Realizing Beneficial End Uses from Abandoned Pit Lakes

Biotic aspects of suspended solid reduction in sedimentation ponds

The reduction of suspended solids is an important aspect of water management, especially in the mining industry. The ecological potential of sedimentation ponds has been identified, but many aspects of the coexistence of ecological and technological functions of these reservoirs are still unresolved. The aim of the study was to determine the effectiveness of suspended solid reduction in an open pit mine drainage system under the influence of biotic factors, including fish, taking into account the hydrochemical background. The dynamics of biotic factors and the sedimentation process were identified using the normalized total suspended solid reduction index (TE) and the trophic group status (TGS) indices of planktivorous, benthivorous, and carnivorous fish. The hydrochemical background in both ponds provided similar habitat conditions for biota and suspended solid reduction. Although a moderately trophic ecosystem has developed, the typical relationship between water transparency (SDD) and chlorophyll a concentration characteristic of natural waters was not observed. The sedimentation complex was found to be highly effective in reducing total suspended solids by 89-93%. Spatial variation of sedimentation rates was related to the ichthyobiotic factor, mainly to the biomass of bentophages and the status of this trophic group in the fish community. Stable relationships in the structure of the ichthyofauna with a balanced proportion of planktivorous fishes favored the sedimentation process. The results obtained are part of the development of research on the possibility of combining the technological function of artificial ponds with other applications and ecological benefits for industrialized areas.

The role of pit lake thermal dynamics on the thermal performance of ground heat exchangers

The addition of ground heat exchangers (GHEs) to a pit lake's basin has the potential for abundant, clean and renewable geothermal energy extraction using shallow geothermal systems. Basin-embedded GHEs avoid direct interaction with mine water, which has been shown to impact efficiency and longevity in mine open-loop geothermal systems negatively. The now accelerated closure of open-pit coal mines presents itself as an opportunity to use this technology. However, no guidelines currently exist for designing or operating GHEs embedded in the sediment of water bodies. Furthermore, the two-way coupling between the complex annual thermal fluid dynamics that lakes are naturally subjected to and heat fluxes on the sediments and the GHE system has not been explored. In this study, we develop and validate finite element models to assess the relevance of lake thermal stratification in the performance of a geothermal system embedded in water bodies basins, e.g., on open-pit mine closures, under temperate residential thermal loads. The results show that the pit lake's role as a thermal sink improves significantly when the lake's thermal dynamics are accounted for, with an increase of up to 292% in the lake's available energy budget. A minor variation in energy budget (~8%) was found whether the lake is modelled explicitly or simplified as a transient Dirichlet temperature boundary condition. This small difference vanishes if horizontal circulation along the lake is considered, highlighting the lake's thermal energy potential. Finally, the impact on the GHE Coefficient of Performance (COP) is evaluated, with a maximum of ~15% difference among all cases.

The transformation of river ecosystems caused by mining affects bird breeding in indigenous riparian habitats

Our study considered the excavation of sand and gravel, which modifies the landscapes of riparian ecosystems. It promotes the creation of water bodies with surrounding vegetation, but it also results in the loss of natural habitats. We investigated the species richness, composition, and abundance of aquatic and terrestrial breeding birds and their interaction with three habitat types: managed and abandoned flooded pits, and oxbow lakes. We surveyed 117 sites in medium-sized river valleys in the foreground of the Bohemian Massif (Czech Republic), and in the Carpathian Mountains (Poland, Slovakia) in 2022. Flooded pits were suitable for open-water and colonial birds. Managed flooded pits were also suitable for early successional land birds, but they did not provide habitat for birds that use marshes and wet meadows, or riparian woodlands. The majority of species preferred to breed in oxbow lakes with riparian forests and these areas hosted the highest number of threatened species. We concluded that high levels of disturbance in riparian ecosystems promoted some birds (e.g. colonial or breeding in early-successional habitats), but it negatively affected the overall bird diversity, and it led to a species composition shift with the elimination of taxa associated with indigenous riparian habitats. The importance of flooded pits increases with subsequent plant succession. Our results indicate that gravel or sand mine pits, although beneficial for some taxa, are not substitutes for natural habitats in riparian ecosystems, as they do not support birds breeding in indigenous riparian habitats. Natural oxbow lakes with riparian forests are habitats that need to be preserved to effectively promote local biodiversity.

