Stormwater Quantity and Quality: An Assessment of Runoff Probability Estimation Methods and the Effects of Bioretention Soil Mixtures on Aquatic Toxicity

Abstract

Stormwater runoff occurs when precipitation exceeds the available storage of a watershed. Urbanization changes the natural hydrology of a watershed as cover types, slopes, flow paths, and antecedent moisture conditions (AMC) are altered. Generally, there are two concerns associated with increased stormwater runoff: water quality and water quantity. Stormwater quality and quantity may be controlled through several types of stormwater management practices (SMPs). The goal of low impact development (LID) is to manage stormwater in a way that preserves the pre-development hydrological characteristics of a watershed, including its ability to infiltrate, evaporate, filter, and detain stormwater (Dietz, 2007; Prince George’s County, 1999; USDA NRCS, 1986; USEPA, 2000). The runoff threshold of a catchment is often reported in the literature as a measure used to evaluate the effectiveness of LID techniques and is oftentimes determined using linear regression. However, due to the heteroscedastic nature of precipitation and runoff data, linear regression analyses can result in invalid conclusions, so the use of a binomial regression model was investigated. Precipitation and runoff data collected from five studies were assessed for homoscedasticity and applied to four linear regression models for the evaluation of the linear regression runoff threshold (LRRT) and the associated 95% bootstrapping confidence intervals. Log-transformation corrected the heteroscedasticity of two of the five precipitation and runoff datasets. While mixed-effect linear models accounted for heteroscedasticity, these models often resulted in extremely large confidence intervals. A binomial regression model was created to determine the likelihood of runoff based on precipitation depth. For each catchment, the 10%-90% runoff probability range (p10-p90) is reported to provide the user with a more comprehensive understanding of when a catchment produces runoff than the LRRT. The p10-p90 range reflects the effects that environmental factors may have on runoff generation. For example, impervious catchments with limited interaction with the vegetation and soil produce a narrow p10-p90 range. Conversely, LID practices encourage the interaction of runoff with environmental factors and result in a wider p10-p90 range. Utilization of the binomial regression methodology presented herein is recommended for evaluation of the likelihood of runoff for each precipitation depth. Heavy metal concentrations in stormwater discharges are regulated and monitored as these pollutants can be toxic to aquatic communities (USEPA, 2007). The total concentration of a heavy metal does not represent the concentration of a metal ion available to aquatic organisms. For a heavy metal to be bioavailable for uptake through the gill, it must be in a dissolved form. When evaluating the toxicity of stormwater, the bioavailability and speciation of a heavy metal should be considered. The influent and effluent of four bioretention soil mixtures (BSMs) from ten storms were studied for the purpose of examining speciation shifts of stormwater pollutants and investigating potential changes to stormwater toxicity following filtration through BSMs. Further, this study sought to determine which, if any, of the BSMs were more adept at decreasing pollutant bioavailability. Visual MINTEQ 3.1 was used to predict pollutant speciation and the Windward Environmental, LLC Biotic Ligand Model (BLM) (v 3.41.2.45) was used to determine toxic concentrations of heavy metal species. No noticeable speciation shifts were noted within bioretention cell (BRC) effluent for cadmium, copper, lead, and zinc. However, the speciation of chromium effluent was dependent on the initial pollutant concentrations. A multiple-factor analysis (MFA) indicated that the four BSMs do not differ from one another in reducing the BLM-identified toxic limit (relative toxicity) of stormwater effluent for the BLM-selected aquatic organisms. BRCs are most effective at reducing the toxicity of stormwater when the stormwater contains high pollutant concentrations. At low pollutant concentrations, BRCs may increase the toxicity of the effluent stormwater through export of copper.