Dry-weather flows discharging from storm drainage systems can contribute significant pollutant loadings to receiving waters. If these loadings are ignored (by only considering wet-weather stormwater runoff, for example), little improvement in receiving water conditions may occur. Illicit dry-weather flows originate from many sources. The most common sources typically include sanitary wastewater or industrial and commercial pollutant discharges and failing septic tank systems.
Studies have shown that dry weather flows from the storm drain system may contribute a larger annual discharge mass for some pollutants than wet weather storm water flows (EPA, 1983; Duke, 1997; and Lilly et al. 2012). Detecting and eliminating these illicit discharges involves complex detective work, which makes it hard to establish a rigid prescription to “hunt down” and correct all illicit connections. Frequently, there is no single approach to take, but rather a variety of ways to get from detection to elimination. Local knowledge and available resources can play significant roles in determining which path to take. At the very least, communities need to systematically understand and characterize their stream, conveyance, and storm sewer infrastructure systems. When illicit discharges are identified, they need to be removed. In fact, well-coordinated illicit discharge detection and elimination (IDDE) programs can benefit from and contribute to other community-wide water resources-based programs, such as public education, storm water management, stream restoration, and pollution prevention.
Municipal separate storm sewer system (MS4) communities regulated under Phase I and II of the National Pollutant Discharge Elimination System (NPDES) Program are required to develop IDDE programs and undertake steps to correct illicit discharges.
The Center and the University of Alabama produced a national guidance manual in 2004 that presents technical guidance for NPDES Phase II communities on how to build a local IDDE program. The 15-chapter manual contains information on establishing adequate legal authority and local ordinances; developing accurate mapping resources; conducting outfall reconnaissance investigation; using indicator monitoring to find and isolate discharges, techniques to prevent illicit discharges from generating sites, and methods to scope and cost a local IDDE program. Illicit Discharge Detection and Elimination: A Guidance Manual for Program Development and Technical Assessments can be downloaded for free from the Online Watershed Library.
Since 2004, the Center has assisted numerous communities with implementing the guidance in the manual, and has continued to refine and improve IDDE protocols based on this experience. The Illicit Discharge Detection and Tracking Guide, developed in 2011, reflects these updates. This mini-guide contains procedures for detecting and tracking illicit discharges and can be downloaded for free from the Online Watershed Library.
Duke. L. R. 1997. Evaluation of Non-Storm Water Discharges to California Storm Drains and Potential Policies for Effective Prohibition. California Regional Water Quality Control Board. Los Angeles, CA.
Environmental Protection Agency (EPA). 1983. Results of the nationwide urban runoff program. PB 84-185552. Washington, DC: US Environmental Protection Agency, Water Planning Division.
Lilly, L. A., Stack, B. P., and D. S. Caraco. 2012. Pollution Loading from Illicit Sewage Discharges in Two Mid-Atlantic Subwatersheds and Implications for Nutrient and Bacterial Total Maximum Daily Loads. Watershed Science Bulletin 3(1): 7-17.