Selenium - A potential time bomb or just another contaminant?

Environmental issues related to selenium are complex and not universally understood. This paper provides detailed information regarding selenium and associated aquatic environmental issues, and then introduces and comments on five debate/commentary papers discussing selenium in the context of the title of this paper. Selenium has a complex and not fully understood biogeochemistry in the aquatic environment as well as an unusual mode of toxicity (acute via water column exposure; chronic via food chain exposure). It has the narrowest range between nutritional requirements and toxicity of any essential element, and chronic toxicity is not readily predictable. Selenium contamination of waters or even of tissues does not necessarily indicate a ticking time bomb; there are no generally accepted or universally accepted threshold values for chronic toxicity. Assessing risk must be done site-specifically in a risk assessment framework, focusing on reproductive effects to sensitive exposed fish and waterfowl and on "worst case" hydrologic units. Unless selenium inputs decrease or site-specific biogeochemistry can be shown not to change, continued biological monitoring and testing are required. Provided the necessary investigative and monitoring studies are done, any potential selenium time bombs can be defused.