Castro Cove Sediment Remediation Project: 20-Acre Mudflat Cap and Restoration Using Beneficial Reuse Dredging

摘要

Background/Objectives. The project site in north San Francisco Bay presents a verychallenging environment with extensive tidal mudflats adjacent to a large active refinery.Historical discharges from the 110-year-old refinery impacted sediments with mercuryand polycyclic aromatic hydrocarbons and posed a toxicity risk to benthic organisms. Atiered risk assessment identified a 20-acre mudflat area of concern (AOC) and correctiveaction objectives of contamination removal and biological restoration. A 2,700-foot sheetpile enclosure was installed to isolated AOC from SF Bay, and 97,000 cubic yards ofsediment were dredged from the AOC in 2007. The residual exposed sediment did notmeet regulated concentrations requiring the design and construction of a sub-aqueousengineered cap. The two-layer cap designed using leach tests and the Reible chemicalflux model includes a 6-inch thick base sand isolation layer and 18-inch thick claybenthic layer. Local regulations impose a “no-net-fill” requirement limiting the capvolume to 95,000 cy. The capping project presented four major technical challenges:(1) hydraulic conveyance and placement of a thin sand layer over an undulating dredgedsurface without remobilizing residual contamination, (2) hydraulic placement of the claylayer over the sand layer, (3) consolidation of clay layer before removal of the sheet pileenclosure, and, (4) sheet pile removal such that the cap was not compromised by tidalaction.Approach/Activities.1. AOC is too large for mechanical sand placement so a placement barge was designedwith a spreader tray to slow 3,300 gpm, 14% solids sand slurry to a 3 fps horizontaldischarge causing sand to fall under gravity. Slurry was pumped one mile from aharbor accessible daily by a small sand barge. Water discharge restrictions requireda closed loop make-up water system. Sand thickness was measured daily using clearPVC push cores.2. Beneficial reuse of 50,000 cy of bay mud dredged from local yacht harbor andmarina was approved for the clay layer. Clay was mechanically dredged 8,000 feetfrom the AOC, converted to slurry and spread using the placement barge. Thevolume of placed clay was tracked daily and clay layer thickness and distributionmeasured relative to 20 survey markers.3. To ensure hydraulically placed clay layer gained sufficient shear strength to resisttidal and wave action during removal of the sheet pile enclosure, AOC was drainedand in situ shear strength measured for four months using a hand-held shear vaneand compared with ambient shear strength in the surrounding mudflats.