Deciphering natural and anthropogenic effects on the groundwater chemistry of Nago City, Okinawa Island, Japan

Abstract

A qualitative assessment of groundwater resources is significant in islands that largely depend on individual aquifers. In Okinawa Island, Japan, limestone aquifers are valuable groundwater reservoirs. However, these aquifers are sensitive to contamination due to high permeability in the conduit network. Although human activity has increased in recent decades, there remains insufficient hydrological information to assess the impact of anthropogenic loading on groundwater quality in Okinawa Island. To address this, we analyzed 4 seepage, 16 river, and 14 shallow (<10 m in depth) groundwater samples to obtain baseline chemistry and anthropogenic impact information on groundwater resources in central Nago City, northern Okinawa Island. We divided the region into three landscape units: lowland (<30 m asl), eastern, and western areas. Except for a limited number of water samples collected in the eastern mountain and coastal section of the lowland, the hydrochemistry was characterized by Ca–HCO3 type, indicating carbonate weathering within limestone-bearing lithology and Quaternary deposits. Divergent water 87Sr/86Sr values (0.707723–0.712102) with lower Sr concentrations (0.1–1.6 μmol/L) in the mountains and convergent values (0.708859–0.709184) with higher Sr concentrations (0.3–17.6 μmol/L) in the lowland suggest that the water–rock interactions in the lowland aquifer composed of Quaternary deposits are mostly responsible for the hydrochemistry of groundwater resources. The local meteoric water line (δD = 6.38 δ18O + 3.36) indicated that the water originates from precipitation, the altitude effect, and evaporation. The δ15N and δ18O in NO3− indicated the addition of manure and septic waste in the lowland aquifer. The results imply that detecting source areas of anthropogenic NO3− prior to serious groundwater pollution is important (regardless of the NO3− concentration), and isotope analyses would aid in developing appropriate action plans to mitigate or prevent future water pollution by NO3− in island regions.