The inspection gallery of a dam is leached by reservoir water that has infiltrated through the rock-fill dam body from the high-pressure water area upstream (leakage), and groundwater seeping through the foundation bed which has been injected with ordinary Portland cement (OPC) grout. Seepage waters with a pH greater than 12 are characterized by the major components of Ca(OH)2 and alkali hydroxides, lower Mg2+ concentration and higher K+/Na+ molar ratio than those for river and spring waters and groundwater near the dam site, and the extensive carbonate deposits precipitated from the waters. During a low discharge period, NaOH was the major component because Ca2+ was removed as carbonate deposits. Based on the stable isotope ratios of hydrogen and oxygen in the water, which reflect the altitude effect, it was inferred that the seepage waters are supplied from the shallow groundwater near the dam site and that the leakage is a mixture of the seepage water and reservoir water. Alkali hydroxides and Ca(OH)2 cement porewaters formed during the early stages of hydration of OPC are still present in the cement mass after 46 years, and that the OPC still remains partially unhydrated. These highly alkaline porewaters have seeped through gaps of the concrete joint surfaces to discharge into the inspection gallery. The Ca(OH)2 concentration in the seepage was below the portlandite saturation, therefore, the grout-derived solutes are considered to have been diluted as they diffused into the shallow groundwater before being discharged with the groundwater seeping into the gallery due to the hydraulic head difference and mixed with the groundwater. Ca(OH)2 in the seepage reacted with atmospheric CO2 in the inspection gallery, and the carbon stable isotope ratio of the carbonate deposits produced in this way was unusually low, about −25‰ VPDB, due to the large disequilibrium isotope fractionation during the direct reaction of CO2 with OH− in the solution. In addition to the change in its discharge, the hyperalkaline seepage and the resulting carbonate deposits with unusually low δ13C values can be measures of the durability of the grout-treated foundation rock of the rock-fill dam, because the decrease in pH in the future is related to the instability of the grout hydration products by carbonation. •Seepage waters with a pH greater than 12 have been observed in an inspection gallery of an over 46 years old rock-fill dam.•Hyperalkaline seepage waters are characterized by their high calcium and alkali hydroxides contents and by calcite deposits.•Altitude effect of hydrogen and oxygen isotope ratios of water could allow estimating their source waters.•The unusually low δ13C carbonate deposits were produced by the direct reaction of atmospheric CO2 with OH- in the solution.•The hyperalkaline seepage and the carbonate deposits can be measures of the grout-treated foundation rock durability.