A change in the chemical composition of the groundwater from Ca-HCO3 type to Na-HCO3 type with decreasing distance from a fault zone is observed along an active fault survey tunnel in central Japan. The sodium/calcium ratio in the groundwater is maximum (ca. 10--25) in and adjacent to the area of the significantly fractured fault zone. The ratio decreases with increasing distance from the fault zone. This may result from Na release from plagioclase into the groundwater due to the intense chemical weathering in the fault zone. Thus, the Na/Ca ratio in the groundwater around the fault has the potential to be a good indicator of fault activity. Based on the oxygen isotope ratio and tritium content in groundwater, one fault is inferred to be fairly permeable and the other impermeable. This implies that the former is active and the latter inactive.