Predicting total dissolved solids release from central Appalachian coal mine spoils

Legacy coal mining impacts downstream ecosystems for decades in the Canadian Rockies

Mountaintop removal coal mining leaves a legacy of disturbed landscapes and abandoned infrastructure with clear impacts on water resources; however, the intensity and persistence of this water pollution remains poorly characterized. Here we examined the downstream impacts of over a century of coal mining in the Crowsnest Pass (Alberta, Canada). Water samples were collected downstream of two historical coal mines: Tent Mountain and Grassy Mountain. Tent Mountain hosts a partially reclaimed surface mine that closed in 1983. Selenium concentrations downstream of Tent Mountain reached 185 μg/L in a lake below the mine spoil pile, and up to 23 μg/L in Crowsnest Creek, which drains the lake and the mine property. Further downstream, a well-dated sediment core from Crowsnest Lake records increases in sediment, selenium, lead, carbon, nitrogen, and polycyclic aromatic compounds that closely tracked the history of mining at Tent Mountain. In contrast, episodic discharge of mine water from abandoned underground adits at Grassy Mountain drive periodic (but short-term) increases in iron, various metals, and suspended sediment. These results underscore the lasting downstream impacts of abandoned and even reclaimed coal mines.

Trace Elements and Consequent Ecological Risks in Mining-Influenced Streams of Appalachia

Appalachian (eastern USA) coal surface mines fracture geologic materials, causing release of both major ions and trace elements to water via accelerated weathering. When elevated above natural background, trace elements in streams may produce adverse effects on biota via direct exposure from water and sediment and via dietary exposure in food sources. Other studies have found elevated water concentrations of multiple trace elements in Appalachia's mining-influenced streams. Except for Se, trace-element concentrations in abiotic and biotic media of Appalachian mining-influenced streams are less well known. We analyzed environmental media of headwater streams receiving alkaline waters from Appalachian coal mines for eight trace elements (Al, As, Cd, Cu, Ni, Sr, V, and Zn) and assessed the potential consequent ecological risks. Streamwater, particulate media (sediment, biofilm, leaf detritus), and benthic macroinvertebrates (primary consumers, secondary consumers, crayfish) were sampled from six mining-influenced and three reference streams during low-flow conditions in two seasons. Dissolved Cu, Ni, and Sr were higher in mining-influenced streams than in reference streams; Ni, Sr, and Zn in fine sediments and Ni in macroinvertebrates were also elevated relative to reference-stream levels in samples from mining-influenced streams. Seasonal ratios of mining-influenced stream concentrations to maximum concentrations in reference streams also demonstrated mining-influenced increases for several elements in multiple media. In most media, concentrations of several elements including Ni were correlated positively. All water-column dissolved concentrations were below protective levels, but fine-sediment concentrations of Ni approached or exceeded threshold-effect concentrations in several streams. Further study is warranted for several elements (Cd, Ni, and Zn in biofilms, and V in macroinvertebrates) that approached or exceeded previously established dietary-risk levels. Environ Toxicol Chem 2023;42:2651-2665. © 2023 SETAC.

The geochemistry and hydrology of coal waste rock dumps: A systematic global review

Coal has been a major global resource for at least the past 250 years. The major waste product of coal mining is waste rock, which is stored in dumps of various sizes. Although the adverse effects of coal waste rock dumps on ecosystems and human health are widely recognised, there is little information on their internal hydrological and geochemical processes in the peer-reviewed literature. Coal and conventional waste rock dumps share many similarities, but coal waste rock dumps differ in structure, organic matter content, and size, which can affect the timing and rate of aqueous chemical release. In this global systematic review, we identify limited links to climate setting and dump construction, and inconsistent reporting of sampling and monitoring approaches, as limitations to the generalisation of findings. Furthermore, sources of aqueous constituents of interest (COIs) are not routinely or adequately identified, which can lead to incorrect assumptions regarding COI availability and geochemical mobility. Water flow regimes within dumps are dominated by matrix and/or preferential flow, depending on dump texture; these flow mechanisms exert a primary control on patterns of aqueous COI release. The inability to successfully transfer COI release rates from laboratory or field scale trials to operational scale dumps is primarily due to limitations of testing methods and fundamental characteristics of scale. Prediction of future release rates is hampered by a lack of long-term studies that fully characterise geochemistry (e.g., source and COI production rates) as well as dump hydrology (e.g., water balance, water migration). Five critical elements to include in best practice investigations are climate setting, dump physical characteristics, geochemical processes, water regime, and environmental load over time, as aqueous release of COIs from coal waste rock dumps occurs over decades to centuries. Key considerations are identified for each of these elements to guide best practice.

