Authors: Pearce, J; Weber, P; Pearce, S; Scott, P
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Predicting the acid and metalliferous drainage (AMD) contribution from waste rock dumps (WRDs) containing potentially acid forming (PAF) material is a key step when planning for closure. For sites already demonstrating impacts from the generation and release of AMD, estimating final water quality and flow rates emanating from WRDs is key to quantifying the level of remediation and/or management required at closure. Predictions of final water quality need to be compared with regulatory limits for closure, stakeholder expectations and any anticipated treatment options (including treatment longevity and costs). In the absence of WRD sample data collected from intrusive investigations, there are often numerous WRD seeps and impacted streams that can be used to determine typical water quality, solubility constraints, flow rates, contaminant loads and thus source terms for PAF WRD drainage. The preceding step critical to the determination of source terms is the development of a conceptual model that incorporates potential/stored acidity components, flow rates and water quality. The developed conceptual model can then be further refined and strengthened with geochemical modelling. The potential acidity component, that is primarily associated with acid generating sulphides, is typically estimated from assay databases and materials placement records. Laboratory derived pyrite oxidation rates can be used to estimate the remaining potential acidity component as well as the formed stored acidity component. The mobilisation of stored acidity and other oxidation products is often constrained by solubility controls, particularly in older WRDs. These solubility controls are often associated with the formation and dissolution of melanterite-type soluble acidity, jarosite-type sparingly soluble acidity and other secondary phases such as gypsum. The determination of these mineral and/or the proportion of which they make up the estimated oxidised sulphur content allows for more accurate determination of the stored acidity component for source term derivation. Geochemical testwork can then confirm the presence of such minerals, which is incorporated into an acid base accounting modelling process and the determination of three key phases of closure water quality; (1) the draindown water quality phase; (2) the transition water quality phase; and, (3) the long-term water quality phase. During the WRD draindown phase, after cover system installation, the seepage quality can be assumed to be equal to the derived WRD source term with the duration of this phase determined by numerical modelling. Seepage quality for the transition phase is determined from the stored acidity (or metalliferous oxidation products), which also incorporates elemental loading. The long-term water quality can be determined by forward reaction path modelling or by using key mineral dissolution kinetics (first principal approach). Combining these three phases then produces a model for the prediction of long-term water quality after operations, which can be utilised for closure planning. This paper presents a number of case studies that utilise the above methods for the prediction of site water quality at closure.

Keywords: AMD, WRD loading, waste management

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