文地球化学模拟等方法,以面状和线状入渗为主线,首次开展了灌溉影响下地下水中砷的迁移、富集规律研究,揭示了灌溉水-地下水-含水层介质间的水文地球化学过程,提出了灌溉影响下的砷释放概念模型。本研究丰富和发展了我国高砷地下水研究的内容和方法,对于进一步提升我国对高砷地下水成因的认识水平,科学指导高砷地下水分布区水资源的合理开发利用,具有重要理论和实际参考价值。
本文取得了如下研究进展：
1.研究区灌溉水水样的水化学特征分析表明研究区内引黄灌溉水本身并不是地下水中As浓度增加的来源,灌溉水中As含量很低(3.7μg/L)。
2.排干沟承担着排泄灌溉余水的作用。按照“源头-上游-中游-下游”的采样原则,在研究区三个主要的排干沟沿程采集了12件排干水。排干水的水化学特征表现为：除总排干源头之外,其余各点的排干水总As含量均高于灌溉水中的；各排干中的TOC的含量以及全盐量均高于灌溉水中的；总N、总P和NH4-N无明显变化规律。导致排干水水化学特征发生变化的原因包括：(1)从灌溉到排水的过程中,水-土之间发生了相互作用；(2)生产、生活废水等排入排干沟,排干沟的水质受到影响。
3.为了研究排干水中As的来源,我们在研究区排干沟附近共采集了6件不同深度的地表土并进行总As含量的测定。测定结果显示,研究区表层土壤As含量超过了内蒙古表层土壤As含量背景值6.12mg/kg,说明土壤中的As能够在灌溉水的淋滤下迁移并随灌溉退水排入排干沟中,As含量较高的表层土壤可以为排干水中As浓度的增加提供物源。
4.为了研究杭锦后旗地下水中As的来源,我们于2007年在研究区钻凿了三个深度约50m的钻孔。沉积物化学组成分析结果显示,高砷区沉积物中的As含量高于非高砷区的。As含量较高的沉积物所在层位主要岩性为粘土、亚粘土。从垂向分布上来看,沉积物中As与Fe、Mn、Sb、B、V的含量变化规律较为一致。沉积物中P含量高的层位,As含量相对较低。这可能是磷酸盐与As进行竞争吸附的结果。
5.研究区高As地下水主要分布在总排干流经的地带。这里是研究区的低洼地带。多数高As地下水呈弱碱性,电导率变化较大。高As地下水中TOC与HCO3-、总N与TOC、总As与TOC以及总As与总P之间存在较好的正相关关系。
6.总排干附近地下水中As含量升高的来源包括：(1)研究区北部狼山一带含砷多金属硫铁矿床的开采引发As的释放,随着大气降水的淋滤进入地下水,并向南部山前冲洪平原前缘的低洼地带(总排干附近)流动。此外,山区一些风化的含砷矿石、岩石在大气降水、洪水的搬运作用下向平原方向移动。在水动力条件缓和的冲洪积平原前缘的低洼地带,含砷碎屑物逐渐沉积下来,随着大气降水或地下水的淋溶,其中的As有可能释放并进入地下水。(2)平原区因农业灌溉而使用的含砷农药也会在灌溉水的淋滤作用下释放As而进入地下水。这两种补给来源不同的高As地下水最终在总排干流经地带汇流,因此导致总排干附近地下水As含量升高。
7.总排干附近地下水As富集的原因为：(1)总排干内底泥As含量高,是潜在的地下水As污染源,当排干水的pH值、氧化还原条件等因素发生变化时,底泥上吸附的As会随之释放进入地下水中；(2)残留在土壤中的过量磷肥在灌溉水和雨水的淋滤下渗入地下。随着夏灌、秋浇的持续交替进行,渗入地下的磷素在地下水水位周期性涨落的情况下被逐渐带入地下水中,由于磷酸盐竞争吸附沉积物表面的吸附点位,导致原本吸附在沉积物表面的As释放并进入地下水中。随着地下水向总排干方向流动,因此,释放到地下水中的As逐渐在总排干附近富集；(3)还原环境的形成也为总排干富集高As地下水创造了条件。由于灌区不合理的漫灌方式,导致总排干附近低洼地带的地下水位抬升明显,在强烈的蒸发浓缩作用下,土壤积盐现象严重,极易造成土壤盐渍化,因此土壤的透气性降低,空气中的氧气难以进入含水层,为含水层还原环境的形成创造了条件。此外,由于总排干中沉积底泥含有大量有机质,也为还原环境的形成提供了条件,这将有利于沉积物中铁锰氧化物的还原性溶解,从而As随之释放到地下水中。
8.利用正向水文地球化学模拟方法中的混合模型对研究区灌溉水-地下水-含水层介质之间的化学热力学平衡进行了模拟。制定了几组不同的灌溉水与地下水的混合比。模拟结果显示,灌溉水入渗到含水层的量增大时,将促进铝硅酸盐矿物的非全等溶解,从而形成更多的次生矿物,它们将会吸附更多的As。当含水层环境发生变化时,这些吸附在粘土矿物表面的As将释放到地下水中。
9.结合地下水和沉积物的地球化学特征分析结果和水文地球化学模拟结果,建立了研究区农业灌溉影响下的砷释放概念模型：大量灌溉水的渗透促进了土壤中的As向水相中迁移。As向下迁移,粘土矿物成为潜在的As的源或汇。随着灌溉水下渗量的增加,地下水位抬升了许多,此时更多的粘土矿物与地下水接触。另外,土壤中因过量施肥而残留的磷素也随着灌溉水下渗。抬升的地下水位使得更多的磷酸盐向地下水中迁移。由于磷酸盐的竞争吸附,使得原本吸附在粘土矿物表面的As向地下水中释放,因此导致地下水中As含量增加。
本文的主要创新点：
(1)提出了灌溉影响下的地下水中砷的释放、迁移与富集模式；
(2)应用正向水文地球化学模拟定量揭示了灌溉水-地下水-含水层介质间的水文地球化学过程。成果丰富和发展了高砷地下水研究的内容和方法。
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.
