The carbon isotopic composition (13C and 14C) of the inorganic carbon dissolved in the waters of a small, largely unconfined aquifer in unconsolidated sediments on the Canadian Shield has been investigated. Three principal carbon sources are recognized: soil CO2, rock carbonate, and biogenic CO2. The average δ13C value of the soil CO2 is close to −21.0 ± 1.5%, and present-day 14C activities of the soil CO2 vary between 130 and 162% modern 14C. Very minor amounts (< 1.0%) of carbonate minerals (δ13C = −0.6‰) are present within the aquifer and react with this soil CO2 to produce a dissolved inorganic carbon (DIC) with 14C activities which are as much as 50% below the initial activities of the soil CO2. The third carbon source, a biogenic CO2, could be detected only indirectly, and its presence is primarily deduced from the occurrence of methane in the deeper parts of these aquifers. The large isotope fractionation which occurs during bacterial coproduction of CO2 with this methane results, however, in a 13C-rich CO2 and thus a DIC with high δ13C values. Values of δ13C as high as +11‰ have been measured in groundwaters on the Canadian Shield. Since the origin of the destroyed organic matter is not yet known, no assessment of the importance of this CO2 for the 14C contents of the DIC is possible.