History matching and predictive modelling of TMA operations at a postash mine site in Saskatchewan

Development of decommissioning plans by the potash mine sites in Saskatchewan require the assessment of long-term brine impact to the environment and to the aquifer systems in the vicinity of potash tailings management areas (TMA). Three-dimensional numerical simulations provide the only means of providing credible predictions of groundwater flow and contaminant transport for TMAs. The objective of this study was to use a 3D groundwater flow and transport model to match the operational history of a selected mine site and estimate the extent of long-term brine migration from the TMA within the groundwater flow system. Geological, hydrogeological, and physiographic data were collected, compiled, and simplified to formulate a conceptual model of the site. This conceptual model was used to construct a 3D mesh that incorporated this information and could be used to simulate groundwater and contaminant transport. Conceptualization of the groundwater flow system and brine migration from the site necessitated the utilization of a 3D saturated-unsaturated, variable-density, flow and transport code. FEMWATER, a finite element method code, developed for the U.S. Environmental Protection Agency (EPA) and maintained by U.S. Army Engineer Waterways Experiment Station (WES), was selected to simulate groundwater flow and contaminant transport. The steady-state model was developed using data that pre-dated mining activity and quantifies the natural flow system in the modeled area. Three aquifers are important in the natural system at the selected mine site: 1) a surficial sand aquifer; 2) the Floral lntertill Aquifer; and, 3) the Hatfield Valley Aquifer. The surficial sand aquifer is an important shallow aquifer in the immediate vicinity of the TMA, receiving an estimated 1,200 m3/d recharge from natural infiltration over the entire model region. Most of the flow in this shallow system discharges locally to surface streams and sloughs. The Floral lntertill Aquifer is a confined local aquifer at considerable depth (> 50 m) below the TMA with a relatively low natural inflow/outflow estimated at 300 m3/d (for the modelled area). The aquifer heads are near ground surface in the Floral lntertill Aquifer. Wells in this unit can be flowing artesian, particularly to the south of the TMA. The Hatfield Valley Aquifer is a major regional aquifer and is located 10 to 20 m below the Floral lntertill Aquifer beneath the TMA. The Hatfield Valley Aquifer has much larger inflow/outflows than the other aquifers in the modelled region, estimated at 3,300 m3/d. Flow directions in the deep aquifers are generally to the south and southeast. There is a predominantly upward gradient from the deep aquifers over most of the modelled domain. History matching and predictive results must be regarded as first or second estimates rather than "fits" or calibrated transient solutions. A major factor considered at the TMA was porewater pressure, generated by the loading applied to the system by the tailings pile. The transient calibration process is ongoing and future improvements are expected, nevertheless the preliminary results are in reasonable agreement with observational data and the model is regarded as an effective tool for comparative evaluation of alternatives. Brine migration depends primarily on the advective velocity field in the more permeable aquifer units. In the thick tills beneath the site, porewater pressure responses to loading by the tailings pile are large, but density effects and diffusion (rather than advection) are likely the controlling factors on brine migration. Overall, lateral gradients generated by porewater pressure dissipation in the aquifers appear to have more influence on directing brine migration than the larger excess porewater pressures in the aquitards. In the history-match and predictive simulations, brine advances laterally in the surficial sands as a shallow advective plume and predictions suggest that the slurry wall and drainage ditch containment systems are effective in intercepting such movements. The thick till sequence inhibits downward movement of brine and the deeper aquifers are unaffected by brine in the 100-year model-timeframe. Large changes in the natural flow system have been induced by the mining operation as a result of pile loading and freshwater pumping from the Hatfield Valley Aquifer. In respect to groundwater flow, the impacts of the mine site extend from the surficial sediments to the deep aquifers and aquitards. Despite these large and widespread changes in flow patterns, contaminant migration impacts approximately 380 hectares outside the footprint of the TMA to an approximate depth of 20 m after 100 years. Note:Missing: Appendix D title page

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