Importance of surface geologic condition in regulating As concentration of groundwater in the alluvial plain

The arsenic (As) concentrations in the groundwater of alluvial plains generally show high spatial variability. We geochemically explore the factors causing the spatial variability in an alluvial plain that is mainly used for rice cultivation, the commonest land-use pattern in alluvial plains of Asia. We investigate the chemical processes, sources of chemicals that affect the behavior of As, and their relationships with the geologic conditions at seven multilevel sampler sites. All sites showed As concentrations that increased with depth at shallow levels and decreased at greater depths, which is the typical vertical pattern in alluvial aquifers where Fe-(hydr)oxide reduction is the major As release mechanism. Data show that NO3 and SO4 originating from the land surface play important roles in suppressing the increase in As concentration by buffering the redox potential at shallow depths and by precipitating As with sulfide minerals at deep depths, respectively. The As concentration in the intermediate depth range was also low in the presence of SO4, because its reduction can occur together with Fe-(hydr)oxide reduction in a wide range of redox potentials. As a result, the maximum As concentrations at the sites where the land was covered with a thick silt layer (∼5 m) were 3- to 5-times higher than those at other sites due to the supply of NO3 and SO4 from land surface being largely limited by the silt layer. This indicates that the surface geology could be an important indicator for the As concentration in alluvial groundwater.