Hydrochemical and isotopic signatures of groundwater infiltration and legacy nitrogen discharge within Jeju Island aquaculture systems

The terrestrial aquaculture systems in Jeju Island have been increasingly threatened by the intrusion of deteriorated groundwater. In this study, we investigated spatiotemporal variations in hydrochemical properties and isotopic compositions (δ13CPOC, δ15NPN, δ15NNO3, and δ18ONO3) of influent waters from land-based fish farms to trace the sources of particulate organic matter (POM) and nitrate (NO₃⁻) pollution. During heavy summer precipitation (> 350 mm), hydrochemical and isotopic shifts revealed increased infiltration of terrestrial groundwater into fish farm systems. Based on discriminative isotopic values of POM and NO₃⁻ in aquaculture influents, source apportionment results after accumulated precipitation indicated that the contribution of anthropogenic POM (livestock; 39 ± 17%) was predominant than those of other sources (soil; 31 ± 16%, fertilizer; 24 ± 7%, septic waste; 6 ± 1%). In addition, the contribution of NO₃⁻ sources (soil; 67 ± 7%, livestock; 18 ± 10%, septic waste; 9 ± 4%, fertilizer; 5 ± 1%) were largely controlled by specific land-use types adjacent to aquaculture systems. These results suggest that variations in volcanic bedrock permeability and land-use patterns may lead to distinct infiltration pathways and delayed responses between the northern and southern sections of Jeju Island. Therefore, these findings demonstrated that legacy pollutants stored in both the unsaturated and saturated zones may continue to impair groundwater quality, thereby affecting aquaculture water quality, particularly during periods of accumulated precipitation. To effectively manage nutrient-driven degradation in aquaculture systems, we recommend integrating isotopic source-specific monitoring with hydrogeological zoning and the targeted pretreatment of saline groundwater. This approach can mitigate eutrophication risks and enhance the sustainability of fish farming operations in volcanic islands.