DEGRADATION OF GROUNDWATER CONTAMINATED WITH CHLORINATED ETHENES IN A CONSTRUCTED WETLAND: SEASONAL EFFECT ON DEGRADATION PROCESSES

Wetlands have been utilized for the treatment of inorganic pollutants through anaerobic and aerobic processes under environmental conditions. An upward-flowing constructed wetland was built in 1999 as a research facility at Wright-Patterson AFB near Dayton Ohio. The project is aimed at developing an understanding of the microbial degradation processes in a constructed wetland that shows a strong evidence of biodegradation of chlorinated solvents such as perchloroethene (PCE), trichloroethene (TCE), and their daughter products. Samples were analyzed for major ions and volatile organic compounds. Main efforts include the geochemical characterization of the redox conditions within the wetland where the chlorinated ethenes appear to be degrading. The previous research at the site has indicated significant influence of changing seasons and temperature fluctuations on microbial processes. The ongoing research is aimed at characterizing the influence of seasonal variation on the reductive chlorination of PCE to its daughter products as well as biogeochemical processes at the constructed wetland site.

To validate field observations, twelve mesocosms were developed in a greenhouse environment similar to the conditions in a treatment wetlands located at Wright Patterson Air Force Base in Dayton, Ohio. These mesocosms (flow-through reactors) were designed with the wetland soil, three species of wetland vegetation, and dissolved PCE in a 50/50 mix of tap water and distilled water. The microbial and vegetative processes have been under investigation to assess PCE biodegradation in the mesocosms. These assessments are based on the analysis for volatiles such as methane, PCE, and its daughter products using gas chromatograph (GC). Other analyses include dissolved oxygen, ferrous iron, ammonia, and major ions using ion chromatography (IC). Preliminary results in the last 10 months indicate that there is indeed a significant degradation of PCE and evidence of daughter products like trichloroethene (TCE), dichloroethene (DCE), and vinyl chloride (VC) in the mesocosms. These results are indicative of the microbial and vegetation-driven processes occurring in the mesocosms that appear to play a role in the biogeochemical processes affecting the degradation of chlorinated compounds.