Arid climate and seawater intrusion amplify the threat of sulfate rather than nitrate in the coastal aquifer of China's Yellow River Delta

Study region The Yellow River Delta located in Shandong Province, northern China. Study focus The contamination dynamics and potential sources of groundwater SO 4 2- and NO 3 - are potentially affected by two worldwide issues: climate change-induced precipitation and seawater intrusion. This study investigated how seasonal precipitation and seawater intrusion impact SO 4 2- and NO 3 - in the coastal groundwater of the Yellow River Delta. New hydrological insights for the region The results indicated significant degradation in the groundwater quality, with 95 % of the samples exceeding China's drinking water standard for SO 4 2- (mean concentration: 829 mg/L), and 27 % exceeding the standard for NO 3 - (mean concentration: 55.0 mg/L). Health risk assessment confirmed nonnegligible noncarcinogenic risks, which were exacerbated by an arid climate. Notably, the risk was attributed primarily to overlooked SO 4 2- rather than NO 3 - . Isotopic analysis using a Bayesian isotope mixing model revealed that SO 4 2- was derived mainly from evaporite dissolution (47.3 ± 19.7 %), followed by sewage (14.8 ± 6.9 %) and seawater (12.3 ± 3.9 %). This contamination was further promoted by seawater intrusion via stronger evaporite dissolution processes, particularly during the early rainy season. In contrast, NO 3 - was less sensitive to rainfall variation and seawater intrusion and was primarily affected by anthropogenic activities, including those related to sewage (57.9 ± 10.9 %), soil nitrogen (29.0 ± 8.7 %) and fertilizers (8.3 ± 3.1 %). Our work confirmed that the flocculation of SO 4 2- and NO 3 - in coastal groundwater is influenced by natural processes and surface anthropogenic activities driven by seasonal precipitation, more strongly affecting on SO 4 2- than NO 3 - from seawater intrusion. The spatiotemporal findings establish a quantitative framework for managing vulnerable coastal groundwater systems in deltaic regions worldwide that face simultaneous challenges of seawater intrusion and anthropogenic pressure.