| Abstrakt: |
The South Bay Salt Pond Restoration Project (SBSPRP) encompasses over 6,000 hectares of former salt production ponds along the south edge of the San Francisco Bay and represents the largest wetland restoration effort on the west coast of North America. A series of studies associated with Phase 1 (2010-2018) restoration activities that are focused on a historically mercury contaminated slough and series of ponds within the restoration area have recently been completed. This report brings together the key findings of these loosely coordinated studies and integrates the results into a more comprehensive and holistic product that informs future restoration activities associated with the SBSPRP and elsewhere. The focus of the Phase 1 studies was organized around two primary restoration management actions associated with Alviso Slough and its adjacent former salt ponds. The first action was a levee breach associated with pond A6 along the lower reach of Alviso Slough. The second action was associated with an adjustable tidal control structure at pond A8 (A8-TCS) that was constructed to reintroduce muted tidal connectivity between the upper portion of Alviso Slough and an area that comprises three hydrologically interconnected former salt ponds (ponds A5, A7 and A8, referred to as the A8-complex). During a 6-year period (2011-2017), the A8-TCS was gradually opened from one gate (1-gate condition, 1.5-meters wide opening) to eight gates (8-gate condition, 12.2-meters wide opening). This report focuses on addressing the extent to which these two management actions resulted in demonstrable changes in mercury concentrations associated with biota, surface water or bed sediment, and with mercury transport and flux associated with Alviso Slough bed sediment erosion caused by an increase in tidal prism. This report documents key findings associated with the breach of pond A6: (1) a short-term spike in slough fish (Mississippi silverside) total mercury concentration in lower Alviso Slough; (2) a short-term spike in surfacewater particulate total mercury in lower Alviso Slough; (3) significant sediment scour in Alviso Slough adjacent to and downstream of the breach points; (4) a decrease in surfacesediment methylmercury (as a percentage of total mercury) in lower Alviso Slough; (5) the transport of 70 kilograms per year of sediment-associated total mercury into pond A6 during the first 2 years following the breach but with much of this coming from outside of Alviso Slough, presumably from the nearby shallows, Guadalupe Slough, and the larger southern San Francisco Bay area; and (6) a slowing of bed sediment erosion in lower Alviso Slough 3-5 years after the breaching of pond A6. Other key findings associated with the construction and gradual opening of the A8-TCS are documented in this report: (1) a short-term total mercury spike in prey fish (Gillichthys mirabilis [longjaw mudsucker] and Gasterosteus aculeatus [three-spined stickleback]) and tern eggs within the Alviso pond complex, and in Alviso Slough Mississippi silverside, all of which were attributable to construction activities within and immediately adjacent to the Alviso pond complex prior to the initial gate opening of the A8-TCS; (2) multiple lines of evidence that indicate the transition from three gates (3-gate condition, 15 feet [4.6 meters]) to five gates (5-gate condition, 25 feet [7.6 meters]) open at the A8-TCS may represent a critical tipping point beyond which the sudden increase in tidal prism resulted in increased bed sediment erosion, and the reversal of suspended sediment flux direction, towards the bay, for a prolonged period (1.6 years); (3) this period concluded with a substantial spike in surface-water methylmercury in Alviso Slough, which preceded (by 6 months) a significant spike in silverside total mercury concentrations in Alviso Slough that we ultimately attribute to the opening of five gates initiated two years prior; (4) a steady year-over-year decrease in Alviso Slough surface-water total mercury and methylmercury concentrations on a volumetric basis that is due to dilution driven by the increased tidal prism linked to the increased number of A8-TCS open gates; (5) a steady year-over-year increase in wintertime (December--February) particulate total mercury concentration on a gravimetric basis in Alviso Slough that is linked to increasing bed sediment erosion of long-buried sediment horizons containing elevated concentrations of legacy mercury (derived from historic mining activities in the watershed); (6) the mass flux of suspended sediment, particulate total mercury, and particulate methylmercury past the A8-TCS was into the A8-complex during the full period of high-resolution water quality monitoring (February 2016--February 2018), which included the transition from five to eight gates open at the A8-TCS; and (7) the opening of eight gates resulted in a decrease of water flux into the A8-complex and a reversal in model-predicted filter-passing total mercury and filter-passing methylmercury flux from the A8-complex being a sink for these two species during a 5-gate condition to being a source to Alviso Slough during an 8-gate condition. Although this report is not intended to be prescriptive in terms of the next steps the SBSPRP should or should not take, the totality of the findings presented provide critical process-level information regarding the extent and the duration of spikes in mercury levels in water, sediment, fish, and birds, which appeared to result from the two management actions under study. Thus, these results can be used to anticipate similar ecosystem responses associated with similar management actions that may be considered in the future. We also conclude this report by highlighting unanswered questions associated with mercury dynamics as it relates to the restoration project, and possible future directions for research. [ABSTRACT FROM AUTHOR] |
