Nitrate contamination and subsequent hydrogeochemical processes of shallow groundwater in agro-livestock farming districts in South Korea

In last several decades, the nitrogen cycle has been significantly perturbed, largely due to intensification of agricultural activities throughout the world. In this study, we examined the impact of agricultural N inputs on the quality and chemistry of shallow groundwater, based on a large hydrochemical dataset (n ≈ 4000) collected from 100 agro-livestock farming districts in South Korea. The South Korean groundwater, studied mostly in silicate aquifers, shows very high nitrate concentrations (median NO₃ ⁻ = 22.2 mg/L) and acidification (median pH = 5.6). The groundwater nitrate levels tend to increase with the estimated N loadings, and the groundwater pH generally decreases with increasing nitrate levels. The relationship between the concentrations of Ca²⁺ + Mg²⁺ and HCO₃- has a moderate adjusted R² value (0.4), and the molar (Ca²⁺ + Mg²⁺) / HCO₃- ratios (r) tend to increase with nitrate concentrations. This implies that the chemical composition of the groundwater is controlled by multiple hydrogeochemical processes, which include weathering of silicates and carbonates induced by carbonic acid (r = 0.5), nitrification (r ≥ 1) and denitrification (mostly, r < 0.5 in this study). In particular, undrinkable (NO₃ ⁻ > 44.3 mg/L) groundwater (n = 988) shows an average molar ratio of 1.04, indicating that such highly contaminated groundwater experiences the enhanced geochemical weathering of aquifer materials by an anthropogenic process (e.g., addition of N fertilizers). The results of principal component (PC) analysis also support our explanation: the first PC shows significant negative correlations with major components including NO₃ ⁻ while there is a weak positive correlation with pH, indicating anthropogenically enhanced weathering that includes the buffering process by agricultural liming materials, while the second PC shows negative correlations with Eh, DO and NO₃ ⁻ but positive correlations with pH and HCO₃ ⁻, suggesting the denitrification process. A few groundwater samples (about 4%) experienced heterotrophic denitrification, and they have molar ratios (r) of less than 0.5. The results of this study indicate that 1) geochemical weathering of aquifer materials in shallow groundwater systems is enhanced by nitrate contamination caused by high N loadings in agro-livestock farming districts, causing increased salinity, and 2) in silicate aquifers, the prevailing hydrochemical process can be interpreted by the molar (Ca²⁺ + Mg²⁺) / HCO₃- ratios of groundwater.