Regulated (contaminated) Soil/groundwater Dewatering, and Interrelated Stormwater Interaction Controls

Temporary erosion prevention BMPs (differentiated from those same BMPs that might be used to establish final perennial and permanent vegetation) proposed to limit contaminated soil erosion during periods of sequenced temporary construction operations are based on specific manufacturer, DOT, or consultant standard specifications/provisions along with engineering judgement and experience. A full range of temporary erosion prevention, sediment control, and water treatment operational BMPs will be discussed that were designed and implemented based on proposed service life, phase of work, and time of year. The knowledge can be adapted to reverse engineer contaminated soil/ground water scenarios for design, and contract treatment means and methods.

Full Abstract: The normal (non-specific) NPDES construction stormwater permit regulates the discharge of stormwater treated to meet sediment removal requirements (i.e., visually clean). Additional dewatering permits and implementation can be confusing with multiple jurisdictions. Part of the routine NPDES dewatering includes planning, implementing, inspecting/documenting, verifying results, and performing corrective actions. Unless the water is discharged visibly clean, spill-free, scour-free, and no nuisance, the pump must be turned off immediately. While a dewatering program is not hard to implement, when operational setup and maintenance fails to perform, dewatering results can cause quick and significant damage to receiving waters or private property. In MN another permit (Water Appropriation) is necessary if 10,000 or more gallons of water will be extracted or moved per day. This state permit uses language direct from the NPDES permit. When invasive species could be part of water appropriations, a different permit is necessary with the goal of prevention of spread regardless of extracted gallons quantity.
Projects that require ground or surface dewatering within contaminated/regulated soils (soil excavations, corrections, utilities, etc.) require a different permit. It is no longer routine. In addition, when stormwater (snow, rain or overland flow) contacts contaminated soils, it too now must be treated as contaminated. There are many reasons that soils are regulated including, but not limited to contamination with fuels, VOCs/SVOCs, PCB, PFAS, metals, cinders, ash, etc.
Normal BMPs used for the management of sediments are not applicable if the sediments contain contaminants that can be released. If sediment and contaminants pass though or overwhelm standard controls, it now negatively interacts with common SWPPP problem areas such as inlets and exit sweeping. In Minnesota another dewatering permit is required, titled Contaminated Groundwater Discharge NPDES/SDS MNG790000 permit. This permit sets the maximum level of pollutant discharge to surface. Sanitary discharge permits may also be obtained depending on jurisdictional capacities, contaminant levels, etc.
Lines of defense:
• Stage work during times of low precipitation (but this does not help with groundwater intrusion).
• Diversion of upland storm water around the work area.
• Phased excavations that direct and contain rain and storm flow into sump depressions formed in the grade where pumps can be utilized to pump water to treatment system (and/or over excavate so dewatering is not needed/minimized).
• Dewatering treatment system with various settling tanks, clarifiers, polymer injection, bubblers, sand pod and bag filters.
These lines of defense help in the immediate area of work. However, there are areas where stormwater cannot be forced to drain back into the work area for treatment. These include positive slope areas above storm gutters, stockpiles, and entrances/exits.
The goal of the case study is to address regulated soil dewatering systems, special provisions, modified sediment control items like 3-dimensional perimeter controls, diversions, gutter traps, exit controls, amended sediment control logs with various chemical agents to change, bind or trap defined pollutant constituents, and soil erosion prevention covers.