Risk Assessment Methodology for Pit Lakes Instabilities

In this study we present a generic probabilistic risk assessment methodology to evaluate the risk associated with flooding process of a pit. We use the bow-tie analysis to analyze the critical events (we focus on slope failures) and the systemic risk assessment methodology to estimate the risk for the population, for the environment and for the infrastructure. Furthermore, we perform a spatial analysis of the risk by discretizing the affected area into squares, by estimating the risk in each one and finally by creating the risk map. The methodology is implemented by specialized software that has been created in a Matlab environment for the deduction of such risk assessments. The developed methodology was applied in the area of the pit lake Most in Czech Republic.

Preliminary results of investigations related to the ecology of Urdari open-pit lake

Flooding the remaining gaps of lignite open-pits and the formation of so-called open-pit lakes tends to become an increasingly common practice in Romania (in the Oltenia Mining Basin, where the lignite deposit contained in Dacian and Romanian formations was and is extracted through open-pits). This type of ecological reconstruction of former exploitation areas has been applied worldwide for quite a time, and there are many references in this regard in the scientific literature. In this context, the present paper aims to present some preliminary results regarding the ecology of these lakes, being taken as case study the lake formed in the remaining gap of Urdari open-pit, within the Rovinari Mining Basin. However, given the multitude of similar conditions (bioclimate, geological framework, the formation of the remaining gaps, etc.) in which they form, we appreciate that the conclusions of this study can be extrapolated for other lakes in the area (eg. South Peșteana). We point out from the outset that this study presents data on the existing biotope and biocenoses (restricted to species identification), without going into details about populations, ecosystem production, energy and matter flows within it, these investigations being foreseen for a future development stage of the present study.

Growth performance of Tilapia (Oreochromis niloticus) cultivated in water from ex-sand pit lakes by phytoremediation treatments

Utilization of ex-sand pit lakes for aquaculture is difficult due to low water quality and high concentrations of iron (Fe). Phytoremediation using aquatic plants has been proven to be effective in reducing Fe in waters. This study aims to determine the growth, feed conversion efficiency and survival rate of tilapia (Oreochromis niloticus) cultured with ex-sand mining water media with phytoremediation treatment. Phytoremediation treatment was carried out by Eichhornnia crassipes, Azolla pinnata, and Salvinia molesta. Fish culture experiments were carried out in plastic tarpaulin tanks for 40 days, with ad satiation feeding, three times a day using the commercial feed. The results showed that phytoremediation with aquatic plants had succeeded in reducing Fe to a level suitable for fish culture. Fish culture experiments showed an absolute length growth rate of 0.09-0.18 cm/day and an absolute weight growth rate of 0.11-0.16 g/day. The feed conversion ratio was 1.18-1.40 and the survival rate was 98.04-99.08%. The survival rate of tilapia is high, the feed conversion ratio is medium and growth is low. The high environmental temperature and the decrease in water quality due to the absence of water changes and aeration are suspected of causing the low growth of fish. Therefore, further research with water change and aeration experiments and the use of other species of fish is needed to follow up the results of this study.

Isotopic and Chemical Assessment of the Dynamics of Methane Sources and Microbial Cycling during Early Development of an Oil Sands Pit Lake

Water-capped tailings technology (WCTT) is a key component of the reclamation strategies in the Athabasca oil sands region (AOSR) of northeastern Alberta, Canada. The release of microbial methane from tailings emplaced within oil sands pit lakes, and its subsequent microbial oxidation, could inhibit the development of persistent oxygen concentrations within the water column, which are critical to the success of this reclamation approach. Here, we describe the results of a four-year (2015–2018) chemical and isotopic (δ13C) investigation into the dynamics of microbial methane cycling within Base Mine Lake (BML), the first full-scale pit lake commissioned in the AOSR. Overall, the water-column methane concentrations decreased over the course of the study, though this was dynamic both seasonally and annually. Phospholipid fatty acid (PLFA) distributions and δ13C demonstrated that dissolved methane, primarily input via fluid fine tailings (FFT) porewater advection, was oxidized by the water column microbial community at all sampling times. Modeling and under-ice observations indicated that the dissolution of methane from bubbles during ebullition, or when trapped beneath ice, was also an important source of dissolved methane. The addition of alum to BML in the fall of 2016 impacted the microbial cycling in BML, leading to decreased methane oxidation rates, the short-term dominance of a phototrophic community, and longer-term shifts in the microbial community metabolism. Overall, our results highlight a need to understand the dynamic nature of these microbial communities and the impact of perturbations on the associated biogeochemical cycling within oil sands pit lakes.

A Review about the Sustainability of Pit Lakes as a Rehabilitation Factor after Mine Closure

At the end of surface mining activities, the remnant voids are of great concern regarding rehabilitating the final open pits. The investigation of the sustainability of pit lakes in post-mining regions constitutes a challenging research problem. This paper aims to highlight the effectiveness of pit lakes as a rehabilitation factor. In this framework, several cases worldwide and in Greece were examined in detail and evaluated. The results indicate that mine pit lakes must be evaluated as dynamic systems, natural or artificial, which demand rational mine water management to ensure their sustainability. Specifically in Greece, it is of great importance during the transition to the post-lignite era.

