Trench Dewatering and Concrete Washout on a Linear Utility Project
A 6.5-acre water main replacement project disturbs 3.2 acres at any one time along 1.8 miles of road. The project has CGP coverage and a SWPPP. Crews are dewatering a trench at 200 gpm into a vegetated roadside ditch that flows 300 feet to a creek. The discharge is visibly turbid; a field meter reads 1,200 NTU and pH 9.8. A concrete washout pit is 50 feet from the ditch, and its liner is torn. The contractor says the project is under 5 acres of disturbance and therefore does not need CGP coverage, and that the ditch is a vegetated buffer, so no further treatment is needed. The forecast is dry for the next week, and the creek is listed as impaired for sediment.
1. Is the contractor correct that CGP coverage is not required? Explain.
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No. The CGP threshold is based on total acreage disturbed over the life of the project, not the acreage exposed at any one moment. This project disturbs 6.5 acres overall, which is well above the 1-acre threshold, and the fact that only 3.2 acres are open at a time does not exempt it. Linear projects are still evaluated on total disturbance. The contractor may be confusing temporary phasing with the permit threshold. Because the project already has coverage and a SWPPP, the practical issue is not whether coverage is needed but whether the SWPPP and controls are adequate. The discharge to a sediment-impaired creek also raises the level of scrutiny and may require additional controls or monitoring.
2. What controls are needed for the trench dewatering and concrete washout before discharge?
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A vegetated ditch is not adequate treatment for 200 gpm of visibly turbid trench water at 1,200 NTU. The operator should route the water to a sediment basin, filter bag, or other engineered treatment before discharge, and should monitor turbidity and pH. Because the pH is 9.8, the concrete washout is a likely contributor; the torn liner must be repaired and the washout must be contained so that no washwater reaches the ditch. If pH remains elevated, the operator should use pH adjustment or contain the water for proper disposal. The discharge should not cause or contribute to a violation of water quality standards, and the impaired status of the creek increases the need for controls. The SWPPP should be revised to describe the dewatering operation and washout controls.
3. What records and SWPPP updates should the project make after this finding?
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The project should document the inspection finding, the turbidity and pH readings, the corrective actions taken, and the dates they were completed. The SWPPP should be updated to include the trench dewatering location, estimated flow, treatment method, monitoring frequency, and the concrete washout design and repair. If the discharge continues, the operator should keep a log of visual observations and field measurements. The records should also show how the impaired status of the creek was considered. These records matter because they demonstrate whether the operator identified and corrected a known deficiency or allowed it to continue.