The Fate of Nitrates in Biofilters and Isotopic Proxies

nitrates biofiltersIsotopic signatures can be used to optimize bioretention cells in urban and industrial settings.

In a 2022 publication co-authored by Beta Analytic’s Research and Development team, the fate of nitrogenous contamination in stormwater bioretention cells was studied. While nitrogen is naturally present in the environment and is an essential nutrient for life on earth, anthropogenic sources of nitrogen (including fossil fuel emissions, wastewater, and fertilizers) have the potential to be released in toxic quantities (10 mg-NNO3-/L as set by the EPA) into the environment. This can ultimately lead to water impairment and have detrimental effects on plants,  animals, and ultimately human life.

In an effort to reduce excessive nitrogen in urban and industrial settings, runoff is often treated with Low Impact Developments (LIDs) such as green infrastructure including bioretention cells, also referred to as biofilters. Biofilters have shown to reduce levels of nutrients from runoff  from the surface and groundwater systems by capturing and removing suspended loads. Some studies have suggested that biofilters can effectively remove more than 70% of nitrogen (Bratieres et al., 2008). However, the effectiveness may be limited by the denitrification potential, or the ability for microbes to reduce nitrate to N2 gas, of the environment – with more significant denitrification permitting more effective removal by biofilters. Limited settings have been studied and thus, the question remains: Do biofilters perform adequately in more realistic conditions, such as during transient flows and large pulsed storms?

To assess this problem, Feraud et al. (2022) conducted a study of biofilter response to excess nitrogen loads using two full-scale biofilters topped with San Diago sedge (Carex spissa) in California. Transient flow conditions were simulated based on a local hydrograph from Orange County, with a period of time representing a high flow storm (85th percentile storm). The inflow, a mix of local stormwater and sewage, as well as the outflow from the biofilter were collected and analyzed for dissolved ammonia and nitrate concentrations as well as nitrate stable isotopic ratios (δ15N-NO3 and δ18O-NO3). Furthermore, the biofilter was cored and soil microbes assays were conducted to determine enzyme activities and soil denitrifying gene copies to estimate denitrification potentials.

It was first discovered that the outflows were high in nitrate concentration and low in ammonia compared to the inflow. Isotopically, there was a net depletion in relation to inflows and soil eluates during the storm events, which is consistent with decreasing ammonium (NH4+) and increasing nitrate (NO3) concentrations during nitrification or the microbial process where ammonia is oxidized to nitrite and nitrate, an important step in remediation. As the storm simulations continued and the soil was more saturated, the researchers found evidence of denitrification occurring (linear positive relationship between δ15N and δ18O outflows). Later subsequent analyses of bacterial metrics concluded that denitrification was limited for significant, high-frequency storms, resulting in the net export of nitrates (NO3), demonstrating the poor performance in these pulsed storm conditions.

This study showed that nitrate isotopic signatures were revealing the major reactions taking place to reduce nitrogen pollution, suggesting that this proxy may be a valuable tool in assessing the performance and longevity of biofilters. The authors identified a series of steps that could improve the performance of biofilters, including not exceeding drawdown times to avoid ponding and utilizing a stormwater catchment system, all of which can also be assessed using nitrate isotopic signatures.

high-n stormwater

Read the publication directly for more details.

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Bratieres, K., Fletcher, T.D., Deletic, A. and Zinger, Y.A.R.O.N., (2008). Nutrient and sediment removal by stormwater biofilters: A large-scale design optimisation study. Water research, 42(14), pp.3930-3940.

Feraud, M., Ahearn, S.P., Parker, E.A., Avasarala, S., Rugh, M.B., Hung, W.C., Li, D., Van De Werfhorst, L.C., Kefela, T., Hemati, A. and Mehring, A.S., (2022). Stormwater biofilter response to high nitrogen loading under transient flow conditions: ammonium and nitrate fates, and nitrous oxide emissions. Water Research, p.119501.