Waste Rock Storage Facilities (WRSFs) can be a significant source of contamination, especially in long-term mine closure scenarios. Mine wastes can oxidise and leach metals through various processes, including the oxidation of sulphides and release of metals, a process referred to as Acid Rock Drainage (ARD). These contaminants can eventually reach the base of the waste dump and emerge as surface water or migrate into groundwater.
WRSFs are generally unsaturated deposits with highly heterogeneous materials. Depending on the intensity of inflows and the layering of material within the WRSF, infiltrating water will take preferential flow paths. Therefore, not all of the rock mass in a WRSF will come into contact with infiltrating water, and not all inflow pathways will be comparable. Only materials within zones of preferential flow paths will contribute to the solute load downgradient, while those accessing fine-grained paths provide higher concentration loads.
Understanding the proportion of rock mass in contact with mobile water and the differing types of flow processes is crucial for determining the potential overall rate of solute mobilisation and the resulting impacts downstream. Typically, geochemical modelling approaches often make simplistic assumptions about fluid contact with waste rock. The contact mass is typically given as an assumed contact factor to fines material, which is not directly linked to rainfall or infiltration data. The flow that may pass through macropore routes is typically not assessed. This approach limits the remediation scenarios that aim to reduce infiltration, such as the application of soil covers, which may also change flow pathways and waste rock mass in contact with water. To better characterise hydrologic processes within a WRSF, studies must account for factors like climate and weather conditions, cover types, dump geometry, particle size distribution and deposition structure. Often however, assessments are made of proposed sites where measurements and samples cannot be taken or existing sites where data availability is nonetheless limited.
To support the assessment of water flow through WRSFs, particularly new or existing facilities where data may be lacking, SRK is putting together a set of guidelines and a parameter database tool. The guidelines will give advice for detailed assessment and inclusion of cover systems, as well as methods for high-level initial appraisal. The parameter database will be compiled from documented studies and enable users to see information from similar WRSF designs or climates to their project, which may be useful as inputs to their calculations.