Simulating nitrate release from an unsaturated coal waste rock dump

Knowledge of the controls affecting the release of contaminants from waste rock dumps is critical for developing strategies to mitigate downstream impacts on water quality. In this study, a three-dimensional model of a large coal waste rock dump constructed in the Elk Valley, British Columbia, Canada was developed to capture the impact of construction history (1981-2012) and solute transport on nitrate (NO₃^-) release over a 100-year timeframe. The model consisted of 21, one-dimensional finite element models that represented the temporal evolution of the dump. Nitrate, derived from undetonated blast products, was assumed to be present at the time of waste rock placement and was simulated as a conservative species. The simulated pattern of NO₃^- release to the surface water receptor occurred approximately 8 years before its measured arrival. This time lag is attributed to displacement of the water within a basal alluvial aquifer by dump effluent. The simulated patterns of historic releases corrected for the 8-year time lag, compare favourably with monitoring data and suggest the dominant hydrogeological and geochemical mechanisms are captured in the model. The model indicated the flushing of NO₃^- from the dump should be complete by about 2042 with a peak effluent concentration of NO₃^- in 2008. The addition of reclamation covers to the model resulted in an immediate decrease in the annual NO₃^- loading rate but extended the time frame for NO₃^- release from the dump relative to the no cover case. The model also showed that the timing of cover placement had little impact on NO₃^- release relative to the no cover case due to long duration of waste rock placement (~30 years) over a relatively large footprint.

Development of Mine Soils in a Chronosequence of Forestry-Reclaimed Sites in Eastern Kentucky

Surface mining for coal has contributed to widespread deforestation and soil loss in coal mining regions around the world, and particularly in Appalachia, USA. Mined land reforestation is of interest in this and other regions where forests are the dominant pre-mining land use. This study evaluated mine soil development on surface-mined sites reforested according to the Forestry Reclamation Approach, representing a chronosequence of time ranging from 0 to 19 years after reclamation. Soils were sampled in depth increments to 50 cm and analyzed for a suite of soil physical and chemical characteristics. Overall, soil fines (silt + clay) tended to increase over time since reclamation (17% silt at year 0 increasing to 35% at year 11; 3.2% clay at year 0 increasing to 5.7% at year 14) while concentrations of metals (e.g., Al, Mg, Mn, Na) demonstrated varied relationships with time since reclamation. Concentrations of organic carbon (OC) tended to increase with time (0.9% OC at year 0 increasing to 2.3% at year 14), and were most enriched in near-surface soils. Some soil characteristics (e.g., Na, OC, Ca) demonstrated patterns of increasing similarity to the forest control, while others were distinct from the forest control throughout the chronosequence (e.g., Al, clay, Mn, gravel). Future surveys of these soils over time will elucidate longer-term patterns in soil development, and better characterize the time scales over which these soils might be expected to approximate forest soil conditions.

Making 'Chemical Cocktails' - Evolution of Urban Geochemical Processes across the Periodic Table of Elements

