[AHW24-P02] Acidification process of shallow groundwater in an alluvial fan in Ueda City, Nagano, Japan
Keywords:acidified river water, alluvial fan, river water infiltration, shallow groundwater, groundwater acidification, multi-tracer method
Survey on the Koganezawa River water and shallow groundwater (well water and spring water) of the alluvial fan was conducted from August to November 2018. As a result, 1) groundwater acidity is associated with nitrate and sulfate ions for the left and right banks of the river, respectively, 2) groundwater in the right bank shows lower pH than that in the left bank, and 3) oxygen (δ18O) and hydrogen (δD) isotopic ratios are low for the groundwater in the right bank, compared with that in the left bank. These results strongly suggest the sulfate-rich acidified and isotopically depleted river water infiltrates mainly into the right bank, affecting its shallow groundwater quality. The two end-member mixing analysis, based on the concentration of sulfate (SO42-) and nitrate (NO3-) ions, indicates a contribution of the infiltrated river water to the formation of the groundwater in the right bank is in the range 8 to 31 %. Furthermore, from the nitrogen (δ15N) and sulfur (δ34S) stable isotopic ratios, it is estimated that nitrogen and sulfur in groundwater in the left bank is likely to be of manure origin. It can be concluded that acidification of shallow groundwater in the Koganezawa River alluvial fan is due to river water infiltration for the right bank, while manuring for the left bank.