Integrated environmental assessment framework for evaluating and managing nitrogen contamination in complex fractured aquifer systems

A novel multidisciplinary framework integrating hydrochemical, isotopic, hydrogeological, geophysical, microbiological, and statistical approaches investigated elevated nitrogen concentrations in fractured crystalline bedrock aquifers at a research farm in northeastern Seoul, South Korea (2018-2022). Integrated analysis revealed complete hydraulic disconnection between western and eastern regions, with 67-fold transmissivity contrasts (<0.3 vs > 20 m²/day), creating fundamentally different hydrochemical conditions (p < 0.05). Western deep groundwater, characterized by thick weathered zones and high-dip permeable fractures, exhibited high multi-contaminant signatures including elevated nitrite (up to 10.4 mg/L), ammonium, chlorofluorocarbon, zinc, and fecal coliform levels, with extremely low transmissivity (<0.3 m²/day). Conversely, the eastern region exhibited greater dilution capacity and biogeochemical stability due to regional hydraulic connectivity. Dual isotope analysis (δ¹⁵N/δ¹⁸O-NO₃^-) combined with Bayesian mixing models incorporating redox-specific fractionation corrections revealed an overall study area average of 93.4 % livestock-derived contamination (composted manure: 58.1 %, manure slurry: 34.6 %), with individual well contributions ranging from 89 to 97 % but showing no statistically significant spatial variation (Kruskal-Wallis test, p > 0.15). The comprehensive framework successfully identified contamination sources and hydrochemical evolution, demonstrating that physical heterogeneity governs both contaminant transport and biogeochemical transformation in fractured bedrock aquifers, providing essential insights for sustainable groundwater management in fractured bedrock systems worldwide.