Hydrogeochemical characteristics of groundwater highly polluted with nitrate in an agricultural area of Hongseong, Korea

The hydrogeochemical characteristics of groundwater, in an area where widespread contamination by nitrate (NO₃ ^-) was anticipated, were studied using traditional geochemical investigation and multivariate statistical analysis. Widespread NO₃ ^- contamination as high as 67.2 mg/L as NO3-N was observed, and positively correlated with that for chemicals (Cl^-, major cations) with surface origin. Principal component analysis (PCA) revealed that three processes affected groundwater chemistry of the area: (1) leaching of pollutants from the ground surface; (2) reduction of NO₃ ^- in areas with low dissolved oxygen (DO); and (3) ingress of low NO₃ ^- deep groundwater. Five sample groups were identified from cluster analysis, and analysis of land use patterns around each group showed that fate and distribution of NO₃ ^- contamination were mainly controlled by surface topography and predominant land use type. The highest NO₃ ^- concentrations were associated with confined livestock feeding operations in hilly terrain areas, where infiltrating water also had high DO. Lower NO₃ ^- concentrations found in the lowland flat areas were thought to be due to either reducing conditions in rice paddies leading to N attenuation or drawing in of deep groundwater by pumping to meet agricultural needs during periods of low rainfall.