Safe for Aquatic Life

A few contaminants, however, are of significant concern due to concentrations that approach or exceed regulatory goals. Specifically, mercury and selenium sometimes exceed their thresholds and are characterized as Fair/Poor in this analysis.

Mercury concentrations in prey fish (e.g., threespine stickleback, topsmelt, yellowfin goby, Mississippi silverside) exceed a water quality objective established to protect the endangered California Least Tern. Other bird species, such as Forster’s Tern, Caspian Tern, Black-necked Stilts, and American Avocets, also face substantial risk. Selenium concentrations in white sturgeon, another protected species, sometimes exceed a threshold established to protect this species. Copper concentrations are just below thresholds, and ensuring they remain there is a priority for water quality managers.

Other contaminants have not been studied thoroughly enough to establish objectives, but are likely to have a negative impact on aquatic life. PFAS (water and stain repellants commonly known as “forever chemicals”) and organophosphate esters (widely used in plastics as additive flame retardants and plasticizers) do not yet have regulatory thresholds, but have a high probability of impact on the Estuary. Many other contaminants of emerging concern have been identified that have a moderate probability of impact, including a highly toxic degradate of a tire additive (6PPDQ), ingredients in plastics (bisphenols), a fungicide and preservative (carbendazim), the pesticides fipronil and imidacloprid, and microplastics. Efforts to evaluate these contaminants and develop appropriate goals are in progress.

Widespread historic contamination of the Estuary and its watershed poses a major challenge to reducing mercury concentrations in the food web. Trends in precipitation have the potential to exacerbate selenium concerns as drought conditions in the watershed are associated with higher food web concentrations. The San Francisco Bay Regional Water Quality Control Board has established Total Maximum Daily Load (TMDL) cleanup plans for mercury and selenium, and the Central Valley Regional Water Quality Control Board has a TMDL for methylmercury. The TMDLs have been in place for approximately 10-15 years and have prompted actions to reduce mercury and selenium inputs to the Estuary.

Regional water quality monitoring programs in the Bay and the Delta are actively evaluating other contaminants that are of known or potential concern. Tens of thousands of chemicals have been registered for production and use worldwide. Determining which of these chemicals pose the greatest threat to the Estuary is a formidable challenge, but one that the Bay and Delta regional monitoring programs are striving to address.