Nitrate removal in aerobic sediments from freshwater systems

NSF Fellow: Paola Gomez Flores, Bates College

Mentor: Dr. Francisco Cubas-Suazo

Denitrification is a process that transforms nitrate into nitrogen gas. Denitrification is known to be a strictly anaerobic process and is controlled by seasonally varying factors such as temperature and oxygen concentrations. Our goal for this study was to assess if denitrification could also occur in the presence of oxygen. In some cases, denitrification has been observed to occur simultaneously with nitrification suggesting that denitrification might happen in the presence of oxygen. In natural systems, such as lakes and reservoirs, oxygen levels decrease during the months before summer and increase months after summer. Even though there is oxygen present in the water during these months, denitrification may still occur. In this study, we attempted to identify at what levels of dissolved oxygen (DO) in the water will denitrification start, and what other circumstances will it allow for it to continue. In order to do this, four different reactors were set up for several weeks to simulate these processes in freshwater systems. Each reactor had a target DO that would help show at what level would denitrification start to occur. Measurements of DO, pH, and oxygen-reduction potential (ORP) levels were recorded throughout the duration of the experiment. Water samples were collected to measure for nutrients levels and profiling of the sediments was conducted. The data gathered showed that nitrate removal occurred when DO was lower than 4 mg/L, suggesting that denitrification may occur in the presence of oxygen under certain conditions. Conditions that would favor nitrate removal in the presence of oxygen include high oxygen depletion rates, which can be triggered by high organic matter concentrations, low DO diffusion to the sediments, and high nitrate concentrations.