CWP, through an on-call engineering agreement for watershed planning services, assisted the Baltimore County Department of Environmental Protection and Sustainability with developing an outfall screening plan for the County’s more than 12,000 stormwater outfalls. Baltimore County’s stormwater permit requires that the County develop a plan to prioritize outfall field screening efforts in areas with the greatest potential for illicit discharges—defined as discharges that are not composed entirely of stormwater—which are prohibited by the County’s regulations.
CWP conducted a series of geospatial analyses that began with an Outfall Inventory to ensure the data associated with stormwater outfalls in the County’s geodatabase were as up to date as possible. Next, CWP conducted a Risk Factor Analysis that assigned a score indicating the potential for illicit discharges to each of the County’s subwatersheds. Ten risk factors were selected that represent increased potential for illicit discharges, based on CWP’s 2004 Illicit Discharge and Detection Elimination Guidance Manual as well as more recent research. Individual risk factor scores and weights were used to calculate an average subwatershed Illicit Discharge Potential (IDP) score. Ranges of these scores correspond to priority levels that indicate the risk for illicit discharges within each subwatershed.
The most highly urbanized areas of Baltimore County were found to have an increased potential for illicit discharges. The subwatersheds with the highest risk were located in urbanized areas adjacent to Baltimore City.
Using the results of the Risk Factor Analysis as well as the County’s previous outfall screening results, CWP developed a Prioritization Scheme that assigned a screening priority to each individual outfall. Lastly, CWP created an Outfall Screening Scenario Tool for evaluating the schedule and staffing needs for various outfall screening scenarios. This Excel-based tool can be continuously updated based on field screenings and investigations.
The final Outfall Screening Plan describes the analyses and outlines how Baltimore County can use the products to: 1) screen at least 150 outfalls per year, 2) ensure screening efforts reflect priorities based on outfall-specific pollution potential, and 3) adaptively manage the prioritization of outfalls based on completed screenings/investigations or changed conditions.
To learn more about this project, contact Amanda Pollack (ahp at cwp dot org).
