Comparison of hydrogeochemical processes controlling arsenic enrichment between shallow and deep aquifers in the Hetao Basin, Inner Mongolia

摘要

Although As contamination in shallow groundwater has been well documented in the Hetao Basin, Inner Mongolia, in deep groundwater it remains unclear. Two types of wells were used for groundwater sampling: shallow wells (<50 m) and deep wells (50-100 m). Results showed that high As concentrations were presented in both shallow and deep aquifers. The average As concentration in deep aquifers (95.6 g L~(-1)) was lower than shallow aquifers (118 g L~(-1)). Shallow groundwaters had higher concentrations of DOC, HCO_3~- and TDS than deep ones. The pH value ranged from 6.8 to 9.0 in shallow aquifers, which was higher than the deep ones. Reducing conditions were the major geochemical characteristic of both groundwaters. Negative correlations were observed between As and Eh, NO_3~-, SO_4~-, while a positive correlation was observed between As and Fe. It demonstrated that the reductive dissolution of Fe oxyhydroxides would control As concentration of groundwater in both shallow and deep aquifers. In addition, desorption may also play a role in As enrichment in groundwater. Due to higher pH in shallow aquifers, desorption would play a more important role in As release in shallow than deep aquifer.