An integrated stable isotope-source apportionment approach for quantitative risk assessment of ammonium and...

Coastal groundwater systems are increasingly threatened by multi-source contamination driven by intensive industrialization, seawater intrusion, and complex hydrogeochemical processes. However, quantitative linkages between contamination sources and associated health risks remain poorly constrained. This study developed an integrated isotope-statistical framework that couples Stable Isotope Mixing Models in R (SIMMR) with positive matrix factorization (PMF) and source-specific health risk assessment to quantify the sources and risks of ammonium and metal (loid) pollution in an active coastal industrial park located at the Pearl River Delta estuary, South China. Ammonium nitrogen (NH₄⁺-N) concentrations ranged from 0.13 to 28.32 mg/L, with 97.6 % of samples exceeding the Chinese groundwater quality standard (0.5 mg/L). The SIMMR-PMF coupling enabled robust quantitative apportionment of ammonium sources, revealing that accumulation primarily originated from anaerobic mineralization of sedimentary organic matter, enhanced by seawater-induced reducing conditions, with additional contributions from localized anthropogenic inputs. For metal (loid)s, PMF identified four major sources, with manganese (Mn) and arsenic (As) emerging as critical pollutants, exhibiting exceedance ratios of 69.1 % and 57.1 %, respectively. Source-oriented health risk assessment indicated that non-carcinogenic hazard indices (HI) for both children and adults exceeded 2.8, dominated by Mn, while carcinogenic risk for adults reached 1.2 × 10^-3, surpassing the commonly accepted threshold of 1 × 10^-4, predominantly attributable to As from agricultural and industrial activities. These findings demonstrated the novel integration of SIMMR and PMF to quantitatively link contaminant sources with human health risks, offering a transferable methodology for diagnosing contamination mechanisms and guiding targeted groundwater protection in complex coastal aquifers worldwide.