[H-TT16] Development and application of environmental traceability methods

[HTT16-05] Boron isotope geochemistry as a tracer to evaluate groundwater contamination in the Quaternary Ryukyu Limestone aquifer in the southern region of Okinawa Island, SW Japan

KE-HAN SONG¹, *Ryuichi Shinjo^3,2, Yuto Ito¹, Yumi Moromizato³, Jun Yasumoto⁴ (1.Graduate School of Engineering and Science, University of the Ryukyus, 2.Research Institute for Humanity and Nature, 3.Department of Physics and Earth Sciences, University of the Ryukyus, 4.Department of Regional Agriculture Engineering, University of the Ryukyus)

Keywords:boron isotope, groundwater, Nitrate-nitrogen contamination, Ryukyu Limestone

Boron isotopes are effective tracers for solving environmental problems such as the quantitative evaluation of contaminants in river water and groundwater. To ensure the accuracy of isotope analysis, well-characterized reference materials (RMs) are required. However, although there are guaranteed values for boron concentration, there are still few RMs with known boron isotopic compositions. We report, for the first time, the boron isotopic compositions of Japan-made certified reference materials (CRMs) for river water (NMIJ CRM 7202-c, JSAC 0301-4a) and tap water (NMIJ CRM 7203-a). We developed a simple boron separation method using only a 0.1 ml cation-exchange resin column. This method was simplified by cumbersome separation, confirming the recovery of boron yields of >97% and a separation efficiency of >98% for matrix elements. The δ¹¹B values of CRM 7202-c, 0301-4a, and 7203-a were -8.81 ± 0.19‰, 14.33 ± 0.49‰, and 17.86 ± 0.28‰, respectively. The developed method is simple and rapid; hence, it enhances the boron measurement efficiency.
This research presents the first quantitative analysis using the boron isotope in groundwater to study environmental contamination in the Quaternary Ryukyu Limestone aquifers of the southern region of Okinawa Island, SW Japan. 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.