SEASONAL VARIATIONS IN DIEL BEHAVIOR OF DISSOLVED INORGANIC CARBON IN A COAL MINE DRAINAGE STREAM

Diel (24 hour) cycling of dissolved inorganic carbon (DIC) is well documented in freshwater settings, such as karst and mountain streams, but few data exist that characterize the diel cycling of DIC in coal mine drainage. Consequently, we evaluate diel cycles of DIC, CO₂, and δ¹³C_DIC in mine drainage in north-central West Virginia in late winter, mid spring, and summer at two sites along the drainage stream. The upstream site (50 m from the mine portal) is lined with limestone riprap and is exposed to less direct sunlight, has less emergent vegetation, and exhibits a faster flow rate than the downstream site (138 m from the mine portal), which is in an altered natural wetland. The differences in characteristics were expected to affect the diel cycling of CO₂ and DIC, and thus impact other solute compositions. The data were fit using a cosine model to assess the magnitude and phase of diel cycles, and the goodness of fit of the model was determined using an F-test statistic. DIC concentrations were higher at the upstream site (2.09 mM – 3.35 mM) than the downstream site (1.11 mM – 1.77) as CO₂ degasses downstream. Over the three sampling periods, DIC and CO₂ concentrations remained approximately constant at the upstream site but decreased slightly over time at the downstream site as temperature increased from winter to summer. The downstream site in late winter exhibited the best fit cycle with the DIC and CO₂ increasing during the nighttime concurrent with a decrease in pH and δ¹³C_DIC values. These changes are consistent with the release of CO₂ during plant and sedimentary respiration, while the daytime increase in δ¹³C_DIC values during the day is consistent with photosynthesis. Significant cycles (at a < 0.01) include (1) DIC–late winter and summer at the downstream site; (2) CO₂–only during late winter at both sites; and, (3) δ¹³C_DIC –during spring and summer at the downstream site. Mechanisms likely controlling diel cycles of these parameters are temperature-dependent CO₂ degassing, photosynthesis- respiration diel cycles in plants and sediments, and interactions with the metals present at high concentrations in the mine water.