Nitrate pollution sources and associated biogeochemical mechanisms in coastal groundwater affected by seawater...

Nitrate pollution is of significant global concern as a potential threat to eco-environmental safety and human health, especially in coastal zones where freshwater resources are limited. However, the information regarding the contribution of seawater intrusion to NO₃^- in coastal groundwater, as well as NO₃^- pollution driven by geological conditions remains limited. Here, an integrated method combining hydrochemical parameters, multi-isotope techniques (δ¹⁵N_nitra, δ¹⁸O_nitra, δ³⁴S_sul, and δ¹⁸O_sul), and source apportionment models (PMF and MixSIAR) was applied to reduce uncertainties in estimating NO₃^- sources and to clarify biogeochemical mechanisms controlling NO₃^- pollution in coastal groundwater. The results revealed that manure & domestic sewage and soil nitrogen can account for over 80% of the total NO₃^- pollution sources (86.3 ± 4.44% from PMF and 83.5 ± 5.42% from MixSIAR), implying the considerable potential of anthropogenic emissions for nitrogen seepage. In contrast to the substantial role of surface inputs in supplying NO₃^- to groundwater, seawater intrusion contributed negligibly to groundwater NO₃^- (~5%). Nitrification mainly attributes to the intensification of NO₃^- pollution. The HNO₃ from nitrification is involved in carbonate weathering and may enhance the simultaneous release of NO₃^-, Ca²⁺, and HCO₃^- into coastal groundwater. Whereas denitrification and/or DNRA processes occur to a limited extent only in local areas where fresh groundwater is distributed. Our study highlights the significance of natural factors in NO₃^- pollution in coastal groundwater, contributing to be a better understanding of nitrogen cycling in global coastal zones.