Natural occurrence of arsenic in shallow groundwater, Shanyin, Datong Basin, China.

In Shanyin, Shanxi province, China, As concentration in shallow groundwater exceeds guide concentrations, set internationally and nationally at 10-50 microg/L, and may reach 1932.0 microg/L, which has resulted in severe clinical symptoms of arsenic toxity. In this article, chemical characteristics of groundwaters containing anomalous As and mechanisms of arsenic release to shallow groundwaters were studied. Groundwaters containing anomalous As in the study area were characterized by higher pH, higher concentration of phosphate, higher concentration of naphthenic acid, and lower concentrations of sulfate and nitrate. Microbial metabolism of sedimentary organic matter, which is present as high as 1.0% organic C, results in the lower concentrations of sulfate and nitrate. The reactions decrease Eh and produce CO2, which promotes the dissolution of carbonates and increases pH. The clay minerals and colloids including organic matters scavenging arsenic would release arsenic to groundwater in higher pH and lower Eh environment. In addition, the competitive absorption between As and anions (such as phosphate and fluoride) contributes to the release of As from kaolinite, montmorillonite, illite, and Fe oxyhydroxides. Naphthenic acid also promotes mobilization and translocation of As in groundwater systems.