Temporal variability of nitrate concentration in groundwater affected by intensive agricultural activities in a rural area of Hongseong, South Korea

Nitrate in groundwater from alluvial and weathered granitic aquifers was monitored for 1–1.5 years on a monthly basis in an agricultural area with a high density of livestock feedlots to identify the main factors that control temporal variations in nitrate concentration. The baseline-loading group had median NO₃–N concentrations of 5–7 mg/L, with temporal variations of 5–34 % indicating less impact of nitrogen sources. This group was mainly located in paddy fields, areas that have limited rainfall recharge during summer monsoon. Upland wells and those near livestock facilities had median NO₃–N concentrations of 11–41 mg/L with temporal variations of 10–87 %, which were designated as the elevated-loading group. Overall, nitrate concentrations in groundwater decreased during dry/growing season of spring and fall due to the mixing of the groundwater in these areas with deeper groundwater because of heavy pumping, whereas nitrate concentrations increased during summer monsoon due to infiltration of the nitrate concentrated in the soil zone, and the level was maintained or rebounded during dry/fallow season. Multiple linear regression showed that nitrate was positively explained by SO₄, Cl, and DO, and negatively explained by pH and HCO₃ indicating groundwater recharge and mixing of shallow and deep groundwater are important factors for nitrate contamination. These results show that nitrate concentrations in groundwater were controlled more by hydrologic processes than by biogeochemical processes, because most wells were considerably oxic. However, some of the wells were suboxic, and they exhibited increased Cl/NO₃ ratio and concentrations of HCO₃ and Mn(II) and decreased nitrate concentrations. Furthermore, NH₃–N was detected up to 2.6 mg/L with a sharp decrease in nitrate concentrations in one well, suggesting that dissimilatory nitrate reduction to ammonia and denitrification contributed to reduction in nitrate concentrations. This study revealed the effects of hydrologic and biogeochemical processes on temporal variations in nitrate concentrations in groundwater with high N loadings due to agricultural activity and a low potential for nitrate attenuation.