Water samples from 52 shallow wells and 2 rivers in the Tonami plain (Shogawa fan), Toyama, were sampled for chemical analysis,
13C/
12C measurement of the dissolved carbonate and
18O /
16O analysis of the water. Wells far from the area of recharge from the rivers have δ
13C values of -19 to -21‰ and total carbonate contents of upto 3 mmol/l, whereas the rivers have δ
13C values of -8 to -9‰ and total carbonate contents about 0.5mmol/l at the recharge area. The relationship between the δ
13C value and total carbonate content indicates that the additional carbonate is of soil CO
2 origin. The partial pressure of CO
2 in the aquifer estimated from the total carbonate content and pH of the groundwater amounts upto about 3×10
-2 atmosphere, 2 orders of magnitide higher than the atmospheric value, supporting that the additional carbonate is derived from the soil CO
2. The.δ
18O value of groundwater increases, as the δ
13C value decreases away from the recharge area, implying that a significant amount of precipitation percolates into the aquifer through the soil zone. The rela-tionship between the HCO
3- and Ca
2+ Mg
2+ contents of the groundwater suggests that the dissolu-tion of soil CO
2 to the recharge water of precipitation origin results from reaction of soil CO
2 with Ca (OH)
2 and CaSiO
3 which are widely used for artificially neutralizing the acidified soil.
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