Fundamental information is lacking on the natural dissolved organic carbon in ground water, information both necessary to understand pollutant transport and degradation and pertinent to isotope dating of ground water and reconstructions of groundwater history. Here we report, for the first time, carbon isotope ratios of fractions of natural organic compounds in ground waters isolated from the Stripa mine (Sweden) and the Milk River aquifer (Alberta, Canada). High-molecular-weight and low-molecular-weight fractions of the organic carbon were characterized and these, along with dissolved inorganic carbon, were analysed for δ13C and 14C. The 14C results suggest that the dissolved organic carbon originates from a combination of soil organic matter and kerogen in the aquifer matrix. The high-molecular-weight fractions show a predominant soil origin, whereas the low-molecular-weight fractions are often strongly influenced by kerogen. In both systems studied, groundwater dating by traditional 14C analyses of the dissolved inorganic carbon is difficult. The 14C activities of the high-molecular-weight fractions, which were identified as fulvic acids, follow trends consistent with 14C dissolved-inorganic-carbon analyses and, in some cases, provide additional information on groundwater age.
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