Mercury speciation and other constituent data for surface water and bed sediment associated with the Hamilton Airfield Wetland Restoration, Novato, California. USA (ver. 5.0, June 2024)

The Hamilton Wetland Restoration Project (HWRP) is a joint venture between the US Army Corps of Engineers (USACE; San Francisco District) and the California State Coastal Conservancy. The site is located on the property of the former Hamilton Army Airfield in Novato, California, along the western edge of San Pablo Bay, which is part of northern San Francisco Bay. The initial stages of wetland restoration, which included dredged sediment reuse, took place during 2009-2011. From 2011 until April 2014, the site was maintained as partially flooded through a temporary one-way culvert that allowed bay water onto the site. On April 25th, 2014, the outboard levee towards the north-eastern corner of the site was breached to restore tidal connectivity from the bay to the restoration area. The anticipated period for complete site restoration and marsh evolution is 10-15 years. During this period an active wetland vegetation replanting program was established by the USACE to augment the natural seeding by native plants. The various habitats on the restoration site include open-water sub-tidal, mudflats, emergent marsh, grassland, and upland transition zone. San Francisco Bay is contaminated with legacy mercury (Hg) from historic use in mining areas throughout the watershed, as well as from contemporary atmospheric and point-source inputs. Since wetlands are known to be particularly effective zones for the production of toxic methylmercury (MeHg) from inorganic Hg(II), the National Environmental Policy Act (NEPA) and California Environmental Quality Act (CEQA) documentation associated with the HWRP restoration identified MeHg production as a potentially significant impact of restoring the site to wetland habitat. As part of the USACE Monitoring and Adaptive Management plan (MAMP) for the restoration, the USACE was directed by the Regional Water Quality Control Board (RWQCB) and the San Francisco Bay Conservation and Development Commission (BCDC) to assess MeHg concentrations at the HWRP site. To fulfill this requirement, the USACE entered into an agreement with the U.S. Geological Survey (USGS) to conduct a multi-year and multi-phase investigation of mercury in biota (fish), water, and sediment at the HWRP site. The Sample and Analysis Plan (SAP) covers a period of 13 years and is divided into three phases. Phase 1 included the monitoring of sediment and water pre-breach [2013] and post-breach [2014], and the monitoring of fish pre-breach [2013 only]. Phase 2 covers th