ORGANIC MATTER REMINERALIZATION IN THE SEDIMENTS OF AN ACID MINE DRAINAGE LAKE

The Rocky Forks Conservation Area near Columbia, Missouri was the site of coal mining during the late 1950s and 1960s. After strip mining of the area stopped in 1972, the old limestone coal pits filled with water and formed lakes. Lakes in the area are generally circum-neutral (pH ~7) except “Red Lake,” which has a pH value of 3-4. The leading question in this study is why has Red Lake not been neutralized by the carbonates in the limestone bedrock. Our hypothesis is that organic matter remineralization rates in Red Lake are much slower than in surrounding lakes. To test this hypothesis, a lake comparison study was done with “Green Lake,” a circum-neutral lake that is approximately the same size, slightly higher in elevation, and located 60m to the west of Red Lake. To determine organic matter remineralization rates, concentration depth profiles of the redox species, dissolved manganese (Mn²⁺), ferrous iron (Fe²⁺), sulfate (SO₄^2-), and dissolved inorganic carbon (DIC), were measured during the summer 2012. In addition, the top 10 cm of sediment were collected, homogenized, and the above pore water constituents were analyzed over time to directly determine rates. Depth profiles ultimately showed Red Lake has significantly higher metal and SO₄^2- concentrations in the pore water than Green Lake. Average pore water concentrations in Red Lake were 0.12 mM Mn²⁺ , 0.27 mM Fe²⁺, and 24.1 mM SO₄^2-, whereas they were 0.03 mM Mn²⁺, 0.02 mM Fe²⁺, and 11.2 mM SO₄^2- in Green Lake. In Red Lake both Fe²⁺ and DIC concentrations increased from the sediment-water interface to about 10cm depth. In Green Lake, only DIC concentrations increased with depth. The other constituents in both lakes remained relatively constant and similar to surface water concentrations. The 8-day incubation study showed similar results in that Fe²⁺ and DIC values increased and SO₄^2- concentrations remained constant over time for Red Lake. Green Lake incubations showed increasing DIC concentrations and constant Fe²⁺ and SO₄^2- levels with time. Studies will be carried out in the winter months to determine variations and seasonal changes in chemical processes.