Proposing a New Method of Stormwater BMP Assessment and Evaluating the Toxicity of Forebay Sediments

Abstract

Stormwater experts agree that the currently used Best Management Practice (BMP) percent removal methodology metric has many flaws, because it does not account for background water quality, eco-region differentiation, or irreducible concentrations. Some have suggested utilizing a BMP effluent concentration metric. Chapter 1 establishes an effluent target concentration for BMPs that relates to the health of macro-invertebrates in receiving water. 193 ambient water quality monitoring stations in North Carolina were paired with benthic macro-invertebrate health ratings collected in very close proximity. Water quality for the sites ranged from Excellent to Poor and was divided into three distinct eco-regions: Mountain, Piedmont, and Coastal. Statistically significant relationships were found in one or more eco-regions for DO, Fecal Coliform, NH3, NO2-3 - N, TKN, TN, and TP. BMPs can then be selected and designed to meet these target effluent concentrations. Based upon this research, a development, and therefore set of BMPs, in Piedmont North Carolina could be required to release TN and TP effluent concentrations of 0.99 mg/L and 0.11 mg/L, respectively. These concentrations are both associated with "Good" Benthos health. The new method was most effective in the Piedmont eco-region, however with more data collection, the Mountain and Coastal eco-regions may also benefit. The removal efficiency metric inherently assumes a definite association between influent and effluent pollutant concentrations. Such a relationship has been minimally studied for bioretention, the most common stormwater control measure associated with Low Impact Development (LID). Chapter 2 analyzes influent and effluent TN and 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 concentrations. Both findings indicate that the percent removal metric is probably a faulty means of evaluating bioretention performance. 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 an irreducible effluent concentration (0.10 to 0.18 mg/l). TN data also may have been tending toward a common concentration; however, the value was not as discernible. After developing a new set of standards for water quality (Chapter 1) and verifying the need for a new method of BMP assessment (Chapter 2), the feasibility of these innovative standards was examined (Chapter 3). Which BMPs, if any, can meet effluent standards set by the WQABI metric established in Chapter 1? A compilation of Mid-Atlantic effluent total nitrogen (TN) and effluent total phosphorus (TP) concentrations were compiled from existing, published studies conducted on the nine most commonly installed BMPs, including: bioretention cells, dry detention, green roofs, level spreaders-vegetated filter strips (LS-VFS), permeable pavement, sand-filter, vegetated swales, wetlands, and wet ponds. Of all BMPs examined in this study, none met “E†WQABI standards for TN (≤ 0.69 mg/L), 4 BMPs (bioretention, permeable pavement, wetlands and wet ponds) met “G†WQABI standards for TN (≤ 0.99 mg/L), and 6 BMPs (bioretention, dry detention basins, LS-VFS, permeable pavement, wetlands and wet ponds) met “GF†WQABI standards for TN (≤ 1.17 mg/L). Only green roofs and grassed swales were unable to achieve “F†TN water quality standards (≤ 2.16 mg/L). Of all BMPs selected in this study, only permeable pavement met “E†WQABI standards for TP (≤ 0.06 mg/L) and 4 BMPs (bioretention, permeable pavement, wetlands and wet ponds) met “G†and “GF†WQABI standards for TP (≤ 0.11 mg/L and ≤ 0.13 mg/L, respectively; Table 5). A final, distinct study (Chapter 4) evaluated the toxicity of forebay sediments and suggested an appropriate method to dispose of excavated sediment. Forebays, small settling basins placed at the inlet of Stormwater Best Management Practices (BMPs), encourage sedimentation with the intention of pollutant consolidation and capture. To test for the potential toxicity of forebay spoils, 30 stormwater wetland and wetpond forebays, of varying age, size, and upstream landuse were sampled across North Carolina and analyzed for 7 metals: cadmium, chromium, copper, iron, lead, nickel, and zinc. The relative toxicity of all sampled sediment metal concentrations was evaluated using existing aquatic health sediment guidelines and U.S. Environmental Protection Agency (EPA) standards for the land application of biosolids (40 CFR503). Twenty-two of 30 forebays exceeded sediment guidelines for aquatic health with respect to one or more metals, emphasizing the need for routine forebay sediment removal. All samples were less than 40 CFR 503 limits with factors of safety ranging from 5 to 13, indicating that land application of forebay sediment is an acceptable means of disposal.