Analysis of the reduction processes at the bottom of Lake Meirama: a singular case of lake formation

The formation of natural lakes is a process that takes place over thousands of years, although the volumetric formation depends on hydrological and climatological phenomena, reaching a stationary hydraulic regime, the evolution of hydrochemistry is more complex and obeys not only phenomena of stoichiometry and chemical kinetics but also diffusion processes. Depending on the depth of the lakes, the anoxization process originating from the bottom is the first phase of the lake's methanogenesis. For this, the course of many thousands of years is necessary, so the studies carried out in the lakes are limited to the current knowledge of the state in which they are, without being able to have real information in this process of methanogenesis. There are no data available on the generation process of a natural lake in its primary stages. In this case, taking advantage of the rehabilitation of the old open-pit mining of Meirama (Northwest Spain), consisting of the controlled flooding of the hole by groundwater, by stopping the perimeter pumping, and the derivation of the nearby streams, whose contribution was the majority with respect to the subterranean contribution, there has been the opportunity to physically and chemically monitor the complete filling of the said hole. The present study focuses on the analysis of the evolution of the different processes initiated in the methanogenesis of the lake bottom identified in the well-known Redox ladder: obtaining oxygen from the reduction of nitrogenous compounds and metallic oxides, from the reduction of the sulfate and the generation of methane from carbon compounds, the latter phase without reaching. Although the methanization process is very slow, it has had the opportunity to know the formation of a lake at its origin, from the hydrochemical point of view. It has been possible to verify that the methanization processes at the bottom, given the anoxia conditions, are in a very primitive phase with the reduction of nitrate and nitrite to ammonium and beginning a reduction of metal oxides and sulfate.

Use of Wild Rice (Zizania palustris L.) in Paddy-Scale Bioassays for Assessing Potential Use of Mining-Influenced Water for Irrigation

As surface water resources become more intensely used, and occasionally non-useable, consideration of non-conventional water resources for anthropogenic use has become more prevalent. Potentially critical non-conventional water sources include flooded mine-pit lakes. However, water in these lakes can contain potentially problematic concentrations of contaminants of concern. We evaluated the potential use of elevated sulphate (SO₄) mining-influenced waters with low to non-detect metals concentrations for irrigation of wild rice (Zizania palustris L.; WR), a culturally and economically important species. Two flow-through in-situ paddies were developed adjacent to two mine-pit lakes with differing chemical water characteristics; specifically, Pit A contained ≈350 mg SO4 L^-1 and Pit C contained ≈1350 mg SO4 L^-1. Throughout the course of multiple consecutive growing seasons, no adverse WR responses to these mining-influenced water exposures were observed. Based on data and observations from this study, potential use of mining-influenced waters containing elevated SO₄ as the primary contaminant for appropriate irrigation purposes is supported. However, site-specific conditions and potential environmental risks must be considered prior to use of mining-influenced waters for anthropogenic applications.

SUPPLEMENTARY INFORMATION

The online version contains supplementary material available at 10.1007/s10230-022-00908-0.

Automated versus Manual Mapping of Gravel Pit Lakes from South-Eastern Romania for Detailed Morphometry and Vegetation

In recent years, the accelerated development of the remote sensing domain and the improvement of the resolution and frequency of satellite images allowed the increase in the accuracy of the evaluation of morphometric characteristics and the spatiotemporal distribution of pit lakes, including the small ones. Our study quantitatively analyzes small-scale pit lakes in the piedmont and subsidence plains from contact with the Getic and Curvature Subcarpathians from Romania using the normalized difference water index (NDWI) and data series, with different resolutions, from Landsat 8, Google Earth, and Sentinel 2A. The problems encountered in extracting the contours of the gravel pit lakes were determined by the different resolution of the images, the uneven quality of the images exported from Google Earth, and an additional challenge was given by the diversity of the analyzed land surfaces, the land use, and the optical properties of the lakes. A comparison of the obtained NDWI values using data series from Sentinel 2A and Landsat 8 highlighted the importance of resolution and also showed a larger spectral difference between the identified water bodies and the surrounding land in favor of Sentinel 2A. Regarding the vegetation-derived indices, superior leaf area index (1.8–3) was recorded in low-lying plains and mixed areas (tall shrubs, wetlands, etc.) because the river banks have increased moisture that supports taller species with denser foliage and the sparsely vegetated areas are located in agricultural crops and in/near villages. Changes in vegetation richness and abundance can be spatiotemporally monitored using indices derived from the spectral bands of satellite imagery.

