Influent pollutant concentrations as predictors of effluent pollutant concentrations for mid-atlantic bioretention

摘要

The water quality performance of best management practices (BMPs) has been frequently assessed by the removal efficiency metric. Recent findings show that the removal efficiency metric is flawed because it does not account for background water quality, eco-region differentiation, and background, or irreducible, concentrations. Additionally, the removal efficiency metric inherently assumes a definite association exists between influent and effluent pollutant concentrations. Such a relationship between influent and effluent concentrations has been minimally studied for bioretention, the most common storm-water control measure associated with low-impact development (LID). This study analyzes influent and effluent total nitrogen (TN) and total phosphorous (TP) concentrations from 11 bioretention cells in the mid-Atlantic United States. Pooled data showed only a slight association between influent and effluent TN. Essentially no relationship exists between influent and effluent TP concentration. Both findings indicate that the percent-removal metric is a faulty means of evaluating bioretention performance. Twelve general linear models (GLMs) were created where influent TN and TP were the predictors of respective effluent TN and TP concentrations. Only one GLM was considered to be good, defined as 67-90% of the variation in effluent concentrations being explained by respective influent concentrations (R~2=0.72). In addition, there were two fair models, five poor models, and four very poor models. No very good models were found for TN or TP. Furthermore, as influent nutrient concentration in runoff increases, the removal efficiency increases for TN and TP. Dirtier influent TP concentrations were effectively reduced; conversely, cleaner TP influent concentrations increased, both tending toward a (possibly media-controlled) baseline effluent concentration (0.10 to 0.18 mg/l). TN effluent data also may have been tending toward a common concentration; however, the value was not as discernible.