Using spatially explicit data, geochemistry and modeling to inform ecological risk assessment for a proposed pit mine expansion

Background/Question/Methods

A screening level ecological risk assessment (SLERA) was conducted for a proposed Pit Lake, an expansion of an existing Pit Mine owned and operated by Newmont Mining Corporation in the arid Great Basin ecosystem of northwestern Nevada. As is typical for SLERAs, very conservative assumptions for both toxicity and exposure to the ecological community were initially used to evaluate the potential for unacceptable risk. As a result, the initial screening approach provided minimal information to refine the list of chemicals of interest needed to focus risk management and reduction strategies. To provide a more complete evaluation of potential risk, expanded and spatially explicit modeled and empirical site data was incorporated into the assessment for multiple media including soil and expected future sediment and surface water conditions. These data were used in conjunction with an expanded list of toxicity values and exposure pathway models to evaluate risks to ecological receptors at the site. Multiple exposure scenarios were used to evaluate both baseline risks and potential mitigation of risks using shallow surface material with lower chemical concentrations in order to reduce exposure to areas with higher levels of chemicals of interest.

Results/Conclusions

The results of exposure modeling indicated that a handful of metals, including aluminum, antimony, arsenic and mercury, showed potential for risk to some aquatic ecological receptors. However, an evaluation of geochemical conditions helped to further inform the risk assessment by showing that expected conditions at the future Pit Lake are likely to limit bioavailability to ecological receptors, reducing the potential for unacceptable risk. The results of this assessment illustrates the importance of a robust, spatially explicit data set, an in-depth understanding of site-specific geochemistry, and consideration of site management alternatives that may be available, in order to develop an accurate understanding of the potential for unacceptable ecological risk at mining sites.