Nickel listing was originally based on possible impairment of beneficial uses, including fish spawning and estuarine habitat. Total recoverable nickel concentrations frequently exceed the Basin Plan objective for saltwater (7.1 ppb), but USEPA regulations have recently changed to express metal criteria as dissolved rather than total recoverable. An evaluation is needed to determine the the most appropriate nickel water quality objective.
Regional Monitoring Program data have demonstrated that dissolved nickel concentrations in most regions of the estuary are below the USEPA water quality criteria for dissolved nickel (8.2 ppb) specified in the National Toxics Rule. On the western margin of San Pablo Bay, episodic exceedance of the dissolved criterion occurs during high-flow periods. Wintertime concentrations in that region range from 6-10 ppb, but can sporadically reach as high as ~40 ppb. Concurrent enrichment of dissolved manganese and cobalt in western San Pablo Bay suggests that the observed nickel inputs come from diagenesis of anoxic sediments in adjacent wetlands. Nickel is known to be naturally enriched in coast range sediments due to the abundance of ultramafic minerals in the Franciscan formation. Therefore, the elevated nickel in western San Pablo Bay may result from natural processes.
Biological effects of nickel are related to free ionic concentrations, rather than dissolved concentrations. Studies in South San Francisco Bay have demonstrated that nickel conveyed by municipal wastewater is strongly complexed by organic ligands, and that these complexes are inert on the time scale of estuarine mixing. There is currently no information on the speciation of nickel in the rest of the estuary.
The Regional Board is using the Santa Clara Basin Watershed Management Initiative as a stakeholder forum for development of a TMDL for nickel in South San Francisco Bay. Studies conducted by the City of San Jose in accordance with USEPA guidance to support a SSO are currently being reviewed. The nickel SSO proposed for the South San Francisco Bay is based on recalculated acute-chronic ratios and may be relevant to the entire estuary.