Groundwater flow and transport and potential sources of groundwater nitrates in the Ryukyu Limestone as a mixed flow aquifer in Okinawa Island, Japan

Abstract

We investigated the groundwater flow and the transport and potential source of groundwater nitrates in the typical karst setting of the Ryukyu Limestone aquifer in the southern part of Okinawa Island, Japan. Analysis of groundwater hydrographs indicated that this is a “mixed flow” aquifer with the coexistence of slow diffuse flow in the matrices and rapid conduit flow in the caves and caverns. This relationship is indicated by the travel time of groundwater flow: 70 days in the matrices of the aquifer and 6 days through the caves and caverns. The conduit flow system was also confirmed by the distribution of relatively low concentrations of 222Rn near caverns. The sampling sites were categorized into upland field (UF) type and residential area (RA) type according to the land-use ratio on the upstream side with a 600-m influential radius, and cave and cavern (CC) type according to the hydrogeologic setting near two large caverns, even though the CC type should be categorized as the UF type from the viewpoint of land use. Cross plots of NO3-N versus SO4 2− showed that the predominant source of UF groundwater nitrates was chemical fertilizer. A difference was observed in average δ15N values between UF (8.9‰) and RA (10.0‰). On the other hand, the average δ15N value for CC (10.5‰) was similar to that for RA, indicating that CC nitrates were not related to the surrounding land use. This phenomenon is considered as evidence that CC groundwater nitrates were carried by rapid groundwater flow through caves and caverns from residential areas located higher upstream compared to the influential areas. According to previous studies, animal and human waste was considered the predominant sources of RA and CC groundwater nitrogen. The contribution ratio of chemical fertilizer (R CF) was calculated using mass balance equations under assumed predictability. There was a relatively high correlation between the rate of upland areas and of residential areas and R CF. Average R CF for UF, RA and CC was 41, 27, and 25%, respectively.