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.