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Best Management Practices for Chemical Treatment Systems for Construction Stormwater and Dewatering

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2. Rationale for Choosing Chemical Treatment Systems (CTS)

Sedimentation ponds are commonly used to treat construction site runoff prior to discharge, but they are only effective at removing larger particles by gravity settling. Smaller particles, such as clay and fine silt, tend to remain in suspension for a longer duration than typical design retention times for sedimentation ponds. Chemical treatment can reliably provide exceptional reductions of turbidity and associated pollutants and should be considered where turbid discharges to sensitive waters cannot be prevented using other traditional BMPs.

Chemical treatment involves the application of chemicals to stormwater to aid in the reduction of turbidity caused by fine suspended sediment. The technology is used to treat stormwater from temporary construction sites, as opposed to permanent stormwater treatment facilities.

Typically, chemical treatment (flocculant) is limited to waters with turbidity limits or other water quality standards. However, at times, there may be no regulatory reason to deal with turbid water, but there may be aesthetic, social, or political reasons to reduce sediment discharge and resulting turbidity in receiving waters.

The following are situations where CTS may be needed:

1. Sediment Retention Pond - Chemical treatment is an enhancement option when a pond of a required size or design cannot be constructed or is otherwise not sufficient to control turbidity. This may occur because of topographical constraints, difficult foundation conditions, or the presence of natural habitats of ecological value.
2. Physical Characteristics of Sediment Retention Pond - In some situations a pond of a required volume can be accommodated but the design of the pond cannot be optimized in terms of shape, depth, location of inlet and outlet, or energy attenuation of the inflow.
3. Soil Type - Most soils contain sufficient clay and silt to produce substantial turbidity in runoff. However, some soil types produce suspended sediment in stormwater runoff which is particularly difficult to settle in typical sediment retention ponds due to particle size and charge.
4. Sediment Generation Potential of Earthwork Area - In areas with highly erodible soils or steep long slopes, there is a high risk of increased erosion and sediment runoff from rainstorm events.
5. Use of the Earthwork Site & Construction Schedule - Some common uses of earthwork sites, particularly repeated machinery movements (e.g., on haul roads) can result in high sediment loadings in stormwater. Large areas of disturbance which cannot be stabilized with ground covers due to operational constraints will likely generate significant turbidity in stormwater runoff.
6. Performance of the Sediment Retention Pond - If a sediment retention pond does not perform adequately, addition of chemical flocculants may improve performance.
7. Construction Dewatering - Dewatering activities from construction sites, such as pumping accumulated water from excavation areas, can produce highly turbid discharge water, as distinguished from stormwater.

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