江汉平原地下水中铁的分布特征及其成因研究

Distribution and Causes of Iron in Groundwater in Jianghan Plain

江汉平原水质型缺水问题严重。作为重要水源的地下水存在着铁含量过高等水质问题,因此,开展江汉平原地下水中铁分布特征和影响因素研究,对于认识地下水中铁的来源、保障饮用水水质具有重要的理论和现实意义。基于江汉平原典型沉积相钻孔沉积物、浅层地下水的采样、测试,该文分析了湖积相、冲积相和残坡积相地下水中铁的分布规律,结果显示,湖积相和冲积相地下水中Fe(Ⅱ)/Fe比值分别为0.77和0.81,均高于残坡积相地下水Fe(Ⅱ)/Fe比值0.36,表明湖积相和冲积相以Fe(Ⅱ)为Fe在地下水中存在的主要形态,而残坡积相地下水则以Fe(Ⅲ)为主。进一步的沉积物铁形态提取和水化学分析表明,Fe的还原性溶解是高铁地下水富集的重要因素。主要表现在以下3个方面:(1)不同沉积相类型钻孔沉积物铁含量存在较大差异,湖积相钻孔各深度有效态铁含量平均值达到9.544 mg/g,远高于冲积相及残坡积相。湖积相沉积物以有机质结合态铁为主;冲积相钻孔主要以Fe-Mn氧化物结合态为主;残坡积相则呈现浅层以水溶态和Fe-Mn氧化态为主、深层只以Fe-Mn氧化物结合态为主的分布模式。(2)湖积相和冲积相地下水均以Fe(Ⅱ)为主要形态,Eh呈现强还原环境;而残坡积相地下水以Fe(Ⅲ)为主,呈现偏氧化环境。(3)不同沉积相的地形地貌、风化沉积过程差异将会导致含水层封闭性、水动力条件、质地、有机碳含量及铁矿物形态分布的分异,进一步导致了铁矿物相还原性溶解过程的不同,从而影响了高铁地下水的富集。

Surface water pollution is a serious problem in Jianghan Plain.At the same time,the groundwater,which is an important water source,also has water quality problems such as excessive iron content.It is of great significance to study the distribution characteristics and influencing factors of iron in groundwater of Jianghan Plain.Different forms of Fe in the groundwater and sediments of three sedimentary facies were studied in Jianghan Plain.The results on groundwater show that the value of Fe(Ⅱ)/Fe ratio in groundwater of lacustrine and fluvial facies is 0.77 and 0.81,higher than the ratio of Fe(Ⅱ)/Fe in the eluvial facies.Further studies have shown that the reductive dissolution of Fe is an important factor in the enrichment of high-iron groundwater.It is mainly manifested in the following three aspects.(1)The concentration of iron in sediments of three sedimentary facies types was significantly different in Jianghan Plain.The average value of bio-available fraction iron content in lake facies reached 9.544 mg/g,which was much higher than that of alluvial facies and residual slope facies.Lacustrine sediments are dominated by organic bound iron.The main form of Fe in fluvial alluvial facies is Fe-Mn oxide bonded iron.In the eluvial facies,water soluble iron and Fe-Mn oxide bonded iron forms dominate in the shallow layer,while Fe-Mn oxide bonded iron dominate in the deep layer.(2)The main form of Fe in groundwater of lacustrine facies and fluvial alluvial facies is Fe(Ⅱ),and present strong reducibility.By contrast,the groundwater of eluvial facies is dominated by Fe(Ⅲ),and the value of Eh indicates oxidizability.(3)Differences in topography,landform and weathering deposition process of different sedimentary facies will lead to the differentiation of sealing property of aquifer,hydrodynamic conditions,texture,organic carbon content and distribution of iron oxides,which further leads to the differences in the reducing dissolution process of iron oxides,thus affecting the enrichment of high-speed iron groundwater.