Geologically controlled agricultural contamination and water-rock interaction in an alluvial aquifer: Results from a hydrochemical study

Hydrogeochemistry data collected from three multi-level monitoring wells in a sandy alluvial aquifer located in the Keum River watershed, South Korea, are used in this study to evaluate groundwater chemistry change in relation to geologic controls of groundwater recharge, redox condition, water-rock interaction and contamination susceptibility. A silt layer at a depth of about 10 m is a hydrologic barrier which prohibits the continued downflow of groundwater and divides the groundwater into two distinct masses (i. e., shallow 'oxic' groundwater and deeper 'sub-oxic' groundwater) with different redox states and ion chemistry. Compared to 'sub-oxic' water, 'oxic' water has lower SiO₂/(Na + K) and higher Ca, NO₃ and SO₄, indicating significant contamination from agrochemicals and manures. The higher Ca and NO₃ but lower HCO₃ concentrations in 'oxic' water result from lime dissolution and nitrification. In contrast, 'sub-oxic' water shows increased SiO₂ and HCO₃, suggesting the effect of water-rock interaction in the aquifer. Potential sources of cations are Na from albite, Mg from chlorite and illite, and K from illite. Vertical fluctuations of cationic composition in 'sub-oxic' water suggest heterogeneity of aquifer mineralogy with depth. Mineral stability calculations show that the chemistry of 'oxic' water plots in the kaolinite field, while that of 'sub-oxic' water plots in the Ca-montmorillonite field and near the kaolinite/montmorillonite equilibrium boundary. Concentrations of dissolved inorganic carbon (DIC) in 'sub-oxic' water are much higher than total alkalinity (HCO₃ + CO₃), suggesting that CO₂ generated by redox reactions such as denitrification, iron reduction and sulfate reduction causes increasing DIC, which also affects the dissolution of silicate minerals in an aquifer free of carbonate minerals.