San Francisco, California
The Offshore Sediment Area is adjacent to the uplands of the Potrero Power Plant Site located along the waterfront in San Francisco’s Central Waterfront Area. Historical upland site uses include industrial activities, a manufactured gas plant (MGP), and power plant activities, while historical overwater site uses include dock activities and a shipyard. Although the shipyard slipways were later backfilled, concrete piers bracketing the shipways and rails used for marine ship building are still present near the sediment surface. The site will be redeveloped to support residential apartments, commercial office space, retail and recreational open space. COCs include MGP-related contaminants including PAHs.
Sevenson mechanically dredged the contaminated sediments from the bay using a mechanical long stick dredge with an environmental self-sealing clam shell bucket on a spud barge. Sevenson performed shoreline restoration by removing sediments and debris, and placement of clean riprap. Sediment was transported to a containment pad that stabilized the sediments with Portland cement. Loaded trucks for off-site disposal and treated all water for discharge to the POTW.
A turbidity curtain was deployed around the work area and turbidity monitors were used to monitor turbidity outside the work area. Sediment was transferred to the sediment processing area on the shore where the sediments were dewatered via gravity and Portland cement. For site restoration, Sevenson installed a 142,000 SF multilayer sediment cap that consisted of sand, stone, rip rap, AquaGate®, organoclay, and Reactive Core Matting. Sevenson received The Big Nut – Environmental Remediation Award of Excellence from PG&E for the successful completion of this project ahead of schedule, under budget, without any safety issues.