河水入渗过程中近岸带地下水中砷的形成与影响因素研究

沈阳黄家水源地是我国北方地区典型的傍河地下水水源地,近岸带地下水中铁(Fe)、锰(Mn)、砷(As)含量严重超标。为查明地下水中As的来源与影响因素,对研究区河水、地下水以及土壤样品进行采集与测试,分析了水样常规指标与碳硫稳定同位素、土样中典型矿物、砷的含量及赋存形态。结果表明,研究区河水中As含量很低,而地下水中As含量普遍超标。河水入渗初期,氧化性河水使部分含As矿物发生氧化而释放As;随着河水入渗,地下水向还原环境转变,含As的Fe/Mn矿物发生还原性溶解,地下水中As含量逐渐升高。研究区典型矿物有黄铁矿、菱铁矿、软锰矿、赤铁矿、针铁矿、菱锰矿等,通过可交换态砷解吸、有机质结合态砷氧化、铁锰氧化物结合态砷还原性溶解等,介质中的As释放至地下水中。地下水中As含量与酸碱度(pH)、氧化还原电位(Eh)呈一定负相关,与溶解有机碳(DOC)、 {\mathrm{HCO}}_3^{-}、Fe、Mn含量呈正相关。

Sources and influencing factors of arsenic in nearshore zone during river water infiltration

The Shenyang Huangjia water source area is a typical riverside groundwater source area in northern China. In its nearshore zone the iron (Fe), manganese (Mn) and arsenic (As) contents were well above the safety standard. In order to find out the sources and influencing factors of groundwater As in the affected area, samples of river water, groundwater and soils of the study area were collected and tested. Water quality indexes and carbon and sulfur stable isotopes of water samples, and the contents and occurrences of typical minerals and As in soil samples were analyzed. The results showed that As contents were very low in river water samples but generally exceeded the standard in groundwater samples. In the early stage of river water infiltration, some As-bearing minerals were oxidized by oxidizing river water and As was released. Later on, the groundwater environment became reductive with the infiltration of river water, and reductive dissolution of As-bearing Fe/Mn minerals resulted in the gradual increase of groundwater As. Typical minerals in the study area were pyrite, siderite, pyrolusite, hematite, goethite, rhodochrosite, etc. Through desorption of exchangeable arsenic, oxidation of organic matter-bound arsenic, and reductive dissolution of iron-manganese oxide-bound arsenic, As was release into the groundwater. To a certain extent, As content in the groundwater was negatively correlated with groundwater pH and redox potential (Eh) and positively correlated with the dissolved organic carbon (DOC), {\mathrm{HCO}}_3^{-}, Fe and Mn contents.