A parking lot wash beside a storm drain, step by step
What should a Wisconsin crew do when the lot it is washing slopes to a storm drain?
The job as quoted
Take a hypothetical job. A strip mall in a Wisconsin city wants its lot and sidewalks washed. The lot is asphalt, and a storm inlet sits at the low corner, ten feet from where the crew starts.
Nothing in the quote mentions water. The crew plans to use a mild surfactant and a surface cleaner, and it has a 300-gallon tank on the truck.
This page follows that job. It uses the state permit's rules, not a local ordinance, and it does not name any real property or company.
Wisconsin's low-impact permit may cover mobile commercial power washing, and it applies to discharges to waters of the state. — Wisconsin DNR, Low-Impact Discharge general permit WI-0066575-01-0, retrieved 2026-09-29
Where the water would go
Follow the slope. Water from the wash runs across the asphalt to the inlet. From the inlet it travels through the storm sewer to a creek or lake, without treatment.
That is a discharge through a storm sewer, and the permit counts storm sewers as conveyances to surface water. It carries solids, surfactant and whatever the lot has collected, such as grease from parked cars.
EPA describes the storm drain system as leading directly to local waterways. The same fact is why municipalities run illicit discharge programs, which Wisconsin requires as part of municipal storm sewer permits.
Wisconsin municipalities with storm sewer permits must run illicit discharge detection and elimination programs. — Wisconsin DNR storm water runoff regulation page, retrieved 2026-09-29
The choices the permit gives you
On pavement, the permit gives three ways to handle the water before washing. Settle it in a basin or tank, block the inlet and let solids settle, or send the water to grass, soil or gravel.
Here the crew has a tank, a plug for the inlet and a strip of gravel by the dumpster enclosure. It plugs the inlet, pumps the wash water into the tank as it goes, and lets solids settle before releasing clear water to the gravel.
Whether the crew also needs coverage depends on what still reaches a water of the state. If nothing does, the tank and the gravel keep the job clear of the storm sewer.
Impervious-surface washing must settle washwater, block channeled flow such as storm sewers, or direct it to grass, soil or gravel. — Wisconsin DNR, Low-Impact Discharge general permit WI-0066575-01-0, retrieved 2026-09-29
The soap and the oil spot by the dumpster
The surfactant is the crew's choice. The permit asks for biodegradable soap in the minimum amount, or a low-phosphate product, and it bars foam beyond trace amounts.
The oil stain by the dumpster is a separate question. Oily degreasing beyond one engine or oily item a month at a site is outside the permit, so heavy grease work at a lot like this one needs its own plan.
The crew picks a lighter-duty plan for the stain: absorbent first, a scrape, then a wash with the water held in the tank. It hauls the tank water to a treatment works that accepts it.
The permit requires biodegradable soaps at minimum quantity, or low-phosphate products, so discharges carry no more than trace foam. — Wisconsin DNR, Low-Impact Discharge general permit WI-0066575-01-0, retrieved 2026-09-29
What to write on the work order
End the job with a record. Note the inlet that was plugged, where the water went, the soap product and amount, and the discharge look at the end.
A short record of that kind fits the permit's inspection log and shows the customer that water was handled. The full checklist lists the other items, and the exclusions page covers when a job leaves the permit.
Before the next quote, decide your default destination. Crews that pick one for lots, one for roofs and one for oily work quote faster and stop arguing about the drain at the curb.
The permittee must keep visual inspection logs on file, with discharge observations recorded, available on DNR inspection. — Wisconsin DNR, Low-Impact Discharge general permit WI-0066575-01-0, retrieved 2026-09-29
Questions
Is this scenario a real case?
No. The strip mall, the crew and the tank size are invented to show how the permit's rules apply. Every rule quoted comes from the DNR's low-impact discharge permit or its storm water page.
Would plugging the inlet be enough by itself?
The permit lists blocking channeled flow such as storm sewers as one option, but the water must still go somewhere and solids must settle. A plug with nowhere for the water to go turns the lot into a pond and the wash water into a problem.
Who enforces the storm drain rule at a city lot?
The municipality runs its illicit discharge program under its storm sewer permit, and the DNR administers the wastewater permit. Both can be involved, so check the city ordinance as well as the state permit.