Driving forces of effluent nutrient variability in field scale bioretention

Nutrient exports from urbanized watersheds have been identified as a major contributor to surface water degradation worldwide. As efforts to implement stormwater treatment controls, such as bioretention, become increasingly common, understanding how the functionality of these systems varies based on environmental conditions is critical. Quantifying this variability and its causes will aid in developing more robust models and watershed restoration plans which incorporate the uncertainty of bioretention function. This study investigated effluent nutrient concentrations from ten bioretention areas in North Carolina, USA. Nitrogen species were found to have higher variability than phosphorus species, with changes in effluent concentrations being significantly influenced by environmental conditions. In particular, effluent concentrations of total nitrogen (TN) and nitrate (NO3-N) were strongly correlated to antecedent rainfall depth and temperature. As antecedent rainfall depth increased, NO3-N and TN concentrations decreased. These trends have been noted in other laboratory-based studies and are logical based on the influence of dry conditions on microbial communities. Increases in ambient temperature were shown to increase effluent TN and NO3-N concentrations in conventionally drained systems. However, a negative correlation was found between temperature and effluent NO3-N concentrations in systems with an internal water storage (IWS) layer, suggesting increased denitrifying microbe activity. These results show that variability in effluent nutrient concentrations can be explained by environmental conditions, and that the effects of these conditions may differ based on system design.