Mechanism of denitrification in subsurface-dammed Ryukyu limestone aquifer, southern Okinawa Island, Japan

Denitrification crucially regulates the attenuation of groundwater nitrate and is unlikely to occur in a fast-flowing aquifer such as the Ryukyu limestone aquifer in southern Okinawa Island, Japan. However, evidences of denitrification have been observed in several wells within this region. This study analyzed environmental isotopes (δ¹⁵N_NO3 and ẟ¹⁸O_NO3) to derive the rationale for denitrification at this site. Additionally, the presence of two subsurface dams in the study area may influence the processes involved in nitrate attenuation. Herein, we analyzed 150 groundwater samples collected spatially and seasonally to characterize the variations in the groundwater chemistry and stable isotopes during denitrification. The values of δ¹⁵N_NO3 and δ¹⁸O_NO3 displayed a progressive trend up to +59.7 ‰ and + 21 ‰, respectively, whereas the concentrations of NO₃^--N decreased to 0.1 mg L^-1. In several wells, the enrichment factors of δ¹⁵N_NO3 ranged from -6.6 to -2.1, indicating rapid denitrification, and the δ¹⁵N_NO3 to δ¹⁸O_NO3 ratios varied from 1.3:1 to 2:1, confirming the occurrence of denitrification. Denitrification intensively proceeds under conditions of depleted dissolved oxygen concentrations (<2 mg L^-1), sluggish groundwater flow with longer residence times, high concentrations of dissolved organic carbon (>1.2 mg L^-1), and low groundwater levels during the dry season with precipitation rates of <100 mm per month (Jun-Sep). SF₆ analysis indicated the exclusive occurrence of denitrification in specific wells with groundwater residence times exceeding 30 years. These wells are located in close proximity to the major NE-SW fault system in the Komesu area, where the hydraulic gradient was below 0.005. Detailed geological and lithological investigations based on borehole data revealed that subsurface dams did not cause denitrification while the major NE-SW fault system uplifted the impermeable basement rock of the Shimajiri Group, creating a lithological gap at an equivalent depth that ultimately formed a sluggish groundwater area, promoting denitrification.