Anaerobic Oxidation of Methane in Tres Rios Constructed Wetlands

Methane (CH4) is a prominent greenhouse gas that contributes to the negative impacts of global warming and climate change, whose emissions have more than doubled since the Industrial Revolution primarily due to anthropogenic sources. The main pathways in which methane moves through the environment are methanogenesis and methanotrophy. Methane is primarily generated by acetoclastic methanogenesis in wetlands while it can be oxidized both aerobically and anaerobically. Wetlands are important methane emission sources at 177 - 284 Tg CH4 year-1. The Tres Rios Wetland (TRW) is a constructed facility to complete nutrient removal of treated municipal wastewater, and has shown low emissions of methane. Whether such low emissions could be achieved through active anaerobic oxidation of methane (AOM) is not known, and the main objective of this work is to evaluate the rates of AOM in TRW. In this study an isotopic method and a mass balance method were utilized to determine the rate of AOM from top sediments found at Tres Rios at various locations and in two sets of sampling. The results showed that evidence of AOM occurred in the sediments of both sampling events conducted. The first sampling set showed evidence of AOM at all locations along a transect, showing that oxidation of methane is indeed occurring in Tres Rios sediments. Evidence from both methodologies suggested that high methanogenesis rates occurred at the outside location closest to the water. The second sampling set showed that the highest rate of AOM occurred at the outlet location, with the lowest rate occurring in the middle location. DNA extractions and PCR images resulted in a poor DNA yield, and inability to extract DNA. It was determined that the isotopic approach was less accurate than the mass balance approach due to unexpected delta CH4 values. It was determined that dilutions of CH4 ppm lead to less accurate isotopic measurements needed to estimate AOM rates using a 13C pulse technique. Literature review suggests that factors including water presence, temperature, redox potential, and plant presence can be influential in the oxidation of methane. This AOM assay can be beneficial in better understanding how methane cycles at Tres Rios, and can provide opportunities for future research in determining which factors influence the oxidation of methane in different locations throughout wetlands.