Resumen
The hydrogeochemical characteristics of groundwater, in an area where widespread contamination by nitrate ( NO-3 ) was anticipated, were studied using traditional geochemical investigation and multivariate statistical analysis. Widespread NO-3 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 NO-3 in areas with low dissolved oxygen (DO); and (3) ingress of low NO-3 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 NO-3 contamination were mainly controlled by surface topography and predominant land use type. The highest NO-3 concentrations were associated with confined livestock feeding operations in hilly terrain areas, where infiltrating water also had high DO. Lower NO-3 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.