| Summary: | Regular features of ground and surface waters affected by drainage from mine waste include their acidity and elevated concentrations of dissolved metals, with their attendant negative effects on drinking water quality and aquatic life. One parameter that aids in buffering these waters against acidity and sustains aquatic life is dissolved inorganic carbon (DIC). In this study, the chemical and isotopic (δ<sup>13</sup>C) composition of primary calcite and DIC (δ<sup>13</sup>C<sub>DIC</sub>) in groundwater and surface waters within and downstream, respectively, of abandoned Cu–W–F skarn tailings at Yxsjöberg, Sweden, were used to trace the biogeochemical processes controlling their respective δ<sup>13</sup>C<sub>DIC</sub> signatures. In addition, the δ<sup>13</sup>C signatures of the inorganic (carbonate) fractions of the tailings were used to verify the formation of secondary carbonates within the tailings. Lower average δ<sup>13</sup>C values of the carbonate fractions (δ<sup>13</sup>C<sub>carb</sub> = −2.7‰) relative to those of the primary calcite (δ<sup>13</sup>C = +0.1‰) from the orebodies from which the tailings originated pointed to the precipitation of secondary carbonates. These lower δ<sup>13</sup>Ccarb signatures were assumed to represent mixed-source C signals involving isotopically light CO<sub>2</sub> from the atmosphere, the degradation of organic matter in the upper part of the tailings and HCO<sub>3</sub><sup>−</sup> from calcite dissolution. The groundwater δ<sup>13</sup>C<sub>DIC</sub> values (−12.6‰ to −4.4‰) were far lower than the hypothetical range of values (−4.6‰ to +0.7‰) for primary calcite and secondary carbonate dissolution. These signatures were attributed to carbonate (calcite and secondary carbonate) dissolution and the degradation of dissolved organic carbon (DOC) from various organic sources such as peat underneath the tailings and the surrounding forests. Downstream surface water samples collected in May had low δ<sup>13</sup>C<sub>DIC</sub> values (−16‰) and high DOC (14 mg C/L) compared to the groundwater samples. These signatures represented the oxidation of the DOC from the wash out of the mires and forests during the snowmelt and spring flood. The DOC and δ<sup>13</sup>C<sub>DIC</sub> values of the surface waters from June to September ranged from 6–15 mg·C/L and −25‰ to −8.6‰, respectively. These signatures were interpreted to reflect mixed C sources, including carbonate dehydration by acidity from Fe<sup>3+</sup> hydrolysis due to the mixing of groundwater with surface waters and the subsequent diffusive loss of CO<sub>2 (g)</sub>, aquatic photosynthesis, photooxidation, DOC degradation, as well as microbial respiration. Although the <sup>13</sup>C<sub>DIC</sub> signatures of the downstream surface waters seemed to be seasonally controlled and influenced by variable groundwater contributions, the lack of data with respect to DIC concentrations, coupled with multiple potential biogeochemical processes that could influence the DIC pool and <sup>13</sup>C<sub>DIC</sub> values, made it difficult to identify the major regulating process of the <sup>13</sup>C<sub>DIC</sub> signatures. Therefore, other complimentary isotopes and elemental concentrations are recommended in order to decipher the dominant biogeochemical process.
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