Limitations on Nitrogen Removal by Treatment Wetlands Under Maritime Climatic Conditions
Thesis (M.S.) Division of Science and Environmental Policy
The use of fertilizer has in part elevated nitrogen concentrations within surface and groundwater. High nitrate concentration can negatively impact human health as well as lead to algal blooms. Treatment wetlands can reduce the negative impacts of nitrate runoff, but the drivers of nitrate removal within maritime climates maybe limited by the cool winter temperatures despite a year-around growing season. We sought to understand the key limitations on nitrate removal within an experimental treatment wetland located within in California’s Central Coast region. We collected water samples and analyzed them for nitrate, ammonium, and dissolved organic carbon, temperature, pH, salinity and dissolved oxygen. We modeled outlet nitrate concentrations using inlet nitrate, temperature and dissolved organic carbon as a priori predictor variables, using a Tobit distribution to account for the positive zero-truncated distribution of water quality data. We compared models using Akaike's Information Criterion (AIC). The best-supported model included all a priori predictors. There was decisive evidence for a dependence of nitrate removal on high inlet nitrate concentrations and high temperatures, and some evidence for a dependence on high dissolved carbon concentrations. Nitrate removal was limited in winter, despite source waters containing elevated nitrate concentrations year-round. To better optimize nitrate removal, wetland design should include ways to increase water temperature and available carbon.