The Importance of Groundwater in the Mining Life Cycle
by: Christoph Wels, Ph.D., M.Sc.
The life of a mining project can be viewed as a sequence of developmental stages: starting with exploration and feasibility studies, followed by start-up and operation and finally concluding with reclamation and closure. During each phase of the mining life cycle, the miner is confronted with unique challenges and opportunities related to the presence of groundwater. This paper examines the various hydrogeological issues that are commonly encountered during the life of a mining project.
The following hydrogeological topics will be covered in this review:
- Baseline Characterization
- Mine Dewatering
- Water Supply for Processing
- Tailings Seepage Control
- Contaminant Migration Control during Mining
- Mine Closure & Aquifer Remediation
- Post-Closure Aquifer Protection
The early phases of project development, characterized by exploration and feasibiity studies, present an opportunity to study the hydrogeological conditions encountered at the proposed mine site under natural (or pre-mining) conditions. This information is critical for an assessment of (i) available groundwater resources for mill and drinking water supply, (ii) potential dewatering rates into the open pit and/or underground workings, and (iii) pre-mining groundwater levels and water quality. The last aspect is critically important for future environmental assessments as it provides a benchmark against which to assess the environmental impact of the mining project on groundwater during operation and to judge any control and/or remediation efforts, which might be implemented during operations, at closure and/or post-closure.
The requirements for the scope of a baseline characterization study depend on the regulatory framework, the scope of the proposed mining project and the local hydrogeological conditions. A baseline characterization study typically includes the following activities:
- Geological & Structural Mapping;
- Drilling and Monitoring Well Installation;
- Groundwater level and Streamflow Monitoring;
- Water Quality Monitoring (groundwater, springs & streams);
- Climate Monitoring (precip, evaporation, temp etc.);
- Hydraulic Testing (pump/slug tests, packer tests); and
- Preliminary Flow Analysis.
In most projects, significant savings can be realized if the baseline characterization study is coordinated with the exploration drilling program. For example, exploration holes can often be completed as water level and/or water quality monitoring wells for only a small additional cost. Similar synergies exist between the hydrogeological baseline characterization study and the data requirements for an environmental assessment (e.g. drill cuttings may be essayed for ABA to determine ARD potential in the local rock). As a result the initial characterization studies during exploration and feasibility assessments should be planned by an interdisciplinary team familiar with mine exploration, mine engineering, and environmental issues related to surface water and groundwater.
The extent of interpretation of the field data and flow analyses again depend on local regulatory requirements and the mining project. However, it is customary to obtain at least preliminary estimates of aquifer properties (permeability, storativity, potential yield) and potential groundwater inflows to the open pit and/or underground mine. The hydraulic properties of the aquifer are often estimated from the hydraulic testing results using Aquifer Testing Software (see Software Tools). Preliminary estimates of groundwater flow and potential inflow into a mine can be obtained by using analytical solutions (see Flow Analysis Tools).
Please contact Christoph Wels if you would like to feature an interesting example of a baseline characterization study for a mining project on this website.
Return to Introduction.
In most mining projects, some active dewatering of the immediate vicinity of the open pit and/or underground workings is required ahead of, or in parallel with, mining. The importance and extent of mine dewatering varies greatly and can range from a single pump back station at a sump pumping less than 100 gallons per minute to complicated dewatering schemes involving dozens of dewatering wells pumping several thousand gallons per minute. In some instances, the cost of mine dewatering can be so prohibitive, that it renders the entire mining project uneconomical.
Mine dewatering is carried out for several reasons including:
- dewatering of the ore and/or waste rock actively mined;
- maintenance of dry pit floor and/or underground workings for operations;
- prevention of sudden flooding and/or mud rushes;
- release of pore pressures in open pit slopes and/or underground workings to prevent failure of pit benches or u/g workings.
Methods of mine dewatering depend on site access, type of mine (open pit, u/g mine), required depth of dewatering, and aquifer conditions (in particular yield and degree of heterogeneity). In general, dewatering of a mine can be achieved by "active" dewatering of the aquifer using dewatering wells and/or drain holes specifically designed for this purpose or by "passive" dewatering where the groundwater inflow to the mine workings is managed but not actively promoted. In many mining projects, "active" dewatering is required early during the early stages of the life of mine and eventually switches to "passive" dewatering as the amount of groundwater inflow declines. This scenario is particularly common in dewatering of unconfined aquifers where the majority of groundwater inflow comes out of storage as opposed to regional groundwateer flow.
In most mining projects, the process of mine dewatering is an iterative process involving the following steps:
- Aquifer Characterization (often done as part of the initial Baseline Characterization Study and/or Water Supply Study;
- Design & Implementation of a Full-Scale Pump Test;
- Development & Calibration of a Groundwater Flow Model;
- Initial Design of a Dewatering System (using the calibrated flow model);
- Drilling of Dewatering Wells and/or Drainholes;
- Initial Performance Monitoring of Dewatering System;
If the dewatering system does not perform as predicted, steps (3) to (5) are repeated, i.e. the groundwater flow model is updated with the information gained during initial performance monitoring. This updated model is then used to select the best location and screening interval for additional dewatering wells and/or drainholes. With the recent advances in numerical model and software development (see Model Tools and Software Tools) the assistance of a groundwater flow model in the design and operation of a mine dewatering system has become commonplace and economical. In many instances, the entire cost of modeling is offset if this modeling exercise can save only one or two unproductive dewatering wells (considering drilling, installation and equipment costs).
In general, the use of drain holes is preferred over the use of pumping wells, because of cost savings and inefficiencies in pumping from a well (well losses). However, drainholes (whether inclined or vertical) require a natural gradient which may not be always available. As a rule of thumb, dewatering of an open pit mine is typically achieved by means of pumping wells whereas underground mines more commonly utilize drain holes which are drilled during exploration and drift development. However, exceptions are abound. For example, dewatering of the Grasberg Open Pit, one of the largest open pit mines in the world, is achieved by a series of subvertical drainholes drilled from drifts developed beneath the open pit.
All groundwater discharging into the mine (either via drainholes or via the underground workings) is collected in central sump stations from where it flows towards a portal or is pumped back to surface. Depending on the quantity and quality of groundwater collected in the underground workings an elaborate system of open channels and/or pipes may be requireds to transport the groundwater to the pump back station. Larger underground mines may have several pump back stations, located at strategic locations and depths.
In most mining projects a dewatering monitoring program is implemented. The scope of this monitoring program depends on the local regulatory requirements and the importance of groundwater to the project (both in terms of operations and potential environmental impact). The following parameters may be monitored:
- Pumping rates of pumps used in mine dewatering;
- Dynamic water levels in pumping wells;
- Discharge of groundwater into mine (from drain holes and/or fractures);
- Water quality of groundwater collected in the mine;
- Water quality of mine water discharged from the mine; and
- Groundwater levels in the local aquifer(s) influenced by mine dewatering.
The main purpose of this monitoring program is to monitor progress of mine dewatering and to document the volume and quality of groundwater pumped from the mine. In addition, a detailed monitoring program (including groundwater levels in the local aquifer) provides valuable information about the impact of mine dewatering on the local groundwater system (e.g. drawdown). This information can be very useful for estimating rates of mine reflooding at the end of mining.
Forward to Water Supply for Processing (under development).
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