珠江口立沙岛地下水水化学特征与高氨地下水溯源分析OA

To elucidate the hydrochemical characteristics,recharge sources,and genesis of high-ammonium(NH4+)groundwater in the complex system of Lisha Island,a typical estuarine sandbar in the Pearl River Estuary,unconfined and confined groundwater samples were systematically collected during wet and dry seasons.A comprehensive study was conducted using hydrochemistry and multiple environmental isotope tracers(δ2H,δ18O,3H,δ13C,14C,and δ15 N-NH4+).The results indicate that the groundwater system exhibits a clear ternary spatial structure of"terrestrial source-mixing-marine source,"with hydrochemical facies evolving from HCO3-Ca type in the terrestrial recharge area to Cl-Na type in the marine-influenced area.Hydrogen and oxygen isotopes confirm that all groundwater originates from atmospheric precipitation but has experienced varying degrees of mixing with seawater.The combined use of 3H and 14C(corrected age 0 to 2 200 a BP)tracers confirms that groundwater salinization results from the combined effects of residual Holocene paleo-seawater and modern seawater intrusion along a preferential pathway on the southwest side.High NH4+concentration is a significant regional characteristic;its δ15N-NH4+values show a wide range(from-2.7‰ to+26.7‰),with most samples far exceeding the typical range(+2‰ to+8‰)for organic matter mineralization.The extremely low NO3-content rules out nitrification as the cause for the 15N enrichment.The material source of the high NH4+is the degradation of natural organic matter in sediments,while the anomalous enrichment in δ15N-NH4+is primarily attributed to the isotopic fractionation effect of the microbial anaerobic ammonium oxidation(anammox)process.Furthermore,analysis of ionic deviation from conservative mixing reveals that NH4+concentration is significantly positively correlated with the positive deviations of Na⁺,K⁺,and HCO3-(p<0.05),and negatively correlated with the negative deviation of SO42-(p<0.05).This study provides a scientific basis for the sustainable development of groundwater resources and the ecological protection of Lisha Island and similar estuarine islands.