Dual isotopes evaluation of nitrate contamination and their relative contribution of multiple sources in Daejeong, Jeju island, South Korea

Nitrate (NO₃^–) is the final product of the microbial nitrification process in the subsurface environment and can play a significant factor in potential contamination, causing seasonal hypoxia or eutrophication in the aquatic ecosystem. Excessive nitrogen (N) input by anthropogenic activities is well known as one of the major nitrogen sources such as dissolution of organic or synthetic (NH₄⁺, NO₃^–) fertilizer, leakage of the municipal and industrial landfill, and manure or septic wastewater. This study implemented dual nitrate isotopes approach to investigate the spatial distribution of nitrate contamination and its relative contribution of multiple nitrate sources in Daejeong region, Jeju island, South Korea. A total of 26 groundwater samples were collected from Sangmo, Hamo, and Dongil areas in Daejeong region and their nitrate concentrations were measured. The isotope ratios of N (δ¹⁵ N_NO3) and O (δ¹⁸ O_NO3) were analyzed using a denitrifier which converts the dissolved nitrate in the groundwater sample into gaseous nitrous oxide (N₂O). The NO₃-N concentration above the DWS (drinking water standard, 10 mg/L) was found in 11 out of 19 wells only in Sangmo, while the average NO₃-N concentrations of Hamo and Dongil were 2.95 and 2.58 mg/L, respectively. More than 80% of land use in Daejeong region was reported as agricultural farming, and massive synthetic fertilizer usage was expected. However, the δ¹⁵ N_NO3 ranged from 8.3‰ to 10.2‰, with an average of 9.22‰ and δ¹⁸ O_NO3 ranged from 3.2 to 6.9‰ with an average of 5.58‰, which are similar values of manure and septic wastewater. Thus, further investigation on land use changes, historical fertilizer consumption and its isotopic shifting in Daejeong region need to be considered for the relative contribution of nitrate sources.