Effective use of farmland soil samples for N and O isotopic source fingerprinting of groundwater nitrate contamination in the subsurface dammed limestone aquifer, Southern Okinawa Island, Japan

Abstract

Nitrate contamination of groundwater due to improper management of fertilizer application and controls during agricultural activities is an escalating worldwide issue. Numerous studies have reported similar contamination in the Ryukyu limestone aquifer in the southern part of Okinawa Island. The study area has a unique setting where the subsurface dams hold the groundwater behind their cut-off walls, probably holding contaminated waters. This study provides updated insights on the source of contamination using stable N and O isotope ratios of dissolved nitrate (ẟ15NNO3 and ẟ18ONO3) in the aquifer waters. To propose a better reference in the source fingerprinting assessment, a special emphasis was placed on the isotopic comparison between the fertilizers and farmland soils where the respective fertilizers were applied. A total of 176 groundwater samples were collected from several spatial and monthly sampling campaigns for the purpose of source determination. Our isotopic comparison detected the enrichment of 15N of nitrate-nitrogen in the soil samples after the application of both chemical fertilizers and manure due to nitrification and volatilization. Unlike results from previous studies, the ẟ15NNO3 compositions of most groundwater samples did not fall into the range of soils mainly controlled by chemical fertilizers (from +3‰ to +5‰), but were comparable to the soils affected by mixed fertilizers including both chemical fertilizers and manure (from +7‰ to +9‰). The soils beneath untreated manure storage sites characterized by the highest ẟ15NNO3 values (from +18‰ to +20‰) affected some groundwater, which shows an even higher value (> +9‰). The contaminated waters tend to be gathered close to the subsurface dams and this tendency did not differ between two subsurface dam settings, Komesu and Giza. However, due to more impact from manure storage sites, the groundwater in the Komesu area tends to show slightly higher δ15NNO3 values than those in the Giza area. Our study demonstrated the usefulness of soil samples for precise isotopic source identification in farmland areas.