Institut de Recherche en Mines et en Environnement, Université du Québec en Abitibi-Témiscamingue, 341 rue Principale Nord, Amos J9T 2L8, Canada.
Institut de Recherche en Mines et en Environnement, Université du Québec en Abitibi-Témiscamingue, 341 rue Principale Nord, Amos J9T 2L8, Canada.
Sci Total Environ. 2017 Jan 1;574:509-519. doi: 10.1016/j.scitotenv.2016.08.210. Epub 2016 Oct 14.
High arsenic concentrations occur in groundwater collected from a fractured crystalline bedrock aquifer in western Quebec (Canada). Sampling and analysis of water from 59 private wells reveal that more than half of the bedrock wells exceed the Canadian guideline value of 10μg/l for arsenic, whereas shallow wells in unconsolidated surficial deposits are not affected by the contamination. The weathering of arsenic-bearing sulfides present along the mineralized fault zone is considered to be the primary source of arsenic in groundwater. High-arsenic wells are generally characterized by mildly reducing conditions (Eh<250mV), weak alkaline conditions (pH>7.4), low Ca/Na ratios, elevated dissolved Fe and Mn concentrations and high proportions of As(III). Private bedrock wells are open boreholes that likely receive groundwater from multiple contributing fractures. Hence, it is proposed that dissolved arsenic is mainly derived from the contribution to the well discharge of reducing and alkaline geochemically evolved groundwater that contains arsenic as As(III). Geochemically evolved groundwater provides favorable conditions to release arsenic by reductive dissolution of iron and manganese oxyhydroxides and alkaline desorption from mineral surfaces. Thus, high-arsenic wells would contain a high proportion of geochemically evolved groundwater, while oxidizing low-pH recharge water causes dilution and sequestration of arsenic. In relation with the chemical evolution of groundwater along the flow path, most contaminated wells are located in confined areas whereas most of the wells located in unconfined recharge areas are not contaminated. The occurrence of boreholes with high dissolved arsenic as As(V) and oxidizing conditions is attributed to extensive sulfide oxidation and alkaline desorption. This work shows that the determination of arsenic speciation provides a valuable tool to investigate the behavior of arsenic in bedrock groundwater.
在加拿大魁北克西部的一个断裂结晶基岩含水层中采集的地下水中，砷浓度很高。对 59 口私人水井的水样进行采样和分析表明，超过一半的基岩水井的砷含量超过了加拿大 10μg/l 的指导值，而未固结表层沉积物中的浅层水井则没有受到污染。含砷硫化物沿矿化断裂带风化被认为是地下水中砷的主要来源。高砷水井通常具有弱还原条件（Eh<250mV）、弱碱性条件（pH>7.4）、低 Ca/Na 比、溶解的 Fe 和 Mn 浓度升高以及较高比例的 As(III)。私人基岩水井是可能从多个补给裂缝接收地下水的开口钻孔。因此，有人提出溶解的砷主要来自对井排放的还原和碱性地球化学演化地下水的贡献，这种地下水含有 As(III)。地球化学演化地下水通过还原溶解铁和锰氢氧化物以及从矿物表面碱性解吸，为释放砷提供了有利条件。因此，高砷水井将含有很大比例的地球化学演化地下水，而氧化的低 pH 补给水会导致砷的稀释和螯合。与地下水流径中地下水的化学演化有关，大多数受污染的水井位于封闭区域，而位于无约束补给区的大多数水井则没有受到污染。存在高溶解砷的 As(V)和氧化条件的钻孔的出现归因于广泛的硫化物氧化和碱性解吸。这项工作表明，砷形态的测定为研究基岩地下水中砷的行为提供了一种有价值的工具。
