Identification of groundwater recharge sources and processes in a heterogeneous alluvial aquifer: Results from multi-level monitoring of hydrochemistry and environmental isotopes in a riverside agricultural area in Korea

We evaluated sources and pathways of groundwater recharge for a heterogeneous alluvial aquifer beneath an agricultural field, based on multi-level monitoring of hydrochemistry and environmental isotopes of a riverside groundwater system at Buyeo, Korea. Two distinct groundwater zones were identified with depth: (1) a shallow oxic groundwater zone, characterized by elevated concentrations of NO₃^- and (2) a deeper (>10-14 m from the ground surface) sub-oxic groundwater zone with high concentrations of dissolved Fe, silica, and HCO₃^-, but little nitrate. The change of redox zones occurred at a depth where the aquifer sediments change from an upper sandy stratum to a silty stratum with mud caps. The δ¹⁸O and δ²H values of groundwater were also different between the two zones. Hydrochemical and δ¹⁸O-δ²H data of oxic groundwater are similar to those of soil water. This illustrates that recharge of oxic groundwater mainly occurs through direct infiltration of rain and irrigation water in the sandy soil area where vegetable cropping with abundant fertilizer use is predominant. Oxic groundwater is therefore severely contaminated by agrochemical pollutants such as nitrate. In contrast, deeper sub-oxic groundwater contains only small amounts of dissolved oxygen (DO) and NO₃^-. The ³H contents and elevated silica concentrations in sub-oxic groundwater indicate a somewhat longer mean residence time of groundwater within this part of the aquifer. Sub-oxic groundwater was also characterized by higher δ¹⁸O and δ²H values and lower d-excess values, indicating significant evaporation during recharge. We suggest that recharge of sub-oxic groundwater occurs in the areas of paddy rice fields where standing irrigation and rain water are affected by strong evaporation, and that reducing conditions develop during subsequent sub-surface infiltration. This study illustrates the existence of two groundwater bodies with different recharge processes within an alluvial aquifer.