The Vangorda and Grum open pit mines operated from 1990 to 1998. This report compares pre-approval acidic mine drainage modeling predictions, actual monitoring data and post-development modeling predictions. The results of the comparison provide insight into the effectiveness of prediction methodologies and guidance for general application of these methodologies.
The Vangorda and Grum mines provided additional ore feed for the mill at the adjacent Faro Mine, developed in the late 1960s. Two pits and three waste rock dumps comprise the major components at the Vangorda and Grum mine site. One waste rock dump contains overburden and is not considered a source of significant contaminant loading. All of the components fall within the Vangorda Creek watershed, a creek unaffected by the Faro Mine.
Based on experience at the Faro Mine and other sulphide base metal mines, regulators and the project developer (Curragh Resources Incorporated) recognized the potential for acidic mine drainage at the Vangorda and Grum mines. As a result, documentation to support project approval included predictions of acidic mine drainage. Monitoring programs during mine operation and after closure tracked water quality throughout the site. In 2003, the Canadian and Yukon governments assumed responsibility for mine reclamation at this site. As part of closure planning, they commissioned updated modeling for acidic mine drainage. This series of modeling and monitoring throughout various phases of the mine lifecycle provides an opportunity to evaluate the effectiveness of acidic mine drainage prediction methodologies.
Pre-development and post-operational modeling followed similar approaches for waste rock sources. Both models assessed receiving water quality by utilizing water and contaminant load balances that incorporated contaminant loads from key sources including pits, sulphide cells and waste rock dumps. The modeling approaches both relied on empirical data for predicting the long-term geochemical performance of source materials, using this information to predict seepage and runoff concentrations for both expected and worst-case conditions. The pre-development modeling utilized empirical data from the adjacent Faro mine, while the post-operational predictions utilized empirical data from the Vangorda and Grum mines. Both exercises considered the results of humidity cell tests, using the results for confirmation purposes. In both cases, the humidity cells predicted worse conditions than the empirical-based models, but the adversity of these conditions did not lead to revision of modeling inputs and assumptions. The empirical approach for predicting waste rock contaminant loads appears to substantially underestimate loading in conditions where the empirical data are not reflective of well-developed acidic mine drainage conditions.
For pits, the pre-development and post-operational predictions relied on substantially different approaches and data sources. The pre-development predictions were based on water quality data from the Faro mine combined with theoretical predictions of inflow rates. The post-operational predictions relied on empirical data to develop water balances for the pits and understand seepage water quality. For sulphide materials, the pre-development predictions substantially underestimated the loading from pit walls.
The comparison of pre-development modeling, monitoring results and post-operational modeling for acidic mine drainage at the Vangorda and Grum mines leads to some conclusions that can help guide future mine planning.
● Modeling results are most sensitive to the predictions of contaminant concentrations from key load sources, though flow rates through waste rock materials are also important.
● Reliance on seepage data from existing facilities as an empirical input for modeling should be done with caution because these data could underestimate the future concentrations and loading.
● The results of laboratory testing (i.e. humidity cells) should be considered carefully. When modeling that utilizes laboratory testing indicates conditions more adverse than those predicted by modeling that uses empirical data, the laboratory based modeling approach may warrant further consideration especially when the empirical data are from sites where acidic mine drainage may not be fully developed, or where the loading may not have reached sampling locations.
● Changes in mine plans and failure to effectively implement key mitigation measures can lead to significant increases in contaminant loading above those predicted in modeling exercises. Measures that are intended to help address future water quality issues are critical and mechanisms need to be in place to make sure they are completed. As mine development progresses and mine design evolves, water quality predictions need to be verified and updated based on monitoring data.
