ANNUAL CHANGES IN BIOGEOCHEMICAL CYCLING AT SURFACE WATER-POREWATER-GROUNDWATER MIXING INTERFACES IN A CONTAMINATED AQUIFER-WETLAND SYSTEM

Changes in chemical indicators of biogeochemical cycling were observed at mixing interfaces between surface water, wetland sediment porewater, and underlying aquifer groundwater in a wetland-aquifer system impacted by landfill leachate in Norman, Oklahoma. In subsurface systems, increased biogeochemical cycling is thought to occur at small-scale mixing interfaces where a gradient of electron acceptors and/or electron donors is present. These processes are controlled by the linked biological, geochemical and hydrological properties of the system. In this study, a vertical profile of water samples was collected using passive-diffusion equilibrators (peepers) to capture the surface water-wetland sediment interface and the wetland sediment-aquifer interface with a resolution of 0.5-cm to 1-cm. (Bio)geochemical indicators including alkalinity, Fe(II), S^2-, NH₄⁺, NO₃^-, NO₂^-, Ca⁺², Mg⁺², Na⁺, K⁺, Cl^-, SO₄^2-, and organic acids such as, acetate, butyrate and propionate were measured in May 2003 and March 2004 during similar hydrologic conditions (“wet” spring seasons). Despite similar hydrologic conditions, differences in biogeochemical processes were observed. At the water-sediment interface sulfate reduction was the dominant process in both 2003 and 2004 but in 2003 higher sulfide was observed (~1000 uM vs. 16.58 uM in 2004) diffusing up into the water column. Lower S^2- in 2004 is likely attributed to reoxidation in the overlying surface water. In 2003 the surface water was covered with dense decaying vegetation and dissolved oxygen content of the surface water was low in contrast to 2004 when sparse vegetative growth and elevated dissolved oxygen was observed. Differences were also present at the sediment-aquifer interface in 2003 and 2004. Most notably, an accumulation of ammonium and organic acids (ex. 8770 uM acetate) was observed in 2003 but not in 2004. The absence of organic acids in 2004 is attributed to the presence of SO₄^2- in the underlying aquifer diffusing upward. The cm-scale sampling provides the ability to resolve sharp changes in biogeochemical indicators and thus quantify the enhanced biogeochemical cycling that occurs at mixing interfaces within the wetland-aquifer system.