ERDC/EL TR-05-15, Pre-Construction Biogeochemical Analysis of Mercury in Wetlands Bordering the Hamilton Army Airfield Wetlands Restoration Site

Over 90 percent of the coastal wetlands in San Francisco Bay have been lost since the industrial revolution. The U.S. Army Corps of Engineers (USACE) is working with the California State Coastal Conservancy and the San Francisco Bay Conservation and Development Commission (BCDC) to reconstruct wetlands at Hamilton Army Airfield (HAAF) and restore 203 ha of tidal habitat to endangered species such as the clapper rail and the saltmarsh harvest mouse. HAAF has subsided below mean sea level and will require 10 million cu yd of fill material to elevate the site to the point where typical marsh vegetation can colonize and the natural sediment trapping, marsh-building physical dynamics can proceed. Wetlands are believed to be sources of methylmercury (MeHg) production and export, and HAAF represents only 203 ha of the additional 23,300 ha of wetlands to be established around the bay by 2055. Means to mitigate MeHg magnification in bay aquatic food webs are needed but currently unknown. This interim report describes site-specific information collected in 2003 on the geochemical/geophysical, microbial, predominant plantand animal-related interactions that affect stabilization and mobilization of Hg and MeHg in the sediments/soils of the area. Total mercury (THg) and MeHg levels were measured in June 2003 in the sediment in relation to depth at intertidal sites at HAAF and the China Camp State Park (as a reference), as well as at inland sites at HAAF and Bel Marin Creek. Other parameters important for the cycling of Hg and MeHg in sediments were determined also to establish site-specific relationships between these parameters and THg and MeHg. The highest MeHg concentrations (range 0.8-4.4 mg g DW) were found in the upper 2.5-5.1 cm of the cores and levels decreased with depth. THg levels (range 160-550 ng g) increased with depth, correlating inversely with MeHg. MeHg correlated directly with redox potential (Eh), total organic carbon, and phosphorus. Net MeHg production is the result of methylation and demethylation rates in the sediment. Methylation and demethylation rates were determined by on-site incubations of mudand vegetated-mud cores with stable Hg isotopes at the HAAF and China Camp sites. Methylation rates were 1.44 ng MeHg g per day in nonvegetated sediments of HAAF. Rates were usually lower in vegetated than in nonvegetated sediments. Rates were usually higher in the light than in darkness. Methylation rates varied with location within the bay on bare and epipelon-vegetated sites and were lower at HAAF than at China Camp. Epipelon is the complex of microalgae, bacteria, and detritus on the sediment surface. Demethylation rates were 1.281 ng MeHg g per day in nonvegetated sediments at HAAF and 0.78 ng MeHg g per day at China Camp. Mean THg concentrations in the macrophytic vegetation, predominated by Spartina foliosa and Salicornia virginica, ranged from 13 to 158 ng g in shoots and from 217 to 297 ng g in roots. Mean MeHg concentration ranged from 0.55 to 4.75 ng g in shoots and from 2.83 to 5.26 in roots. Plant levels usually exceeded those in the sediments in which they rooted. The THg and MeHg levels in plant detritus were far higher than in live shoots, i.e., by a factor of 5 to 8. Significant levels of THg and MeHg were detected in tissues of macrofauna collected at intertidal sites of HAAF and China Camp. MeHg comprised on average 40 percent of THg (range 20-70 percent). Biota to sediment accumulation factors (BAFs) for MeHg ranged from about 3 to 50. Snails were the highest Hg bioaccumulators. Because the diet of these animals is composed largely of plant material, it is likely that MeHg in plants represents an important MeHg source for trophic transfer in the marsh. A preliminary annual MeHg mass balance for a 203-ha HAAF system indicated a net MeHg production of 12.8 kg and MeHg export in the order of 0.1 kg with tidal waters to the bay. These values serve as the basis for research hypotheses and future work. Measures to decrease bioavailability were explored as a management tool. The bioavailability characteristics of Hg species in HAAF sediments were evaluated experimentally. The MeHg body burdens of the experimentally 2-56 days exposed Macoma nasuta clams were only approximately half of those recorded in clams inhabiting bay edge sediments while elimination was slow, suggesting that exposure periods longer than 56 days are needed for Hg to approach apparent steady state in clam tissues. Bioavailability decreased more by sediment amendment with granular activated carbon (GAC) than with Kraft-lignin. A screening-level model pertaining to HAAF (QnD:HAAF) was created to serve as a tool to incorporate ecosystem characteristics and management measures into a user-friendly framework. This model links the spatial components within GIS files to the prevalent abiotic, climatic, and biotic interactions in the ecosystem. It has a simple design and can be upgraded easily. The current QnD:HAAF version 1.0 integrates the field and laboratory data pertaining to HAAF and other related systems. The model was run to simulate two 14-day scenarios, representing the wet and dry season, respectively. Simulated sediment MeHg concentrations exceeded the measured levels while simulated methylation and demethylation rates were on the same order of magnitude as measured values. Elevation proved to be an important factor influencing net MeHg production. The differences between the simulated and measured THg and MeHg levels in sediment and biota may provide leads to identify areas in which more information is needed. Future research efforts will address processes determining net MeHg production, atmospheric flux of Hg, exchange of Hg and MeHg between sediment and tidal waters, biomagnification of Hg up relevant aquatic food webs, data integration, and management issues. 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