Are pit lakes an environmentally sustainable closure option for opencast coal mines?

The aim of the study is to determine if pit lakes are a sustainable coal mine closure option in South African. The water balance, chemistry, limnology, and bacterial population of three selected pit lakes were investigated. The lakes are in the three major coal basins of South Africa and are associated with different lithologies and mining methods. The major factors driving the water balance of the pit lakes are direct rainfall, runoff, inflow from old mine workings, and groundwater infiltration, with the major losses being evaporation or discharge onto surface. The study indicated that pit lakes can be designed as 'terminal sinks' to provide a sustainable mine closure option. The pit lakes sampled have an alkaline pH, and mostly a sodium/calcium sulphate water with total dissolved solids content of less than 3000 mg/l. The phytoplankton and microbiological data indicates that the pit lakes support aquatic life. The study shows that correctly designed pit lakes can be an environmentally sustainable closure option for South Africa's coal mines. A suggested design manual has been developed to assist mine owners and regulators in developing sustainable coal mine pit lakes as a closure option.

Coal pit lakes in abandoned mining areas in León (NW Spain): characteristics and geoecological significance

Mining activity introduces severe changes in landscapes and, subsequently, in land uses. One of the most singular changes is the existence of pit lakes, which occur in active and, more frequently, abandoned mines. Pit lakes are produced by water table interception when open-pit mines deepen. Their characteristics are highly variable, depending on the type of mine, the environment or the climate. In León province there is a long tradition of coal mining that dates back to the nineteenth century, and hundreds of open pits from the 1970s to 2018 have been opened, producing permanent landscape changes. This work analyses the main parameters, including morphological measurements, depth and pH values obtained from aerial photos and field work, of 76 coal pit lakes more than 30 m in length. The vast majority of these pit lakes were unknown until now and were not included in inventories or maps. The data obtained provide baseline knowledge that will allow, in the future, potential uses (storage of water for various uses, recreational use, wildlife habitat, and geological heritage sites) for these pit lakes and establish their importance as a new geoecological environment.

Water Balance of Pit Lake Development in the Equatorial Region

In recent years, Indonesia has become the largest coal exporter in the world, and most of the coal is being mined by means of open-pit mining. The closure of an open-pit mine will usually leave a pit morphological landform that, in most cases, will be developed into a pit lake. One of the main issues in developing a pit lake is the understanding of the pit lake filling process. This paper discusses the hydrological model in filling the mineout void in a coal mine in Kalimantan which is located close to the equatorial line. The J-void is a mineout coal pit that is 3000 m long and 1000 m wide, with a maximum depth of 145 m. The development of the J-void pit lake after the last load of coal had been mined out experienced a dynamic process, such as backfilling activities with an overburden as well as pumping mine water from the surrounding pits. There are two components in the model, i.e., overland/subsurface and pit area. The overland zone is simulated using the Rainfall-Runoff NRECA Hydrological Model approach to determine the runoff and groundwater components, whereas the pit area is affected by direct rainfall and evaporation. The model is validated with the observation data. The main source of water in the J-void pit lake is rainwater, both from the surrounding catchment area as well as direct rainfall. As this coal mine area is characterized as a multi-pit area and, consequently, several pit lakes will be formed in the future, the result of the hydrological model is very useful in planning the future pit lakes.

Effects of Calcium and Aluminum on Particle Settling in an Oil Sands End Pit Lake

Surface mining of oil sands ore in Alberta, Canada has generated fluid fine tailings (FFT) that must be reclaimed. End pit lakes (EPLs), which consist of thick deposits of FFT capped with water, have been proposed for FFT reclamation, and Base Mine Lake (BML) is the first full-scale demonstration EPL. However, FFT particle settling and resuspension contributes to high turbidity in the BML water cap, which may be detrimental to the development of an aquatic ecosystem. This study investigated the effect of Ca and Al treatments on turbidity mitigation. The initial turbidity was reduced from 20 NTU to less than 2 NTU in BML surface water treated with 54 mg/L of Ca or 1.1 mg/L of Al. At a concentration of 1.1 mg/L, Al reduced the initial turbidity to a greater extent, and in a shorter time, than 54 mg/L of Ca. Further, resuspended Al-treated FFT particles were 100–700 nm larger in diameter, and thus resettled faster than the resuspended untreated or Ca-treated FFT particles. The final turbidity values 21 days after resuspension of untreated and 1.7 mg/L Al-treated FFT particles in fresh BML surface water were 20.5 NTU and 2.5 NTU, respectively. Thus, Al treatment may be effective in mitigating turbidity in BML through both Al-induced coagulation and self-weight settling of the resuspended Al-treated FFT particles.