Assessment of major element hydrogeochemistry and agricultural contamination of groundwater in the coastal plain of Gimje, midwestern South Korea

The importance of groundwater in coastal plain areas is gradually increasing; however, the understanding of agricultural contamination and the hydrogeochemical processes controlling the major elemental characteristics of groundwater in these areas is still insufficient. We here examined the effects of well depth, seasonal variation, terrain characteristics, and land-use on major element hydrogeochemistry for 103 wells of surficial and bedrock aquifers in an extensive agricultural plain in the mid-western part of South Korea. We found that 81% of groundwater samples exceeded the threshold value for the national background nitrate level and degree of nitrate contamination was similar for both aquifers, indicating significant contamination in the study area. As agricultural contamination increased, the groundwater type changed from Ca-HCO₃ and Na-HCO₃ to Ca-Cl(NO₃), and the molar ratio of (Ca²⁺ + Mg²⁺):HCO₃⁻ changed from 1:2 to 1:1. Additionally, the correlations between Ca²⁺, HCO₃⁻, and SO₄²⁻; between Na⁺ and Cl⁻; and between pH and F⁻ decreased. Principal component analysis of groundwater hydrogeochemical parameters showed that agricultural contamination was the dominant process, followed by water-bedrock interactions and shallow groundwater mineralization. Hierarchical cluster analysis showed that shallow groundwater exhibited high concentrations of K⁺ and Mg²⁺, whereas deep groundwater in the mountainous agricultural area had high agricultural contamination and elevated concentrations of Ca²⁺, Mg²⁺, Na⁺, K⁺, Cl⁻, SO₄²⁻, and F⁻. It is noteworthy that groundwater hardness increased with the increase in agricultural contamination. Our findings confirm that agricultural activities in the coastal plain not only increased the concentration of agriculture-derived elements, but also had an overall impact on major elements in groundwater of both surficial and bedrock aquifers.