Precision Dredging Techniques Enhance Cleanup at Lake River
The benefits of precision dredging are numerous; I’ve seen them firsthand: streamlined regulatory approval, greater risk reduction, and increased water quality. MFA wrapped up a large-scale dredging project in February 2015 in Lake River, Ridgefield, Washington. Sediment in Lake River was impacted by contaminants from a wood-treating operation that, from the 1960s through 1993, pressure treated poles and dimensional lumber with chemical preservatives. The Port of Ridgefield and the Washington State Department of Ecology entered into a legal agreement in 2014 to collaborate on the cleanup efforts.
The remedy was ambitious in scope and significance. It involved removing up to 100 old, treated wood pilings; dredging 10,000 cubic yards of contaminated sediment (including dioxins—known endocrine disruptors); placing a clean layer of sand in the river; stabilizing the bank with rounded cobbles; and revegetating 2.5 acres with native grasses and about 1,000 shrubs and trees. These actions result in 0.5 mile of clean shoreline habitat.
MFA engineers insisted on applying “precision dredging” techniques in the design. This approach uses sophisticated global positioning system equipment to verify removal of toxic sediment, and a bucket design to limit disturbance and spreading of fine-grained sediment during construction.
The selected dredging method resulted in improved efficiencies during permitting; in a biological opinion, the National Marine Fisheries Service (NMFS) identified this approach as a desired method for minimizing the loss of sediment into the water column. NMFS is responsible for evaluating potential adverse effects to endangered species, such as juvenile salmon, related to remedial construction. Further, real-time water quality monitoring during precision dredging construction confirmed that water quality was not adversely impacted. Finally, observations indicated that generation of contaminated sediment residuals were significantly limited throughout the project.
The sediment cleanup was preceded by an emergency action that involved the use of steam-enhanced remediation to remove almost 30,000 gallons of wood-treating chemicals from groundwater and kept them from contaminating the nearby Ridgefield National Wildlife Refuge.