Water Quality Characteristics and Time-series Analysis of Groundwater in Northeastern Gyeonggi Province

This study investigated groundwater quality and pollution sources in the northeastern Gyeonggi region, specifically Pocheon, Gapyeong, and Yangpyeong. A total of 3,749 samples collected between January 2022 and December 2024 were analyzed for 15 drinking water quality parameters, including pH, major inorganic ions, and heavy metals. Piper diagram analysis indicated that Ca-HCO3 (approximately 60%) and Ca-Mg-HCO3(approximately 30%) water types predominated, suggesting the presence of fresh groundwater. Statistical analyses using the Mann–Kendall test and Sen’s slope estimator revealed significant increasing trends in nitrate-nitrogen across all three regions (p < 0.05), with Pocheon exhibiting the highest rate of increase (0.020 mg/L/month). Pocheon also showed the highest concentrations of sulfate and chloride, attributed to intensive agricultural activities and urbanization. The NO3−-N/NH4+-N ratio indicated that 97.9% of samples exceeded 10, suggesting that chemical fertilizers are the dominant pollution source. This interpretation was supported by the Cl−/NO3−-N mass ratio, with 62.1% of samples below 5. Overall, these findings provide a scientific basis for developing region-specific groundwater management strategies to ensure the safe use and conservation of groundwater in the upper Han River basin.