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Vol. 35 (1993) No. 2 p. 77-86



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 δ13C values of -19 to -21‰ and total carbonate contents of upto 3 mmol/l, whereas the rivers have δ13C values of -8 to -9‰ and total carbonate contents about 0.5mmol/l at the recharge area. The relationship between the δ13C value and total carbonate content indicates that the additional carbonate is of soil CO2 origin. The partial pressure of CO2 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 CO2. The.δ18O value of groundwater increases, as the δ13C 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 HCO3- and Ca2+ Mg2+ contents of the groundwater suggests that the dissolu-tion of soil CO2 to the recharge water of precipitation origin results from reaction of soil CO2 with Ca (OH)2 and CaSiO3 which are widely used for artificially neutralizing the acidified soil.

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