Urbanization contributes to the formation of novel elemental combinations and signatures in terrestrial and aquatic watersheds, also known as 'chemical cocktails.' The composition of chemical cocktails evolves across space and time due to: (1) elevated concentrations from anthropogenic sources, (2) accelerated weathering and corrosion of the built environment, (3) increased drainage density and intensification of urban water conveyance systems, and (4) enhanced rates of geochemical transformations due to changes in temperature, ionic strength, pH, and redox potentials. Characterizing chemical cocktails and underlying geochemical processes is necessary for: (1) tracking pollution sources using complex chemical mixtures instead of individual elements or compounds; (2) developing new strategies for co-managing groups of contaminants; (3) identifying proxies for predicting transport of chemical mixtures using continuous sensor data; and (4) determining whether interactive effects of chemical cocktails produce ecosystem-scale impacts greater than the sum of individual chemical stressors. First, we discuss some unique urban geochemical processes which form chemical cocktails, such as urban soil formation, human-accelerated weathering, urban acidification-alkalinization, and freshwater salinization syndrome. Second, we review and synthesize global patterns in concentrations of major ions, carbon and nutrients, and trace elements in urban streams across different world regions and make comparisons with reference conditions. In addition to our global analysis, we highlight examples from some watersheds in the Baltimore-Washington DC region, which show increased transport of major ions, trace metals, and nutrients across streams draining a well-defined land-use gradient. Urbanization increased the concentrations of multiple major and trace elements in streams draining human-dominated watersheds compared to reference conditions. Chemical cocktails of major and trace elements were formed over diurnal cycles coinciding with changes in streamflow, dissolved oxygen, pH, and other variables measured by high-frequency sensors. Some chemical cocktails of major and trace elements were also significantly related to specific conductance (p<0.05), which can be measured by sensors. Concentrations of major and trace elements increased, peaked, or decreased longitudinally along streams as watershed urbanization increased, which is consistent with distinct shifts in chemical mixtures upstream and downstream of other major cities in the world. Our global analysis of urban streams shows that concentrations of multiple elements along the Periodic Table significantly increase when compared with reference conditions. Furthermore, similar biogeochemical patterns and processes can be grouped among distinct mixtures of elements of major ions, dissolved organic matter, nutrients, and trace elements as chemical cocktails. Chemical cocktails form in urban waters over diurnal cycles, decades, and throughout drainage basins. We conclude our global review and synthesis by proposing strategies for monitoring and managing chemical cocktails using source control, ecosystem restoration, and green infrastructure. We discuss future research directions applying the watershed chemical cocktail approach to diagnose and manage environmental problems. Ultimately, a chemical cocktail approach targeting sources, transport, and transformations of different and distinct elemental combinations is necessary to more holistically monitor and manage the emerging impacts of chemical mixtures in the world's fresh waters.

Long-term leaching characteristic study of coal processing waste streams

A study of the mobility of major and potentially hazardous trace elements from coal processing waste materials was conducted using two types of leaching tests. The baseline leaching test simulates stable waste storage under water, whereas the kinetic test models the storage of waste under more variable conditions including intermittent exposure to air and variations in humidity. Coarse and fine refuse materials were obtained from three commercial coal preparation plants that were being used to upgrade US bituminous run-of-mine coal containing low-to-high amounts of pyritic sulfur. X-ray diffraction analyses revealed a large variation in mineralogy between the coarse and fine refuse streams due to the mineral fractionation that occurs in the processing units and plant. The coarse refuse samples contained higher pyrite contents while the fine refuse samples had high clay content and a minor amount of calcite. This variation in mineralogy resulted in relatively large difference in the leaching characteristics of the waste streams. The most acidic pH and highest release of trace elements were observed in the leachate of coarse refuse containing medium-to-high amounts of coal pyrite, while the fine refuse samples released lower amounts of trace elements in their circumneutral leachate. The least amount of trace elements was observed in the leachate of low pyritic refuse streams. The test data suggested that the most effective disposal practice for coal waste material is segregation and isolation of the coal pyrite and co-disposal of the coarse and fine refuse streams.

Impacts to water quality and biota persist in mining-influenced Appalachian streams

Elevated dissolved major ions (salinization) from surface coal mining are a common impact to central Appalachian headwater streams. Salinization is associated with alterations of benthic macroinvertebrate communities, as many organisms are adapted to the naturally dilute streams of the region. These geochemical and biological alterations have been observed in streams decades after mining, but it remains unclear whether and at what rate water quality and aquatic biota recover after mining. To address this issue, we analyzed temporal trends in specific conductance (SC), ion matrix ratios, and benthic macroinvertebrate communities over an eight-year period in 23 headwater streams, including 18 salinized by surface coal mining. We found strong, negative correlations between SC and diversity of benthic macroinvertebrate communities. Temporal trend analysis demonstrated limited recovery of water chemistry to natural background conditions. Five of the 18 mining-influenced streams exhibited declining SC; however, annual rates of decline in these streams ranged from 1.9% to 3.7% of mean annual SC, suggesting long time periods will be required to reach established benchmark values (ca. 25 years) or values observed in our five reference study streams (ca. 40 years). Similarly, there was limited evidence for recovery of macroinvertebrate community metrics, even in the few mining-influenced streams with decreasing SC. These findings indicate that salinization and its biological effects persist, likely for decades, in central Appalachian headwater streams. Our work also highlights the value of long-term monitoring data for assessing recovery potential of salinized freshwaters, as well as the need for improved understanding of water quality and biological recovery processes and time frames.

