Dry weather flows, also known as illicit discharges, may contribute more than wet weather stormwater flows to the annual discharge mass for some pollutants (Lilly et al 2012; Duke 1997; Pitt and McLean 1986; US Environmental Protection Agency [USEPA] 1983b). Dry weather flows can stem from car wash discharges, water main breaks, and sewage discharges, among other sources. Brown et al (2004) provides a thorough overview of illicit discharges and IDDE programs and program development.
The cumulative effect of illicit sewage discharges into a storm drain system can have a significant water quality impact by introducing high nutrient and bacterial loads with toxic and pathogenic effects. They are often missed by ineffective and/or inefficiently implemented municipal separate storm sewer system (MS4) illicit discharge detection and elimination (IDDE) programs. Sewage discharges can be relatively small, but persistent, problems that are often missed under existing programs. Incentives for implementing effective IDDE programs have been lacking, but this has recently changed. The USEPA Chesapeake Bay Program approved a "crediting" system through the total maximum daily load (TMDL) process for local governments that fix illicit discharges. More information can be found here.
Lilly et al (2012) quantified dry weather pollutant loading from illicit sewage discharges in two subwatersheds in the Baltimore-Washington area, illustrating the pervasiveness and cumulative impact of dry weather illicit sewage discharges. An investigation in Western Run in the City of Baltimore showed that the elimination of illicit discharges could potentially meet 21% of the phosphorus, 43% of the nitrogen, and 46% of the bacteria TMDL goals. For Sligo Creek in Montgomery County, a similar analysis showed that the elimination of illicit discharges could potentially meet 6% of the phosphrus, 17% of the nitrogen, and 21% of the bacteria TMDL goals. IDDE is an important best management practice (BMP) for achieving goals set forth in TMDLs for impaired waters. It is necessary to increase the priority of sewage discharge elimination within the overall strategy for watershed restoration if we are to achieve our collective goals.
The cost of fixing illicit discharges is much less expensive per pound of nutrient reduced than other methods that treat the same load at the end of the pipe. For example, removing the annual nitrogen load associated with potential illicit discharges found in Sligo Creek would conceivably cost 18 times more if done via a practice such as a dry swale (Lilly et al, 2012). IDDE can be costly in terms of staff time to track down problems, but the water quality benefit that can be achieved outweighs the upfront cost. A holistic approach that effectively integrates both structural and nonstructural practices will be needed to address the many water quality impairments in the United States.
Pollution Loading from Illicit Sewage Discharges