6 Seepage Assessment for Tailings Storage Facilities
An assessment of the quantity of seepage from a tailings facility, both during operations and in closure, requires insight to the hydraulic properties of the deposited tailings, pond geometry, recharge rates on exposed beaches in the facility, structure of the dam and its seepage control elements, and foundation characteristics. Tailings often form lower-permeability deposits that impede the release of supernatant pond water to the surrounding watershed. Some very large tailings facilities, covering many km2 in area, report outflows (unrecovered seepage in Figure 7) as low as 10 to 50 L/s. Seepage rates from tailings facilities are lower than those at water storage reservoirs with similar size and foundation conditions due to seepage impedance provided by the tailings. A tailings facility characterized as having a high seepage rate might have outflows of 200 to 500 L/s. Oftentimes, sites reporting these larger seepage rates have a water pond in direct contact with native ground around some portion of the pond perimeter where tailings might not be present to lower the seepage flux. In this latter situation, it is the hydraulic conductivity of surficial sediments and shallow bedrock that influence the magnitude of the seepage fluxes. Seepage volumes exceeding 1000 L/s are a rare occurrence except where karstic limestone is present within the footprint of the facility.
Tailings beaches play an important role in modifying seepage flows. The geometry of a tailings beach is determined by the location of spigot points where the tailings are discharged into the facility, how the water budget for the facility is managed, and the geotechnical properties of the tailings. To aid in reducing seepage rates through the foundation of a dam and at its abutments, there is a preference to develop a wide tailings beach in front of the dam, if operational conditions permit this geometry to be established. A reduction in seepage rates places a lower demand on the seepage interception system and a reduction in the volume of process water pumped back to the tailings facility from the seepage recovery system. Beach length is also a factor in enhancing dam stability by reducing the hydraulic gradient that could initiate internal erosion and piping and creating a zone of non-liquefiable tailings against the upstream face of the dam where the tailings are only partially saturated. Beach lengths vary widely between facilities; some sites operate with a beach 50 m wide, while other sites might have a beach several thousand meters wide. The tailings beach seen in Figure 1 is approximately 1400 m wide.
During project development, estimates of seepage volumes and the design of seepage interception systems are typically based on representative two-dimensional cross-section models or three-dimensional models that incorporate the dam, the tailings facility including a beach zone, engineered seepage control elements, and the surrounding terrain as part of the model structure (see Figure 15, 16). A decision on the extent of the model domain is tailored to the site-specific hydrogeologic conditions that need to be considered to address the question at hand. Because of the scale of a tailings facility and the neighboring basins where a water quality impact might be possible, some of the most detailed and complex models used in hydrogeologic practice today are developed during the assessment of seepage impacts associated with mining projects.
Once in operation, the volume of pond water reporting to the seepage collection systems is often recorded by flow meter or weir. These measurements provide important data in updating a seepage model for the facility, if one has been developed. Reliable estimation of the unrecovered seepage volume is more challenging. It is usually a small component in an overall water balance calculation for the TSF and therefore, uncertainties in the estimation of the other water balance components can mask the volume of unrecovered seepage. Reliance is often placed on either monitoring the changes in baseflow in nearby streams and dry-season flows at springs, or prediction of unrecovered seepage flows derived from a calibrated groundwater model of the local basin in which the facility is located. There are examples of tailings facilities where due consideration was not given to the potential for the geologic setting to lead to unfavorable foundation conditions and the unrecovered seepage volume was a substantial component of the water balance.