Sources and enrichment processes of groundwater arsenite and arsenate in fissured bedrock aquifers in the Xunhua-Hualong basin, China

Abstract

Although high arsenic (As) groundwater has been widely observed in sedimentary aquifers, arsenite (As(III)) and arsenate (As(V)) mobilization in fissured bedrock aquifers are less documented, and different mobilization behaviors of As(III) and As(V) in groundwater from those aquifers are little known. Thus, geochemical and isotopic characteristics of high As groundwater in fissured bedrock aquifers in the Xunhua-Hualong basin were investigated. Results showed that fissured bedrock aquifer hosted high As groundwater. Total As concentrations in spring waters were up to 628 μg/L being dominated by As(V) with high ORP values (from 87.2 to 212 mV). As(III) dominated dissolved As in deep groundwater (up to 22.5 μg/L) with low ORP values (−217 and −288 mV). The positive correlation between As concentration and δ¹⁸O in spring water revealed that oxidation of As-bearing pyritic minerals in the fissured bedrock aquifers released As into groundwater. In deep groundwater under anoxic conditions recharged by shallow aquifers, high As groundwater usually had high concentrations of dissolved Fe, suggesting that reductive dissolution of Fe (oxyhydr)oxide minerals mobilized As into groundwater. Elevated concentrations of PO₄³⁻ in high As spring water and deep groundwater indicated the competition adsorption between PO₄³⁻ and As(V) (and As(III)). Insignificant correlations were observed between As(V) and pH, HCO₃⁻, and Na⁺/Ca²⁺ in spring water, revealing that desorption induced by pH and HCO₃⁻ and cation exchange between Ca²⁺ and Na ⁺ had negligible effects on As(V) mobilization. However, alkaline pH caused the desorption of As(III), and the presence of HCO₃⁻ was conducive to As(III) desorption in deep groundwater. A conceptual model was established to explain As distribution in fissured bedrock aquifers, emphasizing contributions of mineral dissolution, desorption, and hydrogeological conditions to As mobilization. This paper highlights different mobilization behaviors of As(III) and As(V) in groundwater from fissured bedrock aquifers, requiring the different strategies to ensure the safety of drinking water in those areas.