Selenium Bioaccumulation Across Trophic Levels and Along a Longitudinal Gradient in Headwater Streams

Toxic effects of selenium (Se) contamination in freshwaters have been well documented. However, study of Se contamination has focused on lentic and larger order lotic systems, whereas headwater streams have received little scrutiny. In central Appalachia, surface coal mining is a common Se source to headwater streams, thus providing a useful system to investigate Se bioaccumulation in headwater food chains and possible longitudinal patterns in Se concentrations. Toward that end, we assessed Se bioaccumulation in 2 reference and 4 mining-influenced headwater streams. At each stream, we sampled ecosystem media, including streamwater, particulate matter (sediment, biofilm, leaf detritus), benthic macroinvertebrates, salamanders, and fish, every 400 m along 1.2- and 1.6-km reaches. We compared media Se concentrations within and among streams and evaluated longitudinal trends in media Se concentrations. Selenium concentrations in sampled media were higher in mining-influenced streams compared with reference streams. We found the highest Se concentrations in benthic macroinvertebrates; however, salamanders and fish bioaccumulated Se to potentially harmful levels in mining-influenced streams. Only one stream demonstrated dilution of streamwater Se with distance downstream, and few longitudinal patterns in Se bioaccumulation occurred along our study reaches. Collectively, our results provide a field-based assessment of Se bioaccumulation in headwater food chains, from streamwater to fish, and highlight the need for future assessments of Se effects in headwater streams and receiving downstream waters. Environ Toxicol Chem 2020;39:692-704. © 2020 SETAC.

Variability of subsurface structure and infiltration hydrology among surface coal mine valley fills

Surface coal mining alters landscapes including creating waste-rock fills or dumps. In Appalachia USA, mines fill valleys with waste rock, constructing valley fills that affect water quality and aquatic ecology downstream. Total dissolved solids (TDS) in mine effluent are elevated from exposure of mineral surfaces to weathering. Understanding TDS variability requires understanding valley fill internal structure and its effect on hydrology, yet prior studies focused on point measurements or did not address patterns among fills. Here we investigated subsurface structure and hydrologic flowpaths in two dimensions within four valley fills using electrical resistivity imaging (ERI). We used artificial rainfall to investigate the location and transit time of preferential flowpaths through the fills. We corroborated our ERI interpretations using borehole logs, downhole video, and shallow soil excavation. ERI results indicated variability in substrate type and widespread presence of preferential flowpaths. We estimated an average preferential flowpath vertical length of 6.6 m, average transit time of water along the flowpath of 1.4 h, and average minimum water velocity of 5.1 m/h (0.14 cm/s). These rates are higher than typical for undisturbed lands, and resemble highly preferential flow in karst terrain. ERI successfully distinguished fills using conventional loose-dump construction from experimental controlled-material compacted-lift construction. Conventional fills exhibited finer particles that retain water at the surface, with larger rocks and larger voids at depth. Conventional fills had greater ranges of subsurface resistivity (i.e. substrate types) and greater interior accumulation of water during artificial rainfall, indicating more quick/deep preferential infiltration flowpaths. We show experimental construction significantly alters hydrologic response, which in combination with use of low-TDS waste rock, may affect downstream water quality relative to conventional loose-dump methods. Our soil boring and pits corroborated ERI interpretation, thus demonstrating ERI to be a robust non-invasive technique that provides reliable information on valley fill structure and hydrology.

