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 very challenging environment with extensive tidal mudflats adjacent to a large active refinery. Historical discharges from the 110-year-old refinery impacted sediments with mercury and polycyclic aromatic hydrocarbons and posed a toxicity risk to benthic organisms. A tiered risk assessment identified a 20-acre mudflat area of concern (AOC) and corrective action objectives of contamination removal and biological restoration. A 2,700-foot sheet pile enclosure was installed to isolated AOC from SF Bay, and 97,000 cubic yards of sediment were dredged from the AOC in 2007. The residual exposed sediment did not meet regulated concentrations requiring the design and construction of a sub-aqueous engineered cap. The two-layer cap designed using leach tests and the Reible chemical flux model includes a 6-inch thick base sand isolation layer and 18-inch thick clay benthic layer. Local regulations impose a “no-net-fill” requirement limiting the cap volume 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 dredged surface without remobilizing residual contamination, (2) hydraulic placement of the clay layer over the sand layer, (3) consolidation of clay layer before removal of the sheet pile enclosure, and, (4) sheet pile removal such that the cap was not compromised by tidal action. Approach/Activities. 1. AOC is too large for mechanical sand placement so a placement barge was designed with a spreader tray to slow 3,300 gpm, 14% solids sand slurry to a 3 fps horizontal discharge causing sand to fall under gravity. Slurry was pumped one mile from a harbor accessible daily by a small sand barge. Water discharge restrictions required a closed loop make-up water system. Sand thickness was measured daily using clear PVC push cores. 2. Beneficial reuse of 50,000 cy of bay mud dredged from local yacht harbor and marina was approved for the clay layer. Clay was mechanically dredged 8,000 feet from the AOC, converted to slurry and spread using the placement barge. The volume of placed clay was tracked daily and clay layer thickness and distribution measured relative to 20 survey markers. 3. To ensure hydraulically placed clay layer gained sufficient shear strength to resist tidal and wave action during removal of the sheet pile enclosure, AOC was drained and in situ shear strength measured for four months using a hand-held shear vane and compared with ambient shear strength in the surrounding mudflats.