Groundwater Recharge and Fate of Nitrate and Sulfate in Alluvial Aquifer: Multiple Environmental Isotopic Studies

Nitrate and sulfate in alluvial groundwater are originated from various sources and are controlled by redox condition in aquifer. Recently in Korea, very high concentrations of nitrate are very frequently found in alluvial groundwater. The recharge of nitrate-rich groundwater in alluvial setting may cause the oxidation of pyrite in aquifer, resulting in enriched sulfate. The sources and transformation of nitrate and sulfate in a riverside sandy aquifer in agricultural area, as well as the characteristics of groundwater recharge, are discussed in this study using hydrochemical and environmental isotope data. Hydrochemically two distinct bodies of alluvial groundwater were identified with depth; these are (1) an oxic (upper) groundwater and (2) a sub-oxic (lower) groundwater. They show the different source of groundwater recharge and the different fate of nitrate and sulfate, which are controlled by aquifer geology. The role of pyrite oxidation following denitrification is also discussed.