Construction Stormwater Benchmark, TMDL, and Dewatering
A 45-acre residential subdivision is under construction over 8 months under the Construction General Permit. The SWPPP calls for silt fence, a sediment basin, and a stabilized construction entrance. After a 2.5-inch storm, the outfall turbidity is 420 NTU, above the 280 NTU benchmark. Inspection finds 30 acres of exposed soil, the sediment basin about 50% full of accumulated sediment, no stabilized entrance, and a concrete washout leaking. Foundation dewatering is being pumped at 150 gpm directly to a stream that is on the state’s 303(d) list for turbidity and has a TMDL with a wasteload allocation for construction stormwater. The operator has not documented any corrective actions. The site discharges to the stream through a single outfall. The project schedule has 3 months remaining before final stabilization.
1. Identify the CGP violations and the corrective actions required, including timing. Which finding is the most immediate threat to the receiving stream?
Read the worked answer
The clear CGP problems are failure to maintain BMPs and failure to document corrective actions. Specific findings include the sediment basin at about 50% of capacity, the missing stabilized construction entrance, the leaking concrete washout, and 30 acres of exposed soil without adequate stabilization. Under the Construction General Permit, a sediment basin that has lost 50% of its design capacity is a corrective-action trigger; corrective action must be completed within 24 hours of discovery or as soon as practicable, and it must be documented. Exposed areas should be stabilized within the permit’s stabilization timeframe, generally 14 days after construction activity stops in that area, or sooner if rain is forecast.
Required actions include cleaning or restoring the sediment basin, installing and maintaining the stabilized entrance, repairing the concrete washout so it does not discharge, stabilizing exposed areas, and inspecting all BMPs after the storm. The most immediate threat is the direct, untreated dewatering discharge, because it bypasses the sediment controls and flows to an impaired stream; the leaking washout is also a direct pollutant source and should be stopped immediately.
2. The outfall turbidity exceeded the benchmark. Is that a permit violation? What must the operator do, especially because the stream has a turbidity TMDL?
Read the worked answer
A benchmark exceedance is not automatically a permit violation. The turbidity benchmark is an indicator that the BMPs may not be effective. However, the operator must review the SWPPP and BMPs, revise them if necessary, implement corrective actions, and document the exceedance and response. If the permit or state requires benchmark reporting, that reporting must be completed.
Because the stream is 303(d)-listed for turbidity and has a TMDL with a construction-stormwater wasteload allocation, the operator must check the CGP eligibility and state requirements for discharges to impaired waters. The TMDL may impose additional BMPs, a more stringent numeric limit, or other conditions. The 420 NTU result should trigger an upgraded treatment train, such as additional sediment controls, polymer-assisted clarification only if authorized, and more frequent inspections. The operator should also evaluate whether the discharge causes or contributes to a water quality standards exceedance. Documenting the benchmark exceedance without action would not satisfy the CGP’s requirement to maintain effective BMPs.
3. Is the foundation dewatering discharge authorized by the CGP? What controls or permits are needed before continuing?
Read the worked answer
The CGP primarily authorizes stormwater discharges from construction activity and only certain limited non-stormwater discharges. Sediment-laden foundation dewatering pumped directly to the stream is not authorized merely by the CGP. It is a non-stormwater discharge that may require a separate NPDES permit or state authorization, and it must not violate water quality standards or the TMDL wasteload allocation.
The operator should stop the direct discharge, contain and treat the water, and obtain the required permit coverage before resuming. Treatment could include a settling basin, weir tank, filtration, or other controls sized for 150 gpm, with monitoring for turbidity, pH, and other parameters required by the permit. Because the stream is turbidity-impaired, the discharge must be managed conservatively and coordinated with the state. If the dewatering water is uncontaminated groundwater and the state allows it under a general permit, that determination must be documented; the current direct, untreated discharge is not protective.