Geochemical Stability of Oil Sands Tailings in Mine Closure Landforms

Oil sands surface mining in Alberta has generated over a billion cubic metres of waste, known as tailings, consisting of sands, silts, clays, and process-affected water that contains toxic organic compounds and chemical constituents. All of these tailings will eventually be reclaimed and integrated into one of two types of mine closure landforms: end pit lakes (EPLs) or terrestrial landforms with a wetland feature. In EPLs, tailings deposits are capped with several metres of water while in terrestrial landforms, tailings are capped with solid materials, such as sand or overburden. Because tailings landforms are relatively new, past research has heavily focused on the geotechnical and biogeochemical characteristics of tailings in temporary storage ponds, referred to as tailings ponds. As such, the geochemical stability of tailings landforms remains largely unknown. This review discusses five mechanisms of geochemical change expected in tailings landforms: consolidation, chemical mass loading via pore water fluxes, biogeochemical cycling, polymer degradation, and surface water and groundwater interactions. Key considerations and knowledge gaps with regard to the long-term geochemical stability of tailings landforms are identified, including salt fluxes and subsequent water quality, bioremediation and biogenic greenhouse gas emissions, and the biogeochemical implications of various tailings treatment methods meant to improve geotechnical properties of tailings, such as flocculant (polyacrylamide) and coagulant (gypsum) addition.

Managing and Reforesting Degraded Post-Mining Landscape in Indonesia: A Review

Tropical forests are among the most diverse ecosystems in the world, completed by huge biodiversity. An expansion in natural resource extraction through open-pit mining activities leads to increasing land and tropical forest degradation. Proper science-based practices are needed as an effort to reclaim their function. This paper summarizes the existing practice of coal mining, covering the regulatory aspects and their reclamation obligations, the practices of coal mining from various sites with different land characteristics, and the reclamation efforts of the post-mining landscapes in Indonesia. The regulations issued accommodate the difference between mining land inside the forest area and outside the forest area, especially in the aspect of the permit authority and in evaluating the success rate of reclamation. In coal-mining practices, this paper describes starting from land clearing activities and followed by storing soil layers and overburden materials. In this step, proper handling of potentially acid-forming materials is crucial to prevent acid mine drainage. At the reclamation stage, this paper sequentially presents research results and the field applications in rearranging the overburden and soil materials, controlling acid mine drainage and erosion, and managing the drainage system, settling ponds, and pit lakes. Many efforts to reclaim post-coal-mining lands and their success rate have been reported and highlighted. Several success stories describe that post-coal-mining lands can be returned to forests that provide ecosystem services and goods. A set of science-based best management practices for post-coal-mine reforestation is needed to develop to promote the success of forest reclamation and restoration in post-coal-mining lands through the planting of high-value hardwood trees, increasing trees’ survival rates and growth, and accelerating the establishment of forest habitat through the application of proper tree planting technique. The monitoring and evaluation aspect is also crucial, as corrective action may be taken considering the different success rates for different site characteristics.

Reconstruction of Ancient Lake after Peat Excavation—A Case Study about Water Quality

According to regulations in some European countries, peat is treated as a fossil fuel or soil for mushroom horticulture and its management is subject to the law for mining activities. As a result of the exploitation of peat bogs, the cutaway or pit lakes remain, which when properly prepared can be local water resources. Such post-peat water bodies can be used for recreation and they can be particularly important in areas struggling with water deficiency in the rural landscape. Maintaining good water quality in such reservoirs requires a number of preparatory works, including the removal of the remaining organic matter that would rest at the bottom of the new pit lake, affecting the water quality. Studies of water quality and aquatic plant communities in the studied post-peat lake were carried out during the period 2012–2014 in order to determine the changes in water quality and the usefulness of water for cyprinids. Aquatic plant communities identified in the reservoir showed a simplified species composition, characteristic of initial communities, and they occupied small areas in the water and on the banks. It has been shown that water quality parameters of the studied water body were stable and corresponded to thresholds established for cyprinids.

A Model to Evaluate the Flooding Opportunity and Sustainable Use of Former Open-Pits

As a result of open-pit mining exploitations, impressive size gaps occur in the landscape. Their flooding leads to the occurrence of so-called open-pit lakes and represents an interesting way to reclaim and use sustainably the degraded land. In the literature, there are numerous plans, strategies, and guidelines for mine closure and open-pit recovery, but these are usually developed at the regional or national level and offer general suggestions, which must be evaluated and approached case-by-case. Because there is still no way to evaluate the opportunity of flooding the open-pits, a methodology for assessing this opportunity was developed to identify the open-pits that are suitable for flooding, this being the main objective of the paper. The paper is novel because of the multicriteria evaluation of open-pits and their remaining gaps, the logical succession of the criteria, and the proposed concept, methods, models, and equations that allow a complex assessment of the flooding opportunity. The methodology also aims to ensure maximum safety conditions in the former mining perimeter, the socio-economic and cultural requirements of local communities, the harmonization of the land in accordance with adjacent ecosystems, and the sustainable development of the region.