17:15 〜 19:15
[AHW27-P03] Seasonal controls on denitrification in coastal groundwater of a steep agricultural catchment using push-pull tests
キーワード：Denitrification, Coastal groundwater, Seasonal controls, Push-pull tests
Nitrate contamination in groundwater, primarily driven by agricultural activities and improper waste management, poses a significant threat to human health and ecosystem stability. This study investigated the seasonal variation in the natural denitrification potential of coastal groundwater of an agricultural catchment using push-pull tests (PPTs). The study evaluated the effects of groundwater characteristics, temperature, and the groundwater's oxidation-reduction potential (ORP) on denitrification potential. The research site is located at Ikuchijima Island in western Japan. Two boreholes at depths of 15m and 30m, with hydraulic conductivities of 6.6E-3 cm/s and 7.5E-4 cm/s respectively, were used for the experiments. The experiments were conducted in Winter (December 2023), Spring (March 2024), and Summer (August 2024). During each season, a 250 mL solution containing 1000 mg/L nitrate-nitrogen and chloride ions was injected into the boreholes followed by water sampling within 24 hours. The reductive ratios (%) of nitrate-nitrogen and chloride ions were used to estimate denitrification potential in each season. Obtained results revealed that natural denitrification potential increases with increasing depth and varies seasonally. Denitrification potential was maximum in Spring (10.82% at 15m, 20.65% at 30m) and least in Winter. In contrast, a previous study at the same site reported 0% denitrification at 15m and 51.54% at 30m when injecting a 500 mg/L nitrate-nitrogen tracer solution. Our results suggest that doubling the nitrate-nitrogen concentration increased the denitrification potential in the 15m borehole but reduced it in the 30m borehole. We also observed seasonal changes in groundwater flow characteristics at both boreholes. Hydraulic flux decreased from Spring to Summer to Winter in both boreholes. In Spring, the 15m borehole had a higher Darcy flux (7.35 E-5 cm/s), which was associated with lower denitrification, while the 30m borehole had a lower Darcy flux (6.13 E-6 cm/s), supporting higher denitrification. The results suggest that denitrification potential increases with increasing depth and a lower hydraulic flux. However, despite the lowest hydraulic fluxes observed in Winter, denitrification potential was also lowest during this season, likely due to the high nitrate-nitrogen concentration injected. Increased groundwater temperature and reduced ORP following tracer injection further supported higher denitrification. Specifically, the 30m borehole had the most reducing conditions in Spring, coinciding with elevated groundwater temperature after tracer injection. It can be concluded that denitrification potential is controlled by seasonal groundwater characteristics, particularly the hydraulic flux. Spring is the optimal period for nitrate remediation, especially at greater depths (30m).
Acknowledgements:
This research was supported by JSPS Grant-in-Aid for Challenging Research (Exploratory) (PI: Mitsuyo Saito, No. 22K19869).
Acknowledgements:
This research was supported by JSPS Grant-in-Aid for Challenging Research (Exploratory) (PI: Mitsuyo Saito, No. 22K19869).