MS4 communities in the Bay watershed have permit requirements to screen their stormwater outfalls to detect illicit discharges of sewage and other pollutants, and take actions to eliminate them.

High nutrient levels have been detected in dry weather flows in a number of urban streams in the Chesapeake Bay. Subsequent outfall screening using nutrient based indicators suggest that the much of nutrients are derived from illicit discharges of sewage. Part of the reason is the interaction of flows and overflows from aging sanitary sewers and storm sewers which often run close together.

Recent studies indicate that these discharges may account for a significant fraction of the annual nutrient load of some urban streams (CWP, 2011). This suggests that an aggressive local IDDE program could achieve significant nutrient reductions. IDDE efforts are already required under municipal MS4 stormwater permits.

An Expert Panel was convened in July 2012 to review the available science on the nutrient load generated by illicit discharges in storm drain systems and to evaluate an approach for crediting such discharges that are discovered and eliminated during routine (yet advanced) outfall screening procedures.

On November 10, 2014 the Recommendations were approved by the final review committee at the Chesapeake Bay Program, the Water Quality GIT. This means this credit is now live and available to communities working to meet their Bay TMDL load allocations. Congratulations to our panelists on a job well done! 

To learn more, visit our IDDE webpage here.