The phone call that wakes a stormwater coordinator at 6:15 a.m. is almost always the same. A resident saw something coming out of an outfall that shouldn’t be there. The coordinator now has minutes to figure out who responds, hours to get someone on site, and weeks to either find the source or document why it could not be found.
Illicit Discharge Detection and Elimination is the workflow most likely to expose a program’s documentation gaps. Inspections happen on a schedule. Annual reports follow a calendar. IDDE happens on someone else’s clock. This post is a working playbook for what good IDDE response looks like, where it tends to fall apart, and what auditors actually check when they pull an incident off the log.
What MCM 3 actually requires
Phase II MS4 permits give MCM 3 (Illicit Discharge Detection and Elimination) a few non-negotiable elements:
- A storm sewer system map with outfalls and major drainage areas identified
- An ordinance that prohibits non-stormwater discharges into the storm system
- A program for detecting illicit discharges, including dry-weather field screening
- A plan for responding to discharges when they are reported or discovered
- Public reporting mechanisms (a phone number, a web form, or both)
- Staff training on identification and response
The ordinance and the map are static. The detection, response, and training pieces are where audit findings live. State agencies want to see that the program ran the way the SWMP says it runs, with a trail of records that proves it.
The six stages of an IDDE investigation
Most IDDE incidents move through the same arc. The names vary by agency, but the structure does not.
Stage 1: Complaint intake
The intake record sets the entire investigation. If it is incomplete, every later record is harder to defend.
A clean intake captures:
- Date and time of the complaint
- Reporter contact information (or anonymous, with a note that the reporter declined to identify)
- Location: street address, nearest intersection, GPS if available, and the receiving outfall if known
- What the reporter saw: color, odor, floatables, dead fish, sheen, foam, sediment plume
- Whether the discharge is currently active or had stopped before the reporter called
- Initial classification (suspected illicit, possible permitted, unknown)
Intake is also where the response clock starts. Most permits include a response window, often 7 days from complaint to first action. The clock running silently in the background of an unfilled intake form is how programs end up with late-response findings.
Stage 2: Dry-weather field screening
The first physical response is usually a screening visit. Whether the discharge is still flowing or has stopped, the inspector documents what they observe at the outfall and at any obvious upstream contributors.
Standard screening data:
- Flow conditions (no flow, intermittent, flowing)
- Visual indicators (color, sheen, foam, floatables, deposits)
- Odor (sewage, petroleum, chlorine, none)
- Photographs of the outfall and any visible discharge
- Field measurements where available (pH, temperature, conductivity)
- Indicator panel results if the program runs sampling
The screening record needs photos and a narrative. A future auditor will not have been at the site. The record is what they read.
Stage 3: Source tracing
If the screening confirms a non-stormwater discharge, source tracing begins. This is the longest stage and the one most likely to span weeks. The investigation moves upstream from the outfall through the storm sewer network, looking at manholes, catch basins, and connections.
Each step in the trace should be its own record, all linked to the original incident. Auditors do not want a single summary entry that says “traced upstream over three weeks.” They want to see the dates of each manhole opened, the inspector who did the work, what was observed at each location, and what decision triggered the next step upstream.
Common evidence captured during source tracing:
- Photographs of each manhole, catch basin, or pipe segment inspected
- Field observations of flow, color, odor, and indicator results at each location
- Suspected contributors (industrial connections, illicit hookups, failed septic, dump events)
- Sampling results if collected
- Communication with property owners, utility staff, or other agencies
Stage 4: Source identification and response
When the source is identified, the investigation transitions from documentation into action. Depending on the source, response can include:
- A direct conversation with the responsible party and a request for voluntary cleanup
- A formal notice of violation
- A compliance schedule for corrective work
- Coordination with other agencies (sanitary sewer utility for cross-connections, environmental health for septic, hazmat for spills)
- Cleanup oversight to confirm the discharge has stopped
Each of these actions belongs on the same incident record. If the response escalates to enforcement, the enforcement record links back to the source inspection that triggered it.
Stage 5: Closure
An incident closes when the discharge has stopped, the source has been corrected (or determined to be uncorrectable for documented reasons), and the program has confirmed both. Closure is not just a status flag. It is a record that includes:
- Date of confirmed cessation
- Description of the corrective action taken
- Evidence that the corrective action worked (follow-up screening, sampling, photos)
- Final classification of the discharge type
- Time elapsed from complaint to closure
If the source could not be identified after reasonable effort, the closure record needs to document what was tried, what was ruled out, and why further investigation was not pursued. “Source unknown” without that documentation reads as an unfinished investigation in an audit.
Stage 6: The outfall timeline
The last stage is not really a stage. It is a view. Every incident, screening, and inspection that has ever touched a particular outfall belongs on a single timeline tied to that outfall. When a new complaint comes in for an outfall that has had three previous incidents in the last two years, the responding inspector should see that immediately. Patterns at an outfall are often the fastest path to identifying a chronic source.
What auditors actually check
State agency reviewers asking about IDDE response tend to look at the same three things:
Response timelines. Permits typically require first action within a defined window. The audit pull is usually “show me every incident from the last reporting year, with the timestamp from intake to first response.” Programs that respond promptly but document poorly fail this check. Programs that respond late and then write a summary fail more obviously.
Investigation continuity. Auditors look for incidents that started cleanly and trail off. A complete intake, a screening visit, then nothing. The investigation might have continued in someone’s email or in a phone call, but if it is not on the record, it did not happen for audit purposes.
Closure documentation. Open incidents from prior years are a common finding. So are closures with no corrective-action evidence. The closure record needs to show that the program confirmed the discharge stopped, not just that the file got marked closed.
Where the workflow tends to break
Most IDDE documentation problems are not about doing the work badly. They are about where the work lives.
Notes in email. A coordinator emails the city engineer about a suspected cross-connection. The engineer replies with what they found. That exchange is the source-tracing record, and it lives in two inboxes that nobody else can search.
Photos on personal phones. Field photos sit in inspector camera rolls until someone remembers to upload them. Some never make it.
Manual incident logs. A spreadsheet with one row per incident loses the chronology. The cells expand or do not. The narrative gets compressed into a single text field that loses every step of the trace.
Disconnected enforcement. The investigation lives in one system, the notice of violation in another. The link between them is a coordinator’s memory.
Multi-agency handoffs. Cross-connection investigations often involve the sanitary sewer utility. The handoff happens by phone. The handoff record does not exist.
How software helps
A purpose-built IDDE workflow keeps the incident on a single timeline, with every screening visit, photo, sample result, and communication attached to the original record. Photo and GPS evidence comes off the inspector’s phone directly to the incident. Source-tracing steps stay linked. If the investigation escalates to enforcement, the notice of violation links back. The outfall timeline is automatic.
NPDESTracker structures IDDE this way. Complaint intake captures the call, screening visits attach photos and field readings, source tracing keeps each step on the same thread, and enforcement actions stay linked from initiation through closure. The full IDDE workflow is on the platform page, with the inspector workflow detailed on the inspections page and the GIS-aware investigation context on the GIS workspace page.
The tool does not run the investigation. The coordinator and the field staff do. What the tool changes is whether the chronology survives long enough to be useful: at the next incident on the same outfall, in the next annual report, and during the next state agency review.