Formatted Title:

Enhanced Ecological Decision-Making Framework in Support of Qualifying Fill for Coastal Restoration Projects

Background/Objectives:

The Don Edwards National Wildlife Refuge (Refuge) is in South San Francisco Bay, California. The former salt ponds of the Refuge contain more than 70 miles of levees that require ongoing operations and maintenance (O&M) to maintain flood protection. The Refuge is also part of the South Bay Salt Pond Restoration Project (SBSPRP), the largest tidal wetland restoration project on the West Coast of the United States. The SBSPRP is partnered with other federal, state, and local agencies in the South San Francisco Bay Shoreline Project, which is constructing flood risk management levees on the Refuge. The Refuge utilizes fill from upland and stream sediment removal projects for a variety of purposes, including habitat enhancement features, pond restoration, levee maintenance, levee construction, and other O&M activities on the Refuge. The San Francisco Bay Regional Water Quality Control Board (RWQCB) qualifies terrestrial fill for import and use on the Refuge through a soil screening process outlined in a master quality assurance project plan (QAPP).

The SBSPRP’s planned restoration of former salt ponds to tidal marsh will proceed by breaching ponds and allowing natural sedimentation to raise pond bottoms back to elevations suitable for establishment of marsh vegetation. This approach may not be sufficient to achieve project objectives given the pace of sea level rise and the rates of natural sedimentation. The San Francisco Estuary Institute (SFEI) estimates that the tidal marshes will need 98 million metric tons (53 million cubic yards [cyds]) of sediment between 2010 and 2050 to raise the elevation of deeply subsided ponds planned for restoration as well as to supply sediment necessary for the marshes to keep pace with sea level rise. Only 9 to 12 million metric tons (5-6.5 million cyds) of sediment are expected to be supplied to the marshes by natural processes during that 40-year period (SFEI, 2021). As a result, there is an urgent need to identify means to qualify and import large volumes of sediment from other sources, including terrestrial fill, into the Refuge to supplement natural sedimentation.

Approach/Activities:

The first step toward increasing qualified fill volumes for import involved development of an ecological conceptual site model (CSM) as a supplement to the master QAPP. The CSM provides a scientifically defensible framework to suggest future modifications to the protective fill screening methods and criteria in the master QAPP, with an overarching goal of increasing opportunities for terrestrial fill import to the Refuge without increasing the risk of adversely affecting ecological populations. The CSM provides a foundation for risk-based decision making.

Results/Lessons Learned:

The ecological CSM was used as the basis upon which risk-based exposure assumptions could be forecast as potential risk to targeted terrestrial and aquatic wildlife receptors that inhabit the Refuge. The CSM identified the following opportunities to modify the master QAPP to facilitate qualification of additional terrestrial fill sources which may have formerly been excluded from use.

Modify ecotoxicity basis for Refuge’s master QAPP screening criteria currently based on freshwater receptors and propose modified screening limits based on marine ecosystems.
Differentiate screening criteria based on anticipated ecological exposures at placement horizon as wetland surface or foundation materials.
Develop proposed not-to-exceed limits to use with the mean hazard quotient (HQ) method for metals in material anticipated for foundation placement.

The modified screening criteria along with other risk-based considerations are anticipated to markedly increase qualified terrestrial fill volumes for import.

Zachery Gizicki

Affiliation

Refine Results

Enhanced Ecological Decision-Making Framework in Support of Qualifying Fill for Coastal Restoration Projects