Field-scale prediction of seepage fluxes for assessment of environment impacts

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One of the key challenges in conducting environmental risk assessments for mineral resource development projects is formulating reliable estimates of subsurface water flows that have the potential to carry contaminants beyond the mine site to the receiving environment. This study involves projects designed to advance methodologies for prediction of these seepage fluxes; one project addresses prediction of seepage fluxes in fractured bedrock, the other the partitioning of water flow at the base of a mine waste stockpile where the waste contacts the pre-mining ground surface. The objective of the first project is to identify strategies for averaging local-scale measurements of the hydraulic properties of fractured rock derived from tests in individual boreholes, to estimate the bulk or large-scale hydraulic conductivity of the rock mass which controls, for example, the amount of seepage leaving a tailings impoundment in the absence of natural hydrodynamic containment. The currently exists no rigorous basis to guide this averaging process for mine sites located in a geologic setting dominated by fractured bedrock. The objective of the second project is to develop a quantitative basis for characterizing the processes that determine water pathways at the base of a mine waste stockpile. Waste rock stockpiles, if geochemically reactive, often represent the greatest single risk to the receiving environment. While considerable effort has been directed in recent year to understand infiltration into waste rock stockpiles, little attention has been directed to understanding the partitioning of soluble contaminants once they reach the pre-mining topographic surface, and are then either transported by surface water flow at the interface between the stockpile and the former ground surface, or enter the groundwater system and are carried beyond the toe of the stockpile. The study is based on an analysis of field data sets and numerical simulations to guide the interpretation of controlling processes, and development of procedures for improved assessment of the risk of contaminant release to the environment.