Selenium and molybdenum are naturally occurring trace elements that are often released to the environment at relatively low concentrations as a result of mining industry discharges. At present, Canada’s Metal Mining Effluent Regulations (“MMER”) do not identify limits for either of these elements. In order to independently establish discharge limits for selenium and molybdenum, it is possible to use water quality guidelines established for the protection of aquatic life e.g., 0.001 mg/L for selenium and 0.073 mg/L for molybdenum. This simple approach, however, is problematic in that recently established regulatory limits to protect water quality are very low, reflecting extreme scenarios that are very unlikely to be encountered in the receiving environments at most mine sites in Canada. The regulatory limits for selenium and molybdenum follow a risk-based approach that involves numerous assumptions that are multiplicative in their conservatism. While these limits will ensure environmental protection, they are very likely overly restrictive for most sites. A more site-specific approach, based on ecological risk assessment, to resolve effluent limits is advisable in most cases.
The establishment of risk-based limits (generic or site-specific) for selenium and molybdenum is problematic due to a series of unique aspects of these two elements, including:
• the role of both elements as essential nutrients to plants and animals, and the unique processes that govern their physiological regulation;
• the relatively narrow range between exposure levels that are sufficient and required, from a nutritional perspective, and those that are potentially toxic;
• the variability in partitioning, bio-uptake, and toxicity of selenium and molybdenum, depending on the specific form of these elements;
• the possibility for toxic effects of aquatic releases of selenium and molybdenum on terrestrial wildlife, owing to direct and indirect foodchain exposure pathways; and
• the overall complexity of interactions in the receiving environment that determine the chemical form of selenium or molybdenum, and the partitioning to various environmental compartments, including aquatic and terrestrial foodchains.
Selenium is one of the few elements with an established case history of significant impacts to both aquatic and terrestrial biota owing to initial loading of this element to the water column. For this reason, it has been the subject to more restrictive regulation and focused research to understand the ecological risk implications in Canada and elsewhere. It is important to understand that the documented cases of selenium-related impacts (e.g. agricultural drainage reservoirs in seleniferous regions of the western USA, and coal-affected reservoirs in various locations) are quite unique. While much has been learned from these cases, an assumption that all receiving environments will behave the same is not justified or warranted. This is evidenced by the many cases, including mine sites in Canada, that display exceedances of conservative regulatory thresholds for selenium exposure accompanied by an absence of measurable effects.
The regulatory limits for selenium have arisen from findings at problematic sites. This understanding of environmental behaviour and toxicity of selenium represents site specificity in terms of the media and organisms involved. Thus, the application of these cases for environmental protection elsewhere needs to be cautious and consider key modifying factors. Site data are presented to demonstrate different approaches for site assessment and role of modifying factors shaping the risk from selenium.
There is no body of evidence for molybdenum to suggest that similar significant impacts have ever occurred that are comparable to those attributed to selenium. The concerns regarding potential foodchain impacts of molybdenum released to aquatic systems are largely theoretical, and are based on the potential toxic effects to a particularly sensitive group of animals (i.e., ruminant). Most research and site data indicate an extremely low likelihood that molybdenum releases to water would result in any detrimental effect(s) on sensitive species of terrestrial wildlife associated with the aquatic environment. Some of these site data are presented to clarify this view.
Overall, generic extrapolation of the conservative concerns and toxicological characterizations of selenium and molybdenum are not an effective means to determine whether or not measurable effects might occur for a given aquatic release of these elements. Application of site-specific risk assessment (“SSRA”) is the best means to determine whether or not a given release of either selenium or molybdenum is likely to have impacts. This represents an emerging consensus among many researchers and informed stake-holders.
The SSRA approach can be applied to provide a degree of confidence that adverse effects will be avoided without undue conservatism or excessive preventative management efforts. For selenium, the SSRA needs to consider virtually all environmental transport processes and pathways that may ultimately be linked to the environment and food web. Generic default values of various transport parameters are identified herein, but site-specific values are strongly recommended. Critical elements of the SSRA process for selenium include:
• Characterization of the receiving environment, as the uptake of selenium into the food web is highly dependent on habitat features. Assessment of sediments is critical, as this environmental compartment can represent the key link between selenium in the physical environment and the foodchain.
• Careful selection of bioaccumulation factors (BAFs) that are appropriate for the scenario under consideration. Site-specific values should be used when feasible.
• Use of tissue-based toxicity thresholds requires a confident understanding of uptake, and must consider the receptor’s of relevance at a site and their diet.
For molybdenum, the SSRA process is not likely to require the same level of detail and pathway delineation as required for selenium. This is primarily because molybdenum exhibits a much lower propensity to bioaccumulate than selenium, and exposure via the food web is typically low relative to toxic thresholds. The primary concern is the exposure of ruminants via consumption of plants with high molybdenum content.
For the environmental management of either element, the SSRA needs to effectively consider all exposure pathways of relevance. The SSRA should not be a stand-alone tool, especially for selenium. The SSRA should be part of an overall management plan that includes appropriate site characterization, release planning, and targeted and effective environmental monitoring programs. These latter points are illustrated in case studies of mine sites in Canada.
