7 Seepage Management During Operation of a Tailings Facility
Seepage control measures at a tailings dam serve two critical roles; the first is to ensure the stability of the dam and the second is to reduce the volume of pond water that can pass beyond the dam through the foundation. A tailings dam will often have an internal drainage system and toe drain to maintain a low water table in the downstream shell of the dam; this water is collected by the drainage system and either returned to the tailings facility, treated and released, or directly released to the environment if water quality considerations are met. Depending upon foundation characteristics and dam design, a low-permeability cutoff might be installed to aid in protecting the clay core of the dam from erosion (piping) due to excessive groundwater velocities. This cutoff also has a benefit in reducing the volume of pond water that can pass through the foundation and migrate beyond the downstream toe.
When a cutoff is proposed to reduce seepage bypass through fractured bedrock, a grout curtain is commonly considered. A line of boreholes is drilled to the target depth, usually on a spacing of about 3 to 5 m. In some countries, practice guidelines are established for grout curtain installation. In other jurisdictions, the designer develops a site-specific plan for placement of the grout curtain. Cement is injected into each borehole, first in a short interval at the bottom of the borehole and then moving the injection zone up the length of the borehole. Cement takes (kg of cement per length of injection interval) are recorded as a key matric for identifying the more permeable zones in the borehole. It is often the case that a second row of injection holes, offset from the first, is required to get the specified reduction in permeability. Three and sometimes even four rows of grout holes might be needed to reach design targets for permeability of the cutoff. Check holes orientated at an angle across the curtain are used to confirm the design target for hydraulic conductivity has been met. A grout curtain with an effective hydraulic conductivity of 10-7 m/s is usually considered a reasonable target in fractured bedrock. A value of this magnitude, under normal hydraulic gradients, results in low seepage volumes through the curtain if the curtain is laterally continuous.
A second line of defense on water release to the environment is the construction of a seepage collection pond at a location downstream of the toe of the dam (see Figure 7). The seepage collection pond might also incorporate a grout curtain as a cutoff to promote upward flow of contact water to surface where the seepage can then be directed to the collection pond. In permeable soft sediments, a trench backfilled with low-permeable soils amended with bentonite is commonly used to construct a cutoff wall. In this case, the hydraulic conductivity of the cutoff wall, if properly installed, can be as low as 10-9 or 10-8 m/s. Contact water reporting to the seepage collection pond is usually pumped back to the tailings storage facility or piped to the mill for re-use. Not all tailings facilities incorporate seepage collection ponds beyond the toe of the dam; but their use has become more common as greater attention has been paid to reduction in the risk of potential downstream impacts on water quality.
Tailings deposits have a hydraulic conductivity that will act to limit seepage rates through the base of the facility if the hydraulic conductivity of the foundation unit is higher than that of the tailings deposit. This can occur, for example, when the facility is located on a permeable, sandy till unit or moderately to highly fractured bedrock. At some sites there could be a requirement to further reduce seepage from the tailings facility due to water quality considerations, in which case a geomembrane liner can be placed at the base of the facility. Figure 18 shows a lined tailings storage facility.
Geomembrane liners are not impermeable; installation defects will invariably occur, even with a good quality control/quality assurance program. Experience indicates that the hydraulic conductivity for flow through a geomembrane liner, placed on a properly prepared base, can be expected to be in the range of 10-11 to 10-10 m/s. As an alternative to a geosynthetic liner, some operations control seepage into the foundation by placement of a compacted, low-permeability soil liner 1 to 3 m in thickness prior to startup of operations. A decision on liner type depends upon soil availability, relative costs of different liner systems, and the required degree of seepage reduction. Construction control during placement of a synthetic or soil liner is key to successful use of this method for seepage management.
Figure 18 – Geomembrane liner installed in a tailings storage facility. At the time of this photograph, water was being accumulated in the facility for startup of mill operations.
The permissible amount of contact water that bypasses a seepage collection system (unrecovered seepage) is regulated under various frameworks. In some jurisdictions, a permissible seepage rate to the environment is specified, such as a value not to exceed 5 L/s. The magnitude of this number is typically determined through studies of the assimilative capacity of the downstream receiving waters. In other instances, the allowable seepage is linked to a requirement to honor a maximum permissible concentration of key elements at specified compliance points. For example, the maximum manganese concentration in surface water at a compliance point several hundred meters beyond the toe of the facility, or in a groundwater monitoring well at a compliance point, cannot exceed, say 2 mg/L. In other instances, the tailings facility might be declared as a zero or de minimis discharge facility. This circumstance requires groundwater interception and monitoring systems be in place at all locations where natural hydrodynamic containment does not exist.
If unrecovered seepage is moving down valley beyond the seepage collection pond with concentrations of one or more solutes above natural background values, then additional groundwater interception measures might be required. This will commonly involve the installation of one or more groundwater wells to capture the process-affected water. Beyond the normal design considerations for the interception wells and associated monitoring wells, a key point of discussion becomes what the capture efficiency of the interception well system will need to be in order to meet environmental standards. For example, is it necessary to capture 90% of the impacted groundwater in the well system, or will the efficiency need to be greater than, say 98%? A prerequisite to achieving high capture efficiency is the identification of secure seepage collection points in the basin downstream of the dam. This step requires reliable definition of preferential seepage pathways. In this regard, seepage interception in a basin underlain by limestone with karst development is prone to a much higher degree of uncertainty than is the normal circumstance because of the inherent difficulty in identifying all the potential pathways.
Seepage interception systems are generally implemented within the framework of an adaptive management plan, with monitoring data used to guide refinement in the interception well network through time. The more heterogeneous the hydrogeologic setting, the more challenging it becomes to design and operate a system at high capture efficiency. Verification of capture efficiencies is challenging. It is often based on concentration measurements in receiving surface waters rather than from subsurface concentration data obtained from monitoring wells.