Evaluation of nitrate contamination sources of unconfined groundwater in the North Han River basin of Korea using nitrogen isotope ratios.

To evaluate the nitrate contamination sources of unconfined groundwater in the North Han River basin (127°45′E, 37°55′N), groundwater samples were collected monthly for three years (1997–1999) from 20 wells, and analyzed for the concentration (n=599) and δ¹⁵N (n=96) of NO₃−N. Frequency distribution of NO₃−N concentration showed that 43.9% of samples exceeded the national standard for drinking water (10 mg NO₃−N L⁻¹). During dry season between October and March, only 36.1% of a total of 296 samples had NO₃−N concentration above 10 mg L⁻¹. However, 51.5% of a total of 303 samples exceeded the standard level during wet season between April and September. Concentrations of nitrate increased with direction of groundwater flow. Wells located in vicinity of livestock feedlots showed consistently high nitrate concentration irrespective of precipitation pattern. The δ¹⁵N signatures of NO showed that in general, both¹⁵N-depleted source (e.g., chemical fertilizer) and¹⁵N-enriched source (e.g., compost and manure) impacted groundwater quality concurrently. The δ¹⁵N ranges of groundwater NO were +1.5–+12.9‰ in dry season and +3.2–+9.9‰ in wet season. These δ¹⁵N data suggested that the effects of compost and/or manure on nitrate concentration were apparent in dry season. A positive correlation (r²=0.32) between N concentration and δ¹⁵N of NO in dry season also suggested that nitrate concentration increased with increasing loading of nitrate from compost and/or manure wastewater. On the other hand, the relatively narrow range of δ¹⁵N and high nitrate concentration in wet season suggested that increased flush of nitrate derived from mixed sources such as fertilizer, compost, and manure during storm event resulted in high nitrate concentration. In this area, neither high δ¹⁵N values nor a negative correlation between δ¹⁵N and N concentration of NO, indicating denitrification was observed because of the coarse textured soil matrix. Therefore, reduction of nitrogen inputs through curtailment of fertilizer and compost application rates and appropriate treatment of livestock manure are the most effective and practical ways to prevent groundwater contamination. [ABSTRACT FROM AUTHOR]