Nitrate contamination of alluvial groundwaters in the Nakdong River basin, Korea

Abstract

Nitrate (NO3-) is one of the most serious pollutantsencountered in shallow groundwater system. To secure the sus-tainable development of alluvial groundwaters, therefore, it isimportant to understand the source(s) and extent of nitrate pol-lution. In this paper, we have studied the hydrogeochemistry ofalluvial groundwaters (n=137) that were collected from irrigationand domestic wells in the Wolha, Daesan, and Yongdang areas onthe border of a lower reach of Nakdong River, Korea. The ana-lyzed nitrate concentrations range widely from below the detectionlimit to 383.4 mg/L NO3- (median=55.4 mg/L). About 58% of theexamined samples have the nitrate concentrations exceeding theKorean Drinking Water Standard (44.3 mg/L NO3- ). Agriculturalpractices in the study sites are the main cause of serious nitratepollution and control the hydrochemical facies change from Ca-HCO3 type toward Ca-NO3(Cl) type for the waters. However, thehydrochemical facies change also reflects the water-rock interac-tion and redox reactions in aquifer. The correlations betwennitrate concentration and other physicochemical data can be efec-tively used to understand the hydrochemical evolution and originsof major ions in waters. The d15N values of nitrate in waters (n=18)indicate two sources of nitrate pollution: (1) nitrates (d15N=4.3-6.2) originated from synthetic fertilizers applied in agriculturalland, and (2) nitrates (d15N=15.0-19.9) from animal manureand human wastes that were discharged from adjacent villageslocated at hydrogeologically upstream site. Based on the d15N andnitrate concentration data, we quantitatively evaluate the degreeof hydrologic mixing between the two nitrate sources (and corre-sponding two water bodies), which occurs through local infiltra-tion process along the groundwater flow direction.