17:15 〜 19:15
[AHW27-P04] Natural remediation capacity in the denitrification zone of shallow groundwater, using the in-situ pumping experiment
キーワード：自然浄化、脱窒、地下水、現場揚水
Nitrate pollution in water is a serious environmental issue in the world, especially in agricultural lands due to excessive application of fertilizer. To remediate nitrate pollution in groundwater, it is useful to utilize natural remediation such as denitrification and plant absorption. In our research, we confirmed natural remediation capacity in the denitrification zone of shallow groundwater, using the in-situ pumping experiment. The study area was located on Ikuchijima Island, in the Seto Inland Sea in western Japan. We installed observation wells with internal diameter of 5cm at 1m, 2m, 3m, 10m, and 15m depth. The wells had screens of 20cm to 1m from the bottom. Based on previous research, groundwater within the 2m to 3m depth was anoxic, with DO less than 2mg/L. In this experiment, the groundwater from the 3m deep well was pumped up for 6 days, during which water samples were collected at 3 hours intervals. Groundwater samples were collected both before and after the experiment. Water flux estimated by Darcy’ law between the wells of depths 2m to 3m were 0.03 to 0.24 mm/min. Cl- concentration was lower in the shallow wells (26.8 mg/L at 1m, 13.7 mg/L at 2m, 29.6 mg/L at 3m) but higher at the deeper wells (47.9mg/L at 10m). However, after pumping began, the Cl- concentration at the 3m deep well decreased drastically and became stable 2 days later. This result suggests that groundwater at 3m depth was diluted by groundwater from the shallow depths (1m and 2m) but not by groundwater from the deeper depths (10m). The mixing ratio of groundwater at 2m, 2 days after pumping was estimated to be around 70%. On the other hand, the initial nitrate concentration in the groundwater was 26.5 mg/L at 1m, 1.6 mg/L at 2m, and 2.8 mg/L at 3m. Nitrate concentration at 3m had wide fluctuations during the pumping experiment. When the derived nitrate concentration was lower than the estimated concentration based only on mixing, denitrification was suggested to be occurring. Conversely, when the derived nitrate concentration was higher than the estimated value, it indicated the mixing of surface groundwater, from 1m, with higher nitrate concentrations. Based on the experiment results, we confirmed that the pumping of anoxic groundwater is an effective natural nitrate remediation strategy.
Acknowledgements:
This research is funded by JSPS Grant-in-Aid for Challenging Research (Exploratory) (PI: Mitsuyo Saito, No. 22K19869)･
Acknowledgements:
This research is funded by JSPS Grant-in-Aid for Challenging Research (Exploratory) (PI: Mitsuyo Saito, No. 22K19869)･