3.6 Use of Groundwater Simulation Models in Mining Practice
Groundwater simulation models play an important role in mine design and in the assessment of potential impacts of mine development on the surrounding environment. The most frequent applications at a mine site are in the evaluation of depressurization requirements for an open pit, prediction of underground dewatering requirements and mine inflows, and in seepage assessments at a tailings storage facility. Groundwater models are also applied in geotechnical stability assessments as pore pressures influence of the shear strength of the materials within embankments and in cut slopes. In some jurisdictions, it has become common practice to require the development of a numerical model of the groundwater flow system as part of the process of obtaining a license to construct and operate a mine. Figure 15 illustrates a three-dimensional groundwater model developed to assess groundwater flow paths for a proposed tailings storage facility. Models constructed with more than one million nodes are becoming common.
Figure 15 – Example of a finite element grid developed to assess pre-development groundwater flow paths in the vicinity of a proposed tailings storage facility. The tailings facility would be located in the central region of this model domain. The different colors map distinct hydrostratigraphic units identified in the conceptual model of the site. The red traces are inferred faults in the area.
A groundwater model generally serves two functions: (i) as a tool to aid in the interpretation of field data and to refine conceptual understanding of the hydrogeological system and system response to changed conditions associated with mine development; and (ii) a decision support tool to aid in the evaluation of design options. In common with all applications of groundwater simulation models, the issue requiring the greatest attention is the formulation of a sound conceptual model of the hydrostratigraphy and groundwater flow system, which then gets translated into an analytical or numerical simulation model. With sophisticated and user-friendly simulation codes widely available, the tendency in practice is to see reliance on numerical modeling approaches, with simplified analytical solutions oftentimes being applied as first-order checks on the reasonableness of the predictions developed using numerical models. Modeling applications typically focus on the analysis of the groundwater flow system, with particle tracking procedures used to characterize advective transport pathways for mine-contact water (Figure 16). However, it is becoming more frequent to encounter regulatory requirements that solute transport models be applied in the prediction of solute concentrations at prescribed compliance points. These transport models are linked to the groundwater flow model.
Figure 16 – Example of using particle tracking to characterize groundwater flow pathways (orange traces) from a proposed tailings storage facility (red outline with two containment dams). Particle tracks do not indicate the magnitude of the seepage flux. This figure provides an example of an impoundment, when filled to ultimate capacity, would not have natural hydrodynamic containment around the full perimeter of the facility. Due to the strong influence of geologic structure and compartmentalization on groundwater flow pathways at this site, most of the contact water is predicted to ultimately report to one of two discharge sites.
Simulation models are also used in the design of a management system for a water pool present in flooded underground workings. For example, if the water pool in the mine is contaminated it may be necessary to establish a hydraulic sink using one or more pumping wells to prevent offsite migration. This could be the case if there are near-surface mine wastes that release solutes that are then carried down to the underground workings by groundwater flow. There are numerous sites where the underground workings are used as a water storage facility as part of the mine water supply. Simulation models can aid in identifying an efficient design to operate such a system.
With the widespread adoption of numerical simulation models as an aid in the assessment of mineral resource development projects, several jurisdictions have seen an advantage in formulating practice guidelines to promote sound application of groundwater models. One such example is “Guidelines for Groundwater Modelling to Assess Impacts of Proposed Natural Resource Development Activities”, released by the province of British Columbia in Canada in 2012 (Wels et al., 2012). This report provides a comprehensive description of the processes and issues involved in applying numerical groundwater models within the context of mineral resource development. This report includes guidance in conceptual model development, code selection, model setup, model calibration and verification, model prediction, sensitivity and uncertainty assessment, and review. Numerous examples are included illustrating model setup and prediction. A similar-styled document “Australian Groundwater Modeling Guidelines”, released in 2012 and intended for a range of applications in addition to mining, is used in the Australian mining industry to establish the expected standard for model applications. Guidance on consideration of uncertainty in hydrogeologic models used for prediction of environmental impacts of mine development and for use of groundwater models in risk assessments can be found in Middlemis and Peters (2018).