Illicit discharge detection and elimination (IDDE) is part of the Municipal Separate Storm Sewer (MS4) Permit, and annual IDDE outfall inspections are required for all regulated MS4 communities. An illicit discharge is any discharge into a municipal storm sewer system that is not composed entirely of storm water or uncontaminated groundwater. These discharges are considered “illicit” because the storm sewer system is not designed to accept, process, or discharge waste or contaminants. IDDE inspections are essential for all communities to ensure that stormwater discharges do not pollute our waters.
What do MS4 permittees need to do?
Classify outfalls (major, minor, and/or priority outfall)
Perform annual dry weather field screening at outfalls
Only perform after 72 hours of no rain
Keep track of specific information (visual observation and field analysis)
Need narrative descriptions and testing levels of water present at flowing outfalls
Guidance is provided by WDNR in Program Guidance #3800-2012-01 and is outlined in each MS4 permit.
Inspection frequency
Major outfalls - once per 5-year permit cycle
Priority outfalls – once per year
Any other outfalls in the community that are known to have illicit discharges – once per year
After initial inspections, perform follow-up testing based on the testing results of the outfall. The goal is to identify the source of the illicit discharge (and eliminate it).
WDNR has recommended action limits for the different tests performed (ammonia, detergents, pH, etc.)
Inspection frequency is outlined in WNDR Program Guidance and communities MS4 permit (use whichever is stricter). Make sure to file all annual IDDE inspections once completed.
Inspection Timing
Coordinate IDDE inspections during the dry season, June through September is the best time to collect accurate inspection data. This information is required to be submitted in March as part of annual reporting.
R/M Can Help!
Contact R/M to help perform your IDDE inspections! We can help classify outfalls, write your community’s IDDE program, and perform the inspections!
about the author
abby h. warwick
project engineer
Abby has worked with multiple communities on MS4 compliance. She has developed stormwater management plans and stormwater device modeling. Abby also has experience in field work with CRT erosion control and green infrastructure inspections.
