Construction Stormwater Expansion and Benchmark Exceedance
Ridgeline Contractors is building a 180,000-square-foot warehouse on a 12.8-acre parcel. The original Notice of Intent and SWPPP described 11.2 acres of earth disturbance. During grading, the owner adds a 3.4-acre borrow area, bringing total disturbance to 14.6 acres. The SWPPP was not updated and no additional sediment controls were installed. A 2.1-inch rain in three hours produces a discharge from the sediment basin with turbidity of 620 NTU; the state general permit lists a 280 NTU benchmark. The discharge pH is 9.6; the permit's benchmark range is 6.0–9.0. A concrete washout pit sits 40 feet from a swale and is visibly leaking. The superintendent says, 'Benchmarks aren't limits, so we don't have to report or do anything until the next inspection.' The site is in a state that requires corrective action within 24 hours of a benchmark exceedance and reporting in the next DMR.
1. What permit and SWPPP actions are triggered by increasing disturbance from 11.2 to 14.6 acres?
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Increasing earth disturbance from 11.2 to 14.6 acres is a change to the project described in the NOI and SWPPP. The CGP authorizes discharges for the project as described; disturbance of the additional 3.4-acre borrow area without updating the SWPPP and NOI is an unpermitted change. Before further disturbance, Ridgeline should revise the SWPPP to include the borrow area, update the site map, add erosion and sediment controls (perimeter controls, stabilized construction entrance, sediment basin sizing, and inspection schedule), and amend the NOI with the correct acreage. Many states require the NOI modification before commencing the added disturbance; at minimum, it must be prompt.
The SWPPP must be signed/certified by the operator, and the pollution prevention team should review the changes. The additional area may change the drainage patterns and the design storm for the basin, so the basin capacity and outlet should be re-evaluated. The 14.6-acre total does not by itself trigger an individual permit under the CGP, but the state may have a separate threshold or an antidegradation review. Document the change, the date, and the BMPs installed. If the site discharges to impaired waters, check for additional numeric limits or TMDL requirements.
2. Analyze the turbidity and pH results: are they violations? What corrective action and reporting are required?
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A benchmark is not a numeric effluent limit. The 280 NTU turbidity value and the 6.0–9.0 pH range in the CGP are benchmarks used to evaluate BMP effectiveness; an exceedance is not automatically a permit violation. But it is also not a 'do nothing' result. The state's rule requires corrective action within 24 hours and reporting in the next DMR. The 620 NTU turbidity and pH 9.6 show that the sediment basin and pH controls are not working. The high pH suggests concrete or fresh cement contact, not just soil erosion.
Ridgeline should immediately investigate the source, stop work in the affected drainage area if necessary, repair or enhance the basin (check for short-circuiting, inadequate capacity, failed outlet), and address the alkaline source. Re-inspect and re-sample after corrective action. Report the exceedance and corrective action in the DMR as required. If the state general permit has converted these benchmarks into numeric limits, or if there is a TMDL wasteload allocation, the analysis changes and the discharge may be a violation. Do not rely on dilution or on 'benchmarks aren't limits' to avoid action.
3. What should be done about the concrete washout pit, and how does that differ from the benchmark issue?
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The concrete washout pit is a different and more serious issue. The CGP expressly prohibits discharges of concrete washout water, which is highly alkaline and can raise pH and harm aquatic life. A leaking pit 40 feet from a swale is an immediate threat of prohibited discharge. The operator should stop using the pit, pump out the liquid, repair or replace the liner, and relocate the pit away from drainage channels and the swale. The waste should be allowed to solidify and be disposed of properly; the liquid should not be land-applied or discharged.
This is not merely a benchmark exceedance: it is a prohibited discharge/pollution prevention failure. If there is evidence that washout water reached the swale, the facility should notify the state as required and document the cleanup. Inspect the swale and downstream area for staining, elevated pH, or hardened concrete. Train crews on designated washout locations. In the SWPPP, add a concrete washout BMP with location, capacity, maintenance, and inspection. The benchmark pH exceedance may be partly caused by this pit, so fixing it is also a corrective action for the discharge.