Linking Selenium Sources to Ecosystems: Refining
Science Center Objects
The San Francisco Bay-Delta receives selenium (Se) internally from oil refineries and externally through riverine agricultural discharges. Predator species considered at risk from Se consume the estuary’s dominant bivalve, C. amurensis, an efficient bioaccumulator of Se. Modeling predicts site-specific ecological risk and derives a range of protective Se concentrations for use by decision-makers.
San Francisco Bay-Delta Selenium Model (2000-2006)
Five oil refineries process crude oil and discharge selenium (Se) to the San Francisco Bay-Delta Estuary (Bay-Delta) (Figures 1 and 2). Organic-rich marine basins are the primary source of elevated concentrations of Se in crude oils (see Mining page). In addition to this internal source of Se, external inputs of irrigation drainage from agricultural lands of the western San Joaquin Valley are conveyed through the San Joaquin River to the estuary (see Irrigation page). Planning since 1955 and proposals in 1978 and 2006 to extend an agricultural drain from the western San Joaquin Valley directly to the Bay-Delta as a way of removing Se from the valley were found, both times, to present substantial and broad ecological risks.
Regulatory and planning processes have intervened in the cases of both existing Se sources resulting in a decline in contamination since 1986-1992 when concentrations were maximal. However, the North Bay, the Delta, and segments of the San Joaquin River and some of its tributaries and marshes remain designated as impaired by Se. The presence of a major oil-refining industry in the North Bay, and the substantial accumulated reservoir of Se in the soils and aquifers of the western San Joaquin Valley suggest that the potential for ecological risk from Se within the Bay-Delta watershed will continue into the foreseeable future as Se management and mitigation efforts take place.
Selenium released to aquatic systems can result in Se being bioaccumulated in food webs to levels that cause toxicity in predator species. The general term bioaccumulation can be applied to all of the biological levels of Se transfer through the food web. Linked biological and geochemical reactions affect how readily Se enters food webs, initiates food web transfer, and cycles through particulate matter, sediments, consumer organisms, and predators. Because Se concentrations can be magnified at each step of food web transfer, upper trophic level species are most vulnerable to adverse effects from Se.
The Bay-Delta Selenium Model (Figures 3 and 5) is a systematic approach for quantifying Se source loading to the estuary and conducting forecasts of Se effects on aquatic food webs including higher trophic level animals such as birds and fish. Forecasts consider (1) oil refinery, agricultural drainage, and river loads, (2) water column concentrations, (3) speciation, (4) transformation to particulate forms, (5) particulate concentrations, (6) bioaccumulation, and (7) trophic transfer to predators, in addition to traditional considerations of water supply and drainage demand. The projections of the model are presented by season, where a season is defined as six months of predominantly high river inflows or low river inflows. In combination with flow seasons, forecasts are made for critically dry years or for wet years. Selenium concentrations and forms in the Bay-Delta were used to model bioaccumulation in invertebrates, like clams. Transfer from clams to predators was estimated from field data, and Se effects on predators were then forecast from literature data.
Predator species considered most at risk from Se (e.g., green and white sturgeon, scoter, scaup) in the Bay-Delta consume the estuary's dominant bivalve, Corbula amurensis, an efficient bioaccumulator of Se. An example model forecast (Figure 4) is given for a dry year during the low flow season and Se loading from oil refineries and conveyance of agricultural drainage through a proposed San Luis Drain extension from the San Joaquin Valley directly to the estuary. Selenium concentrations for each media forecast (water, sediment, invertebrate, predator) are shown, along with guidelines or concentrations where biotic effects are expected. The dry years and low flow seasons will be the ecological bottleneck (the times that will drive impacts) with regard to Se. Surf scoter, greater and lesser scaup, and white sturgeon are present in the estuary during the low flow season and leave before high flows subside. Animals preparing for reproduction, or for which early life stages develop in September through March, will be vulnerable.
The Bay-Delta is probably best suited for site-specific Se guidelines and the modeling approach given above could provide a framework for developing new protective criteria. If water quality criteria are to be employed in managing Se inputs, then consideration should be given to the elevated Se concentrations currently occurring in clams and fish of the Bay-Delta even though waterborne Se concentrations in the Bay-Delta are less than current Se criteria.
San Francisco Bay-Delta Ecosystem-Scale Selenium Model (2007 to Present) in Support of:
- fish and wildlife criteria development
- the Regional Ecosystem Restoration Implementation Plan (DRERIP)
The Bay-Delta Ecosystem-Scale Selenium Model combines fundamental knowledge of Se behavior in ecosystems with site-specific knowledge of the estuary to define site-specific Se risk (Figure 5). This type of modeling addresses the need for an updated regulatory approach that 1) formally document system hydrology, biogeochemistry, biology, ecology, and ecotoxicology; and 2) quantitatively links ecosystem media (water, particulate material, and tissues of different food web species) to enable site-specific translation of fish and wildlife tissue Se effect guidelines to dissolved Se concentrations. Additionally, the model serves as a Se component model of a planned comprehensive model for the estuary system, which will cover multiple levels of management inputs. The model uses common elements that are designed to interconnect to achieve the goals of evaluating and informing Bay-Delta restoration actions. Figure 5 shows a summary of critical choices for modeling and a quantitative application of the model for the estuary derived for predators most at risk from Se at the time and place of greatest ecosystem Se sensitivity.
Presser, T.S. and Luoma, S.N., 2006, Forecasting selenium discharges to the San Francisco Bay-Delta Estuary: ecological effects of a proposed San Luis Drain extension: U.S. Geological Survey Professional Paper 1646, 196 p.
Presser, T.S. and Luoma, S.N, 2010, Ecosystem-scale selenium modeling in support of fish and wildlife selenium criteria development for the San Francisco Bay-Delta Estuary, California, U.S. Geological Survey Administrative Report, 101 p. and Appendices A-D. [Published 12/14/2010; released by USEPA (Region 9, San Francisco, California) 8/29/2011]
Presser, T.S., and Luoma, S.N., 2013, Ecosystem-scale selenium model for the San Francisco Bay-Delta Regional Ecosystem Restoration Implementation Plan (DRERIP): San Francisco Estuary and Watershed Science, v. 11, no. 1, p. 1-39.