Quantifying Mechanisms of Elevated Iron and Manganese Concentrations in Coastal Multi-Layer Aquifers via Numerical Modeling

Elevated Fe/Mn in coastal groundwater threatens water safety and ecosystems, yet their coupled natural-anthropogenic drivers remain poorly quantified in complex multi-aquifer systems. Given the quantitative limitations of conventional hydrochemical and statistical analyses, this study developed a numerical modeling-driven framework integrating PMF-based quantitative source apportionment of Fe/Mn with PHREEQC-constrained reactive transport modeling (via GMS) in Zhanjiang City, China. Dataset from 1970s to present revealed persistent Fe/Mn contamination across this groundwater-dependent region. PMF analysis quantitatively differentiated natural sources (dominantly silicate weathering and Fe/Mn-bearing mineral dissolution) from anthropogenic inputs, constraining reaction pathways. PHREEQC inverse modeling further quantified Fe/Mn-bearing mineral dissolution rates, generating essential reaction parameters for transport simulations. Forward modeling assessed Fe/Mn mobility under saline-water mixing and CO₂ equilibrium conditions. Simulations indicated that low seawater mixing best reproduced observed Fe/Mn levels, with Ca²⁺/Mg²⁺ exchange as a key control. Discrepancies between modeled and observed ion compositions implied possible contributions from deep brine upcoming or anthropogenic inputs. 3D numerical simulations characterized flow dynamics and reactive transport. Results revealed spatially constrained Fe/Mn dispersion, indicating limited aquifer hydrogeological connectivity regardless of pollution source location. Large-scale pumping neither induced significant downstream dispersion nor facilitated upstream transport, highlighting the dominance of natural hydrogeochemical controls. Redox conditions and hydraulic parameters were key regulators of Fe/Mn mobility. This framework quantifies redox-driven Fe/Mn contamination mechanisms under combined stressors, offering transferable solutions for global coastal groundwater management.