Hydrogeochemical contrasts between low and high arsenic groundwater and its implications for arsenic mobilization in shallow aquifers of the northern Yinchuan Basin, P.R. China

Hydrogeochemical contrasts between low and high arsenic groundwater and its implications for arsenic mobilization in shallow aquifers of the northern Yinchuan Basin, P.R. China

Little is known about hydrogeochemical contrasts between low and high As groundwaters and their connection to As mobilization in the Yinchuan Basin. Investigations were carried out to evaluate As distribution and geochemical processes for As mobilization in three regions, including piedmont proluvial fans (PA), dry farmland (DF) and paddy farmland (PF). Ninety-two groundwater samples, 4 surface water samples, and 66 sediments samples were collected and analyzed for chemical and isotopic components. Results show that low As groundwater is generally found in PA. However, high As concentrations (up to 105 mu g L-1) are mainly observed in groundwaters from DF and PF, which are associated with reducing conditions. High As groundwater is characterized by high concentrations of NH4+, dissolved Mn, dissolved Fe and Fe(II), and low concentrations of NO3- and SO42-. The intensive irrigation in PF recharges the aquifers by vertical infiltration of the diverted Yellow River water, and leads to the higher redox potentials and the lower dissolved As in comparison with those in DF. Environmental isotopes (delta O-18 and delta D) show that evaporation due to the intensive irrigation plays a minor role in As enrichment. The positive correlation between As and dissolved Fe suggests that groundwater As would result from the reductive dissolution of Fe oxides. Besides, dissolved P may be involved in competing with As for binding sites on Fe oxide minerals. Sediment As ranges between 3.94 and 75.2 mg kg(-1). HCl-leached As accounts for 60% of total As in the sediments, while H3PO4-leached As accounts for 5%. Depth-matched samples show a good correlation between dissolved As and H3PO4-leached As in sediments. Arsenic distribution coefficient (K-d), calculated from H3PO4-leached As and dissolved As, ranges between 5.08 and 17.3 cm(3) g(-1), which generally depends on groundwater redox potentials. In reducing conditions, low values are found with As being preferentially partitioned into groundwater. (C) 2014 Elsevier B.V. All rights reserved.

关于银川盆地低砷和高砷地下水之间的水文地球化学差异以及它们与砷迁移的联系知之甚少。进行了调查以评估三个区域（包括山前洪积扇（PA）、旱地（DF）和水田（PF））中砷的分布以及砷迁移的地球化学过程。采集并分析了92个地下水样本、4个地表水样本和66个沉积物样本的化学和同位素成分。结果表明，低砷地下水通常出现在山前洪积扇区域。然而，高砷浓度（高达105μg/L）主要出现在旱地和水田的地下水中，这与还原条件有关。高砷地下水的特征是高浓度的铵离子、溶解态锰、溶解态铁和亚铁离子，以及低浓度的硝酸根和硫酸根。水田的大量灌溉通过引黄河水的垂直入渗补给含水层，与旱地相比，导致了较高的氧化还原电位和较低的溶解态砷。环境同位素（δ¹⁸O和δD）表明，大量灌溉导致的蒸发在砷富集过程中作用较小。砷与溶解态铁之间的正相关关系表明，地下水中的砷是由铁氧化物的还原溶解产生的。此外，溶解态磷可能会与砷竞争铁氧化物矿物上的结合位点。沉积物中砷的含量在3.94 - 75.2mg/kg之间。盐酸浸出的砷占沉积物中总砷的60%，而磷酸浸出的砷占5%。深度匹配的样本显示溶解态砷与沉积物中磷酸浸出的砷之间具有良好的相关性。由磷酸浸出的砷和溶解态砷计算得出的砷分配系数（Kd）在5.08 - 17.3cm³/g之间，它通常取决于地下水的氧化还原电位。在还原条件下，发现数值较低，砷优先分配到地下水中。（C）2014爱思唯尔有限公司。保留所有权利。