Introduction

Guidelines

Irrigation

Refining

Mining

References

Library/Links

Index Page

REFINING

San Francisco Bay-Delta Selenium Model

The Bay-Delta Selenium Model is a systematic approach for 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. Se 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.

An example forecast is given for a dry year during the low flow season and conveyance through a proposed San Luis Drain extension directly to the Bay-Delta. Se 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 aforesaid model 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 selenium concentrations in the Bay-Delta are less than recommended Se criteria.

Bioaccumulation Model

Selenium released to aquatic systems can result in Se being bioaccumulated to toxic levels in plants, fish, bird eggs, and livestock. 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.