Illicit Control: Arming Your MS4 Illicit Discharge Detection Program

The EPA defines illicit discharges as “any discharge to a municipal separate storm sewer that is not composed entirely of stormwater.” Operators of Phase II-designated municipal separate stormwater sewer system (MS4s) are required to develop a program to detect and eliminate illicit discharges to protect receiving bodies of water.

Storm water permit holders and Municipal Separate Storm Sewer System (MS4) operators are tasked with balancing the compliance needs of their storm water discharge permit and developing effective program elements with the constraints of limited funding.

Jason Maldonado

Jason Maldonado

Most small MS4 permit holders are required to comply with six Minimum Control Measures (MCM):  public education and outreach, public involvement, illicit discharge detection and elimination, construction, post-construction controls, and pollution prevention and good housekeeping.

To address these measures and the associated Best Management Practices (BMPs) used to implement them, MS4 permit holders are required to develop a Storm Water Management Plan (SWMP).  As part of this plan, permit holders must develop a program to detect illicit discharges, track them to their sources, and eliminate them.  Illicit discharges are typically considered a discharge to an MS4 that is not entirely composed of storm water.  Studies have indicated that dry weather flows from a storm drain system may contribute a larger annual discharge mass for some pollutants than wet weather storm water flows (EPA, 1983; Duke 1997 and Pitt, 2004).

Looking downstream from an outfall

Looking downstream from an outfall

It is critical to develop effective investigative methods since it directly affects the cost of resources allocated to find illicit discharges.  An effective illicit discharge program should: (1) identify areas in the MS4 with the greatest potential for generating illicit discharges; (2) develop standardized criteria and methods for finding these areas; (3) consider ways to detect illicit discharges; and (4) track the discharge to the source and eliminate it.  Physical observations and other olfactory senses (e.g. a keen sense of smell), are essential in detecting illicit discharges.

If a permit holder is discharging to an impaired water body, the urgency to eliminate any illicit discharge is high.  Effective illicit discharge detection and elimination programs should use all available information such as free internet tools and on-line data systems.  Many MS4 permit holders operate systems that discharge pollutants such as bacteria, dissolved oxygen, metals, or nutrients into impaired water bodies.  Traditional methods of screening outfalls within the MS4 are labor intensive and require significant resources.  An MS4 permit holder should consider focusing resources in areas with the highest potential of generating illicit discharges.  Finding these areas may not be as difficult as one would think.  Desktop management is an excellent approach to prioritize field screening efforts since it uses free and readily available tools to assist in planning site selections for illicit discharge detection methods.