Linking Selenium Sources to Ecosystems:
Local and Global Perspectives
A predictive global map of latent risk for environmental Selenium loading is part of the conceptual model of Se pollution (see below and Mining). The map indicates that ancient organic-rich depositional marine basins, unrestricted by age, are linked to the contemporary global distribution of Se source rocks. Given the geographic patterns, Se emerges as a contaminant within specific regions of the globe that may limit phosphate mining, oil refining, and drainage of agricultural lands because of potential ecological risks to vulnerable food webs. Selenium also may serve as a geochemical exploration tool that signals an ancient productive biological environment.
Theresa S. Presser, U.S. Geological Survey, 345 Middlefield Road, Menlo Park, CA 94025
Joseph P. Skorupa, U.S. Fish and Wildlife Service, 4401 N. Fairfax Dr., Arlington, VA 22203
The sources and biogeochemistry of selenium (Se) combine to produce a widespread potential for ecological risk. Selenium is an essential micronutrient in bacteria and animals. Although humans can benefit from its role as an antioxidant, Se is the most dose-sensitive of all nutrients. Toxicity occurs via biochemical pathways unable to distinguish Se from sulfur, thus substituting excess Se into proteins and altering their structure and function. Congenital anomalies (monstrosities) in aquatic birds are overt expressions of such toxicity. We present 1) a model of Se pollution annotated with protective guidelines and concentrations in sources, food webs, and predators; and 2) a predictive global map of latent risk for environmental Se loading. Examples from the San Joaquin Valley and San Francisco Bay-Delta Estuary, California; watersheds of the Colorado River; waste-rock sites at phosphate mines, Idaho; and valley-fills associated with mountaintop coal mining, West Virginia include a range of processing activities that call attention to anthropogenic connections to the environment (disposal of irrigation drainage, oil refining effluents, and waste shales), in addition to surface processes (weathering, erosion, and runoff), that can ultimately mediate contamination.
The global distribution of organic-enriched sedimentary rocks, black shales, petroleum source rocks, phosphorites, and coals, depends on the fundamental role of major and trace nutrients in determining primary productivity. Although black shales and their recoverable organic fractions as sources of trace elements are widely recognized, the implications of worldwide reservoirs, site-specific fluxes, and persistent biologic cycling of Se are not. Given the geographic distribution of these source rocks, Se emerges as a contaminant within specific regions of the globe that may limit mineral extraction and agricultural growth or exacerbate environmental toxicity.
Development of technologies for controlling Se pollution and predictive forecasts of ecological effects will become increasingly critical to commercial exploitation, as well as to faunal conservation. Based on our conceptual model, adoption of methodologies to protect fish and wildlife that recognize the full sequence of interacting processes from sources through food webs to vulnerable predators will advance risk management by including all considerations that cause systems to respond differently to Se contamination.
Se sources are predicted from major basins hosting phosphate and petroleum source rocks, not from source-rock age.
A similarity in Se mobility trends (increased flow results in increased concentrations and hence increased loads) in affected watersheds in agricultural and mining regions suggests massive Se storage that is now subject to transport.
Most recently, our model enabled prediction of selenium mobilization during Appalachian mountain top removal coal mining, where waste shales are disposed of in valley-fills. Preliminary data from West Virginia show Se concentrations 1) in some streams and ponds exceed the criterion for the protection of aquatic life; and 2) in some fish exceed levels at which substantive risk occurs.
Accurate forecasting of the environmental fate of selenium is crucial because of the element's effect on reproduction in aquatic birds and fish. Selenium concentrations in fish and bird eggs provide assessments for risk management that incorporate and integrate many potentially confounding site variabilities.
Bioaccumulation from food determines the ecological effects of Se. Direct transfer of Se from solution to animals is a small proportion of exposures. For example, Se concentrations are less than water quality guidelines in the San Francisco Bay-Delta Estuary in the latest surveys. Nevertheless, Se concentrations in food webs are sufficient to be a threat to some species and a concern to human health if those species were consumed.
The San Francisco Bay-Delta Selenium Model is an example of a new type of tool that predicts ecological effects based on the major processes leading from loading through food webs to predators.
The USEPA and USFWS have agreed to develop Se criteria specifically for California that go beyond USEPA's traditional focus on aquatic life such as fish and aquatic invertebrates. The new criteria will consider aquatic-dependent wildlife, such as water birds, as well as aquatic and semi-aquatic mammals, reptiles, and amphibians. This broader ecosystem approach could become a model for national standards.
Model of Selenium Pollution
The model illustrates specific biogeochemical pathways connected to irrigation, refining, and mining in the western United States. The model shows Se cycling from sources through food webs to vulnerable predators.
The model components are:
oceanic depositional environments
organic carbon-rich marine sediments
specific Se source rocks
anthropogenic activities that facilitate transfer to the environment
affected receiving water bodies
food webs that have bioaccumulated Se to toxic dietary levels
predator species whose tissue Se concentrations exceed toxic risk thresholds
Selenium concentration ranges (in dry weight, except as noted) together with averages for the source rock shales.