沉积物地球化学特征和土地利用方式对地下水砷行为的影响

Influences of sediments geochemical characteristics and land utilization on groundwater arsenic activities

银川平原是典型的高砷内陆盆地。然而人为活动及沉积物地球化学条件对地下水砷分布的影响并不清楚。本文选取银川平原北部平罗县作为研究区,在不同研究单元共采集119组地下水样品(冲洪积平原、冲湖积平原水田区和冲湖积平原旱田区分别为54组、31组和34组),分析了其主量和微量组分含量。另在水田和旱田分别钻井一眼,采集66组沉积物(水田和旱田单元分别为31组和35组)样品,分析了粒度分布、化学成分、强(S2)、弱(S1)结合态砷等物化指标。结果表明,高砷地下水主要分布在10~20m深度范围内;冲洪积平原地下水中砷浓度普遍较低,平均浓度1.71μg·L^(-1),约95%的样品中砷浓度在10μg·L^(-1)以下;而冲湖积平原水田和旱田砷浓度较高,90%以上超过10μg·L^(-1),平均浓度分别达到26.6μg·L^(-1)和39.6μg·L^(-1)。沉积物总砷含量为3.94~75.2 mg·kg^(-1),平均值为11.5 mg·kg^(-1),S1、S2含量范围分别为<0.01~0.33mg·kg^(-1),<0.01~19.5mg·kg^(-1),分别约占总砷的0.7%和6.2%。沉积物中砷与铁、锰含量呈正相关,其分布受水动力条件及岩性的控制,黏土中砷含量较高。结合相同深度地下水砷浓度,得到S2、S1对应的平均砷分配系数Kd分别为0.001L·g^(-1)和0.007L·g^(-1)。Kd与ORP呈较好的正相关性,表明还原环境利于砷的释放;PO3-4的竞争吸附也能促进沉积物中砷的释放。在冲湖积平原,同深度上沉积物组成相近的含水层中,旱田地下水砷浓度较水田高,表明土地利用方式对砷的分布和运移也有重要影响。

The Yinchuan plain is a typical inland basin where high arsenic groundwater has been usually found.However,effects of human activities and sediments geochemistry on arsenic behaviors in groundwater are still unclear.The study area is located in Pingluo County in northern Yinchuan plain.One-hundred and nineteen groundwater samples were collected in different hydrogeological units（54,31,34samples from alluvial plain,paddy farmland,and dry farmland of alluvial lacustrine plain,respectively）,and analyzed the major and trace components.Besides,66sediment samples were collected from two boreholes drilled in paddy and dry farmland,respectively.Physical and chemical parameters were analyzed,including grain size distribution,chemical components,and weakly（S1）and strongly（S2）bound arsenic.Results showed that high arsenic groundwater was mainly distributed at depths of 10-20m below land surface.Low arsenic concentrations were found in groundwaters of alluvial plain,with an average of 1.71μg·L-（-1） and below 10μg·L-（-1） in 95%of samples.In contrast,arsenic concentrations in groundwater from dry and paddy unit were generally high,with averages of 26.6 and 39.6μg·L-（-1）,respectively.More than 90%of samples had arsenic concentrations10μg·L-（-1）.Arsenic contents of sediments were between 3.94 and 75.2 mg·kg-（-1）,with an average of 11.5 mg·kg-（-1）.Contents of S1 and S2 ranged from0.01 to 0.33 mg·kg-（-1） and 0.01 to 19.5 mg·kg-（-1）,averagely accounting for 0.7%and 6.2% of total arsenic,respectively.Sediment arsenic had good correlations with total iron and manganese.The distribution of sediment arsenic was controlled by hydrodynamic conditions and lithology.Arsenic partition coefficient values（Kd）calculated from S1,S2 and dissolved As were 0.007L·g-（-1） and0.001L·g-（-1） respectively,which generally depended on groundwater redox potentials.Positive correlation between Kdand ORP indicated that As was preferentially partitioned into groundwater in reducing conditions.Competitive adsorption of phosphate promoted the release of arsenic from sediments to groundwater.Groundwater As concentrations under dry farmland were generally higher than those under paddy farmland at the same depths,which indicated that land utilization affected the fate of arsenic in groundwater.