Pilot and Special Studies

Mercury Synthesis and Conceptual Model Update

Authors: Jay Davis, Lester McKee, Letitia Grenier, Josh Collins, Don Yee and Ben Greenfield (SFEI)

The RMP Mercury Strategy has funded a significant body of work to address the first two questions in the Mercury Strategy (listed below). This included three years of intensive monitoring of spatial and temporal patterns in mercury in small fish (a total budget of $450,000), a two-year study of mercury isotopes ($115,000), and a two-year study of passive samplers for aqueous methylmercury, referred to as DGTs ($80,000). The results of these studies are either just being evaluated now (isotopes and DGTs) or will be available shortly as is the case for the three-year dataset on small fish. In addition, extensive monitoring of other indicators of spatial patterns, temporal trends, fate, and effects have been conducted by RMP and other programs and projects in the last few years. Our state of knowledge has been advancing rapidly.

The RMP Mercury Strategy has articulated management questions to guide a long-term program of studies to support the goal of identifying and reducing high leverage pathways and thereby reducing mercury impairment in the Bay. The studies conducted to date are providing partial answers to some of the questions in the Strategy, but it is clear that we have not yet answered all of them. The optimal next steps to take in answering the questions, however, are unclear. A prudent next step to ensure effective use of RMP funds is to thoroughly evaluate and synthesize all of the information acquired in the last several years and to use this synthesis as the basis for a plan for the next few years of mercury studies.

The complex biogeochemistry and spatial and temporal dynamics of mercury and especially methylmercury pose a challenge for such a synthesis effort. A large body of
information on the various aspects of mercury science has been generated in the Estuary
in the past few years. An interdisciplinary approach to the synthesis is called for, with collaboration of a team spanning the different branches of mercury science. It will be important to include tidal wetlands within the scope of the review, given their potential influence on regional patterns of contamination in the Bay and the potential importance as a zone of methylmercury impact.

Mercury Food Web Uptake (Small Fish)

Authors: Ben Greenfield and Rachel Allen (SFEI)

Since 2005, SFEI has been monitoring forage fish in the San Francisco Estuary to assess the sources and pathways of mercury entering the Estuary. Mercury (Hg) in small fish is a TMDL target, and a long-term monitoring program is needed to determine how successful TMDL implementation and management actions are at reducing hazards to Bay piscivorous wildlife.

The monitoring to date has established a clear spatial gradient in food web methyl mercury, with elevated concentrations observed in the Lower South Bay (near Alviso slough) and declining concentrations moving towards the Delta. Seasonal variation has been demonstrated to occur in Hg bioaccumulation in San Francisco Bay and elsewhere. However, seasonal variation differs among locations, with summer Hg peaks in two South Bay wetlands, spring peaks in Delta tributaries after large storm events, and variable patterns in North Bay salt ponds.

There is a need to better understand the...