Biogeochemical Processes and Seasonal Effects in Flow-Through Mesocosm Reactors Simulating Constructed Wetlands
Abinash Agrawal (Advisor)
Master of Science (MS)
Due to their distinctive environments, constructed wetlands are utilized to remediate groundwater that has been contaminated with chlorinated ethenes (i.e. perchloroethene) and other such contaminants. This research focuses on vertical flow mesocosm reactors housed in a greenhouse at Wright State University, which simulate a constructed wetland on Wright-Patterson Air Force Base. These reactors were studied to investigate the effects of vegetation and seasonal changes on the redox parameters present. Water samples were collected from six different reactors from late September/early October 2006 until August 2007. The reactors were sampled three days in a row and this sampling was done once a month. The triplicate sampling was done to promote accuracy in the data by averaging the three days together. Colorimetric analysis of ammonia and iron found anoxic/oxic redox zones apparent in the reactor. These zones were soil and plant specific. In control reactors, where no plants were present, anoxic/oxic boundaries were found in the lower portions. The same was found in reactors planted with the Carex comosa. An anoxic/oxic boundary was found at the bottom of the reactor as well as the top of the reactor where plant roots were present. Those reactors planted with Scirpus atrovirens also showed similar boundaries. The rhizosphere present in these reactors introduces oxygen and bacteria that is not available in control reactors so intensified Fe(III) is able to occur. The profiles of ammonia and nitrite coincide well together. It was found that where ammonia concentrations were high nitrite was low and as ammonia values began to decrease nitrite values began to increase. This suggests ammonia oxidation is causing the nitrification that takes place near the rhizosphere. There are lower ammonia values found in SA reactors as compared to CC reactors, which indicates that the SA plant roots extend further into the reactor than CC roots. More oxygen is being introduced which inhibits ammonia formation; it is immediately oxidized to nitrite. This coupled with the iron profiles suggests iron and nitrogen cycling is occurring. Colorimetric analysis was also done to determine dissolved oxygen concentrations, however the values found were too large so the analysis was discontinued. Major ion analysis was also done on the water samples collected. From the results found it is apparent that plant roots affect potassium concentrations, as it is a nutrient the roots uptake. Also, the plants create a favorable environment for calcium carbonate and dolomite dissolution because the pH slightly decreases near the root zone. This causes the release of calcium and magnesium into the system increasing their concentrations. The other ions, conductivity, and temperature values were also influenced by the presence of plants in the system. Aside from the Fe(II) profiles there was not much seasonal effect present in the reactors. However, with the results found there is a better comprehension of the environments present in these mesocosms. It was found that plant species have major effects on the redox reactions that occur in wetland soil. The results imply that the ability of a wetland to remediate contaminants is largely based on the species of plants present This knowledge should be applied to the organic analysis that was executed simultaneously with this research so a narrative can be told about how the reactors work. This can be applied to the subsurface of the wetlands to establish how they can be improved upon for more efficient contaminant remediation.
Department or Program
Department of Earth and Environmental Sciences
Year Degree Awarded
Copyright 2007, all rights reserved. This open access ETD is published by Wright State University and OhioLINK.