8 Seepage Assessment for Waste Rock Stockpiles

Following initial wet up of a waste rock stockpile, the principal determinants of the volume of contact water reaching the base of a stockpile are the amount of infiltration that passes beyond the zone of evaporative losses at the crest of the stockpile (sometimes called net infiltration or percolation) and potential losses due to air circulation within the stockpile removing water by evaporation. Evaporative losses at the crest of a stockpile are dependent upon seasonal variations in climatic conditions and solar radiation, the temporal distribution of rainfall and snowmelt, and material properties of the matrix. Average infiltration rates are commonly reported in the range from 10 to 50% of the annual precipitation. At a given site, infiltration totals can vary widely from one year to the next due to differences monthly rainfall totals, the characteristics of individual storm events, and in the factors that control evaporative losses. For example, in northern high latitude sites, a daily rainfall of 5 to 10 mm that occurs during the peak month for incoming solar radiation in July might lead to little or no infiltration to the stockpile, while the same event occurring in mid to late Fall could lead to a significant infiltration event due to significantly lower levels of solar radiation and a consequent small evaporative loss.

Molson and others (2005) provide an early example of a numerical model developed to obtain improved insight into the nature of interacting processes controlling water movement through a waste rock stockpile and the geochemical evolution of the pore water. They represented the waste rock stockpile as a layered system responding as a granular porous medium. Lahmira and others (2017) provide a recent example of a model-based investigation of water infiltration and air circulation through a waste rock pile represented as a heterogeneous porous medium. Current state-of-the-science models are not widely applied in practice due to questions of whether the effort required to formulate and develop a numerical model adds significant value in the decision process. There remains considerable uncertainty in the quantitative description of the material properties of a waste rock stockpile and in obtaining field measurements of the key parameters to inform site-specific applications of these advanced numerical models. When they are used, it is less in a predictive mode than a “what-if” mode to explore design concepts.

A key issue of concern in evaluation of potential environmental impacts of a waste rock stockpile is the prediction of solutes loads at the base of the stockpile. As noted in Section 3.4, solute loads are determined by time-varying water fluxes and solute concentrations. The many geochemical processes that must be considered in prediction of solute loads at the base of a waste rock stockpile are reviewed in a number of papers (e.g., Amos et al., 2015; Wilson et al., 2018) and these processes are not addressed here. There is, however, a key hydrogeological factor that needs to be considered in prediction of the solute load for each potential contaminant of concern. Geochemical predictions are frequently based on estimates of geochemical weathering rates derived from standardized leaching tests carried out in the laboratory on samples where the maximum particle size is 6 mm. These weathering rates are then up-scaled to apply in the field over pile heights of 10’s of m or more. One of the scaling factors used describes the proportion of the rock mass with surface area sufficient to contribute meaningfully to release of metals and other solutes. The reactive surface area is several orders of magnitude greater for the finer particle-size fraction when compared to larger particles. A second scaling factor is the proportion of the rock mass that is flushed by water infiltrating through the stockpile. The former factor is often taken as the fraction of the waste rock that is less than 6 mm in diameter, corresponding to the upper bound particle size specified in laboratory leaching tests. The latter factor is referred to as the water to solid ratio or the water-solid contact area. The water to solid ratio is an empirical ratio between 0 and 1. This factor is difficult to estimate in practice because it varies with nature of the unsaturated flow regime within a stockpile; including how the fraction of immobile water, granular matrix zones, and preferential flow paths interact, and how that interaction varies in time. Values used in practice are usually in the range from 0.2 to 0.4, but there is considerable uncertainty in identifying an appropriate value. This topic continues to be an active subject of research (e.g., Blackmore et al., 2018; Wilson et al., 2018).