Contribution of nitrate-nitrogen concentration in groundwater to stream water in an agricultural head watershed

This study characterized nitrate-nitrogen (NO₃-N) concentrations in groundwater and stream water in an agricultural head watershed in South Korea and identified the pollution load of NO₃-N as a result of the groundwater entering streams using field surveys, analyses of chemical constituents, and numerical modeling. The mean NO₃-N concentration in groundwater was 7.373 mg/L, which is approximately 1.9 times higher than concentrations found in stream water. The groundwater and stream water samples belonged to the Ca-HCO₃ type. The concentration of NO₃-N in groundwater tended to increase in the lowland areas downstream. There was seasonal variations of NO₃-N in both the groundwater and stream water samples, with increases in concentration during the dry season (January-April) and decreases during the wet season (June-October). The NO₃-N load in stream water to that in groundwater (R) was higher during the wet season (September) than the dry season (March), with R distinctly increasing in upstream areas relative to downstream areas, indicating that during the wet season, a large amount of NO₃-N is introduced into stream water from groundwater. By analyzing the relationship between groundwater and stream water and through NO₃-N transport modeling, it was revealed that in the watershed, the nitrate-N load in stream water is greatly augmented by inputs from groundwater, particularly in the middle and downstream areas.