Evaluation of Sulfate Reduction Using a Series of Modified Push-Pull Tests in an Aquifer-Wetland System Impacted by Landfill Leachate, Norman, OK

A series of modified in situ push-pull tests were used to quantify rates of SO42- reduction at the mixing interface between wetland porewater and groundwater from the underlying anaerobic aquifer at the Norman Landfill research site, Norman, OK. At small (cm) scale mixing interfaces, steep geochemical gradients and enhanced biogeochemical cycling have been observed. Quantifying the role of these interfaces on system level biogeochemical cycling, including the rates of sulfate reduction is poorly studied due to the small transient nature of mixing interfaces. In this study, the kinetic controls on sulfate reduction were evaluated using a series of modified ``mini'' push-pull tests to introduce electron-acceptor (SO42-) limited wetland porewater to anaerobic groundwater containing abundant electron acceptor (SO42-), thus simulating the aquifer-wetland interface. A relatively well-sorted, fine-grained sand lens within the reducing wetland sediments was targeted using small-diameter (2.54 cm, O.D.) ``drive-point'' wells with a discrete, internally packed 4.5 cm well screen. A series of push-pull tests were then performed using these wells by injecting the sulfate-rich aquifer water into the targeted zone. Results indicated that 1) SO42- reduction was the dominant terminal electron accepting process (TEAP) initiated by the mixing event 2) in all tests in which sulfate reduction was observed, a lag period was present until mixing of water began and 3) tests indicate that the lag period is neither a function of electron donor (acetate) concentration nor directly related to time in situ. These results indicate that, data retrieved from push-pull tests should be interpreted with caution to ensure that the ``rate'' is actually a function of time and not another parameter such as degree of mixing.