EVALUATION OF PROCESSES CONTROLLING THE ENRICHMENT OF ARSENIC IN GROUNDWATER FROM THE DATONG BASIN, NORTHERN CHINA

In order to understand the mechanism of As enrichment in groundwater, major trace elements and redox-sensitive components analysis were conducted on groundwater from the Datong Basin in Northern China. Factor analysis study indicates that hydrochemistry of the groundwater is governed by processes including irrigation practice, microbial-driven redox reaction and NO3- contamination. Saturation indices (SI) calculated using PHREEQC suggest that amorphous Fe oxide/hydroxide and siderite change from slightly undersaturated to oversaturated. Notably, negative correlations were observed between the saturation index of these minerals with As and Fe in high As groundwater. This indicates the replacement of amorphous Fe oxide/hydroxide by Fe carbonate during ferric and SO42- reduction, and organic carbon oxidation could be the key process controlling the mobilization of As because of higher adsorption ability of Fe oxide/hydroxide than Fe carbonate, with respect to As. Thus, reductive dissolution of Fe oxides/hydroxide accompanied with bacteria-mediated decomposition of organic matter and reduction of SO42- can lead to the enrichment of As in groundwater at Datong. This can be confirmed by the good correlation between As and HS- concentration as well as Eh values.