An integrated geochemical and isotopic framework for nitrate source apportionment in a shallow coastal aquifer under...

Nitrate pollution in coastal aquifers poses growing environmental concerns, yet aquifer responses to coupled natural and anthropogenic drivers remain poorly understood. This study quantifies nitrate sources to support targeted groundwater management in Zhuhai, a rapidly urbanizing coastal area in the Pearl River Delta, South China. An integrated approach combining hydrogeochemical analysis, multi-isotope fingerprinting (δ²H-H₂O, δ¹⁸O-H₂O, δ¹⁵N-NO₃^-, δ¹⁸O-NO₃^-), and quantitative modeling (PMF and MixSIAR) was applied. Stable hydrogen and oxygen isotopic signatures reveal precipitation-dominated recharge with anthropogenic inputs and evaporation effects. Salinity-enhanced geogenic NH₄⁺ release through cation exchange with paleo-seawater Mg²⁺, and its subsequent oxidation to NO₃^- drive coastal nitrogen hotspots. Dual nitrate isotope tracing confirmed that nitrification dominates nitrogen transformation, while denitrification is limited and can be ignored for subsequent models. Qualitative assessments enabled the separation of natural and anthropogenic sources and initial identification of end-members. Quantitative source apportionment using MixSIAR determined that sewage and manure (80.5 ± 9.7%) are the main contributors for nitrate, with PMF and MixSIAR yielding consistent results. This study establishes a "process identification-source apportionment" framework that effectively links nitrogen transformation processes with source quantification. The framework provides methodological support for identifying pollution hotspots and prioritizing control strategies in coastal aquifers under multiple pressures worldwide.