Hydrochemical evolution and nitrate contamination sources in Laiwu groundwater: Insights from hydrochemistry and dual-isotope analysis

Effective groundwater protection fundamentally relies on the accurate identification of primary influencing factors and pollution sources. In this study, 81 groundwater samples from a fault depression basin were analyzed using statistical and hydrochemical methods, the dual-isotope method, and the Bayesian Mixing Model (MixSIAR) to characterize groundwater and identify potential pollution sources and their contributions. Results reveal dominant hydrochemical facies of SO₄·Cl–Ca·Mg and HCO₃–Ca·Mg types, driven by natural processes such as Silicate and Carbonate weathering and dissolution, alongside anthropogenic nitrate inputs. Notably, the concentration of nitrate exceeded the permissible limit in over 90 % of the samples. Spatial analysis of nitrate pollution using the kriging method reveals the most of them were located in the lower reaches of the basin. Dual isotope analysis identified Soil Nitrogen (SN) and Manure and Sewage (M&S) as the primary sources of nitrate in groundwater. The MixSIAR model results further indicated that SN and M&S nitrogen contributed 30.66 % and 42.21 %, respectively, to groundwater nitrate concentrations. This study demonstrates the effectiveness of hydrochemical and MixSIAR methods in identifying and quantifying influencing factors of groundwater, with particular emphasis on the necessity of connecting these methods with the investigation of the industrial sector.