砷(As)是变价元素(常见的是+3和+5),毒性强(As(Ⅲ)>As(ⅤV)),在多数地下水环境中(pH为6.5-8.5之间)As可以多种形式存在。氧化环境中,当pH约小于6.9时,H2AsO4-占优势；当pH值较高时,以HAsO42-为主(H3AsO40和AsO43-可能分别在强酸和强碱环境中存在)。还原环境中,当pH值约小于9.2时主要以电中性的H3AsO30形式存在。As的释放和迁移对人为干扰很敏感。在某些地区,由于抽取地下水用于灌溉、改变农业方式、建造防堤工程等都将改变地下水流场和／或砷的物质来源,从而影响地下水中As的含量。砷的独特的水文地球化学特点决定了其成因是复杂的。
灌溉作为人类的一种重要的农业活动,不仅改变着高砷地下水分布区地下水的水动力场和水化学场,同时可能将地质环境中的砷带入食物链,进而威胁人类的粮食安全。为此,灌溉影响下地下水中砷的迁移富集规律研究成为当今地学界高度关注的热点研究领域。
内蒙古河套平原是我国高砷地下水分布最为广泛和严重的地区之一。地下水中砷的含量最高达1.74mg/L,超过国家饮用水标准(10μg/L)174倍。已发现的高砷地下水分布于巴彦淖尔市临河区、五原县、杭锦后旗、阿拉善盟阿左旗等19个乡镇,近100个自然村,受威胁的人口超过30万，砷中毒患者达2000余人(2002年)。河套平原作为我国重要的商品粮基地之一,有着上千年的引黄灌溉历史,构筑了亚洲最大的一首制自流灌区。巨大的排灌水网孕育着50多万公顷的耕地和100多万河套人民。
自河套平原高砷地下水被发现的十多年间,国内外学者围绕该地区高砷地下水成因开展了卓有成效的研究,但却忽略了灌溉这一重要影响因素。引黄灌溉水是该地区地下水最主要的补给来源,通过灌溉,土壤中的氮、磷、砷等组分将被淋滤而进入地下水环境,有可能对地下水造成污染。该地区任何不考虑灌溉影响的高砷地下水研究是不全面的。
本文以河套平原高砷地下水分布典型区——杭锦后旗为研究区,以地下水系统理论和水岩相互作用理论为指导,在对研究区地下水、灌溉水、土壤、沉积物等系统开展水文地球化学野外调查基础上,综合运用水化学、正向水文地球化学模拟等方法,以面状和线状入渗为主线,首次开展了灌溉影响下地下水中砷的迁移、富集规律研究,揭示了灌溉水-地下水-含水层介质间的水文地球化学过程,提出了灌溉影响下的砷释放概念模型。本研究丰富和发展了我国高砷地下水研究的内容和方法,对于进一步提升我国对高砷地下水成因的认识水平,科学指导高砷地下水分布区水资源的合理开发利用,具有重要理论和实际参考价值。
本文取得了如下研究进展：
1.研究区灌溉水水样的水化学特征分析表明研究区内引黄灌溉水本身并不是地下水中As浓度增加的来源,灌溉水中As含量很低(3.7μg/L)。
2.排干沟承担着排泄灌溉余水的作用。按照“源头-上游-中游-下游”的采样原则,在研究区三个主要的排干沟沿程采集了12件排干水。排干水的水化学特征表现为：除总排干源头之外,其余各点的排干水总As含量均高于灌溉水中的；各排干中的TOC的含量以及全盐量均高于灌溉水中的；总N、总P和NH4-N无明显变化规律。导致排干水水化学特征发生变化的原因包括：(1)从灌溉到排水的过程中,水-土之间发生了相互作用；(2)生产、生活废水等排入排干沟,排干沟的水质受到影响。
3.为了研究排干水中As的来源,我们在研究区排干沟附近共采集了6件不同深度的地表土并进行总As含量的测定。测定结果显示,研究区表层土壤As含量超过了内蒙古表层土壤As含量背景值6.12mg/kg,说明土壤中的As能够在灌溉水的淋滤下迁移并随灌溉退水排入排干沟中,As含量较高的表层土壤可以为排干水中As浓度的增加提供物源。
