Acid and metalliferous drainage contaminant load prediction for operational or legacy mines at closure

Abstract

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 f...

Extremely Acidic Eukaryotic (Micro) Organisms: Life in Acid Mine Drainage Polluted Environments—Mini-Review

“…Around the world, there are mines that have been abandoned and pose a long-term threat to aquatic ecosystems due to the continuous or intermittent flow of acidic drainage water containing high concentrations of various heavy metals [ 23 ]. AMD is predominantly caused when sulfide minerals present in metallic ores, coal beds, or the strata overlying and underlying the coal are exposed to weathering causing oxidation [ 24 , 25 ], which later on is propagated through indirect oxidation by ferric ions produced mainly by chemolithotrophic bacteria [ 26 , 27 ]. Chemical reactions such as hydrolysis and oxidation can transform sulfide minerals into sulfuric acid, decreasing the pH of water at active or abandoned mine sites [ 28 ].…”

Biocorrosion of Carbon Steel under Controlled Laboratory Conditions

“…In contrast, microbes dramatically increase the rate of this reaction by as much as six orders of magnitude, depending on pH [22]. The microbial oxidation of dissolved Fe 2+ to Fe 3+ is a key to understanding the oxidation of sulfide minerals because the redox potential of the ferrous/ferric iron couple is related to pH and is most positive in extremely acidic environments (Eh 1 4 +770 mV at pH 2), implying that ferric iron is an attractive alternative electron acceptor to oxygen in low pH environment [23][24][25]. On the other hand, it cannot be ignored that the inoculum used contains oxidized iron as it comes from waters affected by AMD from the Agustin River.…”

Global acid and metalliferous drainage management standard: BHP’s approach to reducing global acid and metalliferous drainage closure risk

“…The processes involved in generating and releasing AMD are well described (Lottermoser 2010;Miller 2014;Pearce et al 2016a). Due to BHP's global span, it is important to clearly define AMD as the acronym can have various meanings in different regions.…”

“…Acid and metalliferous drainage (AMD) is recognised internationally as one of the most difficult and significant environmental issues facing both miners and regulatory bodies in the pursuit of a sustainable mining industry. This significance is due to the ability of AMD pollution to persist for hundreds, if not thousands of years (Pearce et al 2016a). There are numerous international examples where this ability to persist many years after the cessation of mining operations is demonstrated.…”