Application of using a boron tracer to evaluate groundwater contamination in the Quaternary limestone aquifer in the southern region of Okinawa Island, Japan

Abstract

This research presents the first quantitative analysis using the boron isotope in groundwater to study environmental contamination in the Quaternary limestone aquifers of the southern region of Okinawa Island. Due to the direct discharge of wastewater from human activities into the groundwater, monitoring water quality is of primary concern in this region. In this study, we reported on boron concentrations and δ11B values in groundwater, spring, soil, and possible contaminant samples collected between August 2020 and August 2021. Our results indicated that boron contents co-migrate with nitrate-nitrogen, as confirmed by a positive correlation between their concentrations, indicating that boron is a valuable tracer in the Ryukyu limestone region. We also observed that the four endmembers had different boron characteristics that could help diagnose the trend of contamination processes via mixing lines. The solute samples from our regional monthly monitoring showed that boron concentrations ranged from 12 to 168 ppb and δ11B values ranged from 19 to 38‰. We estimated that the rainwater contained 0.62% seawater, obtaining the best fit of solute samples to two trends on the mixing lines of fertilization (manure and fertilizer). The mixing calculation revealed that contribution rates were less than 1% and approximately 3% in groundwater and soil leachates, respectively. Overall, the quantitative application of the boron tracer allowed us to represent the distribution of contamination caused by fertilization in the Ryukyu limestone watershed.