GROUNDWATER SOLUTE TRANSPORT MODELING: A TOOL FOR MINE TAILINGS LANDFILL DESIGN

In Wisconsin, state laws and regulations governing the permitting of metallic mines stipulate that protection of groundwater quality be assured through all phases of the mining project, including development, active mining, reclamation, and long-term closure. The Wisconsin Department of Natural Resources (WDNR) requires that mine permit applicants use solute transport models to assess project compliance with this requirement. The transport simulations are used to predict groundwater solute concentrations for comparison to existing numerical groundwater quality standards. The work presented here demonstrates that solute transport models should be used during the design-phase of mining facility development because a model is well suited to estimate and demonstrate the effect of various design features on ensuing groundwater quality.

A three-dimensional solute transport model was prepared by GeoTrans, Inc., and submitted to the WDNR by the developers of the proposed Crandon mine, a massive sulfide mine in northern Wisconsin. Our review of this model demonstrates the problematic nature of model calibration for the stated regulatory objective. Recent improvements in numerical solvers have improved the quality of model results. However, the utility of the model to predict project compliance or noncompliance with numerical standards is limited by the range of uncertainty of key model parameters, such as the source term and dispersivity.

While a lack of site-specific data prohibited detailed model calibration, the requirement for model development and ensuing technical review improved our evaluation of project design. Our review of the model included analysis of how specific components of the facility design might affect groundwater solute concentrations. The model demonstrates that changes to a proposed wastewater reclaim pond and the landfill cap would significantly improve the protection of groundwater quality. Although state regulations require that the results of the model review be used to predict compliance or non-compliance of groundwater quality to numerical standards, a substantial benefit of the review process was identification of, and appropriate changes to, design features in the proposed project that enhance groundwater protection.