Monsoon Precipitation Regulates Priority Contaminant Sources, Quality, and Health Risks of Groundwater in a Rapidly Urbanizing Region

Abstract Groundwater supplies ∼50% of global domestic water yet faces intertwined geogenic and anthropogenic threats. The seasonal surge and shifts in priority groundwater contaminants in rapidly urbanizing regions, along with the specific sources driving these pulses, remain poorly understood. Addressing these knowledge gaps is critically important for improving water quality management, guiding sustainable water resource strategies, and mitigating health risks. We conducted triseason monitoring (dry, early wet, postwet) of 159 shallow domestic well samples across China’s Pearl River Delta, integrating multi-isotopes (δD-H2O, δ18O–H2O, δ15N-NO3–, δ18O-NO3–, 208Pb/207Pb, and 206Pb/207Pb) and Bayesian mixing, self-organizing maps (SOM), and positive matrix factorization (PMF). Water isotopes quantified a monsoon-paced shift from greater surface water influence in the dry season to precipitation-dominated recharge in the early wet season, linking hydroclimate to contaminant transport. Four contaminants (NO3–, NH4+, As, and Pb) controlled water quality deterioration and health risk to the local population in the study and worldwide regions. The four priority contaminants peak asynchronously: nitrate maximizes in the early wet season and is dominated by sewage/manure inputs (mean 71%), whereas NH4+ increased postwet under more reducing, DOC-rich conditions. Lead was chiefly geogenic (granite weathering; mean 86%) yet showed stronger anthropogenic signatures postwet, and arsenic remained the dominant carcinogenic risk driver. The resulting connectivity-redox-risk cascade explains seasonal switching of risk composition and identifies sanitation upgrades, storm-resilient urban planning, legacy metal management, and seasonally adaptive monitoring as high-leverage interventions to safeguard groundwater.