Spatial Mapping of Acidity and Geochemical Properties of Oxidized Tailings within the Former Eagle/Telbel Mine Site

Abstract

At some orphaned and abandoned mine sites, acid mine drainage can represent a complex challenge due to the advanced tailings' oxidation state as well as the combination of other factors. At the field scale, several parameters control sulfides' oxidation rates and, therefore, the acidity generation. The objective of this paper is to map the acidity and geochemical properties of oxidized tailings within a closed tailings storage facility. Based on systematic sampling, various geochemical parameters were measured within the oxidized Joutel tailings, including the: Neutralization potential, acid-generating potential, net neutralization potential, neutralization potential ratio, paste pH, thickness of oxidized, hardpan, and transition zones. The different parameters were integrated in geographical information system (GISs) databases to quantify the spatial variability of the acidity and geochemical properties of oxidized tailings. The oxidized tailings were characterized by low sulfide (mainly as pyrite) and carbonate (mainly as siderite/ankerite) contents compared to unweathered tailings. Acidic zones, identified based on paste pH, were located in the eastern portion of the southern zone and at the northern tip of the northern zone.Minerals 2019, 9, 180 2 of 23 released [9][10][11]. This phenomenon is known as acid mine drainage (AMD) or acid rock drainage (ARD). In some cases, drainage waters can be near-neutral in pH and still contain concentrations of elements that surpass environmental standards and regulations. This phenomenon is known as contaminated neutral drainage (CND) [12] or metal leaching (ML). The cost of mine site reclamation is influenced by the type and quality of drainages produced in TSFs [13].The process of sulfide oxidation can result in the generation of protons. This increases the acidity of pore waters and leads to the dissolution of carbonates, which buffer the acid. Depending on the geochemical conditions in the pore water, various reactions may occur, such as: Hydrolysis, precipitation,. During these reactions, the tailings' mineralogical composition can change significantly depending on the dissolution rates of sulfides, carbonates, and, to a lesser extent, silicates. As a result of oxidation, neutralization, and hydrolysis reactions, novel phases may precipitate. The most common secondary phases are ferric oxyhydroxides (e.g., goethite, ferrihydrite, lepidocrocite), gypsum, and iron sulfates. These secondary phases affect the geochemical behavior of tailings considerably [10].Ferric oxyhydroxides are known for their high affinity to limiting contaminant mobility through mechanisms, including co-precipitation, adsorption, and substitution. Under some specific conditions, the occurrence of secondary phases may also affect the hydrogeochemical behavior in TSFs by modifying the tailings' hydrogeological properties, such as porosity. For example, the formation of hardpan within TSFs affects the water balance and flow paths [18,19]. Hardpan formation is more likely to occur at orphaned or aban...

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