4.为了研究杭锦后旗地下水中As的来源,我们于2007年在研究区钻凿了三个深度约50m的钻孔。沉积物化学组成分析结果显示,高砷区沉积物中的As含量高于非高砷区的。As含量较高的沉积物所在层位主要岩性为粘土、亚粘土。从垂向分布上来看,沉积物中As与Fe、Mn、Sb、B、V的含量变化规律较为一致。沉积物中P含量高的层位,As含量相对较低。这可能是磷酸盐与As进行竞争吸附的结果。
5.研究区高As地下水主要分布在总排干流经的地带。这里是研究区的低洼地带。多数高As地下水呈弱碱性,电导率变化较大。高As地下水中TOC与HCO3-、总N与TOC、总As与TOC以及总As与总P之间存在较好的正相关关系。
6.总排干附近地下水中As含量升高的来源包括：(1)研究区北部狼山一带含砷多金属硫铁矿床的开采引发As的释放,随着大气降水的淋滤进入地下水,并向南部山前冲洪平原前缘的低洼地带(总排干附近)流动。此外,山区一些风化的含砷矿石、岩石在大气降水、洪水的搬运作用下向平原方向移动。在水动力条件缓和的冲洪积平原前缘的低洼地带,含砷碎屑物逐渐沉积下来,随着大气降水或地下水的淋溶,其中的As有可能释放并进入地下水。(2)平原区因农业灌溉而使用的含砷农药也会在灌溉水的淋滤作用下释放As而进入地下水。这两种补给来源不同的高As地下水最终在总排干流经地带汇流,因此导致总排干附近地下水As含量升高。
7.总排干附近地下水As富集的原因为：(1)总排干内底泥As含量高,是潜在的地下水As污染源,当排干水的pH值、氧化还原条件等因素发生变化时,底泥上吸附的As会随之释放进入地下水中；(2)残留在土壤中的过量磷肥在灌溉水和雨水的淋滤下渗入地下。随着夏灌、秋浇的持续交替进行,渗入地下的磷素在地下水水位周期性涨落的情况下被逐渐带入地下水中,由于磷酸盐竞争吸附沉积物表面的吸附点位,导致原本吸附在沉积物表面的As释放并进入地下水中。随着地下水向总排干方向流动,因此,释放到地下水中的As逐渐在总排干附近富集；(3)还原环境的形成也为总排干富集高As地下水创造了条件。由于灌区不合理的漫灌方式,导致总排干附近低洼地带的地下水位抬升明显,在强烈的蒸发浓缩作用下,土壤积盐现象严重,极易造成土壤盐渍化,因此土壤的透气性降低,空气中的氧气难以进入含水层,为含水层还原环境的形成创造了条件。此外,由于总排干中沉积底泥含有大量有机质,也为还原环境的形成提供了条件,这将有利于沉积物中铁锰氧化物的还原性溶解,从而As随之释放到地下水中。
8.利用正向水文地球化学模拟方法中的混合模型对研究区灌溉水-地下水-含水层介质之间的化学热力学平衡进行了模拟。制定了几组不同的灌溉水与地下水的混合比。模拟结果显示,灌溉水入渗到含水层的量增大时,将促进铝硅酸盐矿物的非全等溶解,从而形成更多的次生矿物,它们将会吸附更多的As。当含水层环境发生变化时,这些吸附在粘土矿物表面的As将释放到地下水中。
9.结合地下水和沉积物的地球化学特征分析结果和水文地球化学模拟结果,建立了研究区农业灌溉影响下的砷释放概念模型：大量灌溉水的渗透促进了土壤中的As向水相中迁移。As向下迁移,粘土矿物成为潜在的As的源或汇。随着灌溉水下渗量的增加,地下水位抬升了许多,此时更多的粘土矿物与地下水接触。另外,土壤中因过量施肥而残留的磷素也随着灌溉水下渗。抬升的地下水位使得更多的磷酸盐向地下水中迁移。由于磷酸盐的竞争吸附,使得原本吸附在粘土矿物表面的As向地下水中释放,因此导致地下水中As含量增加。
本文的主要创新点：
(1)提出了灌溉影响下的地下水中砷的释放、迁移与富集模式；
(2)应用正向水文地球化学模拟定量揭示了灌溉水-地下水-含水层介质间的水文地球化学过程。成果丰富和发展了高砷地下水研究的内容和方法。
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.
引文
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