Selenium dynamics in headwater streams of the central Appalachian coalfield

Coal mining can cause selenium (Se) contamination in US Appalachian streams, but linkages between water-column Se concentrations and Se bioaccumulation within Appalachian headwater streams have rarely been quantified. Using elevated specific conductance (SC) in stream water as an indicator of mining influence, we evaluated relationships between SC and Se concentrations in macroinvertebrates and examined dynamics of Se bioaccumulation in headwater streams. Twenty-three Appalachian streams were categorized into 3 stream types based on SC measurements: 1) reference streams with no coal-mining history; 2) mining-influenced, high-SC streams; and 3) mining-influenced, low-SC streams. Selenium concentrations in macroinvertebrates exhibited strong positive associations with both SC and dissolved Se concentrations in stream water. At 3 streams of each type, we further collected water, particulate matter (sediment, biofilm, leaf detritus), and macroinvertebrates and analyzed them for Se during 2 seasons. Enrichment, trophic transfer, and bioaccumulation factors were calculated and compared among stream types. Particulate matter and macroinvertebrates in mining-influenced streams accumulated high Se concentrations relative to reference streams. Concentrations were found at levels indicating Se to be a potential environmental stressor to aquatic life. Most Se enrichment, trophic transfer, and bioaccumulation factors were independent of season. Enrichment factors for biofilm and sediments and bioaccumulation factors for macroinvertebrate predators varied negatively with water-column Se. Our results increase scientific understanding of Se bioaccumulation processes in Appalachian headwater streams. Environ Toxicol Chem 2018;37:2714-2726. © 2018 SETAC.

Mapping the yearly extent of surface coal mining in Central Appalachia using Landsat and Google Earth Engine

Surface mining for coal has taken place in the Central Appalachian region of the United States for well over a century, with a notable increase since the 1970s. Researchers have quantified the ecosystem and health impacts stemming from mining, relying in part on a geospatial dataset defining surface mining’s extent at a decadal interval. This dataset, however, does not deliver the temporal resolution necessary to support research that could establish causal links between mining activity and environmental or public health and safety outcomes, nor has it been updated since 2005. Here we use Google Earth Engine and Landsat imagery to map the yearly extent of surface coal mining in Central Appalachia from 1985 through 2015, making our processing models and output data publicly available. We find that 2,900 km² of land has been newly mined over this 31-year period. Adding this more-recent mining to surface mines constructed prior to 1985, we calculate a cumulative mining footprint of 5,900 km². Over the study period, correlating active mine area with historical surface mine coal production shows that each metric ton of coal is associated with 12 m² of actively mined land. Our automated, open-source model can be regularly updated as new surface mining occurs in the region and can be refined to capture mining reclamation activity into the future. We freely and openly offer the data for use in a range of environmental, health, and economic studies; moreover, we demonstrate the capability of using tools like Earth Engine to analyze years of remotely sensed imagery over spatially large areas to quantify land use change.

Seasonal pattern of anthropogenic salinization in temperate forested headwater streams

Salinization of freshwaters by human activities is of growing concern globally. Consequences of salt pollution include adverse effects to aquatic biodiversity, ecosystem function, human health, and ecosystem services. In headwater streams of the temperate forests of eastern USA, elevated specific conductance (SC), a surrogate measurement for the major dissolved ions composing salinity, has been linked to decreased diversity of aquatic insects. However, such linkages have typically been based on limited numbers of SC measurements that do not quantify intra-annual variation. Effective management of salinization requires tools to accurately monitor and predict salinity while accounting for temporal variability. Toward that end, high-frequency SC data were collected within the central Appalachian coalfield over 4 years at 25 forested headwater streams spanning a gradient of salinity. A sinusoidal periodic function was used to model the annual cycle of SC, averaged across years and streams. The resultant model revealed that, on average, salinity deviated approximately ±20% from annual mean levels across all years and streams, with minimum SC occurring in late winter and peak SC occurring in late summer. The pattern was evident in headwater streams influenced by surface coal mining, unmined headwater reference streams with low salinity, and larger-order salinized rivers draining the study area. The pattern was strongly responsive to varying seasonal dilution as driven by catchment evapotranspiration, an effect that was amplified slightly in unmined catchments with greater relative forest cover. Evaluation of alternative sampling intervals indicated that discrete sampling can approximate the model performance afforded by high-frequency data but model error increases rapidly as discrete sampling intervals exceed 30 days. This study demonstrates that intra-annual variation of salinity in temperate forested headwater streams of Appalachia USA follows a natural seasonal pattern, driven by interactive influences on water quantity and quality of climate, geology, and terrestrial vegetation. Because climatic and vegetation dynamics vary annually in a seasonal, cyclic manner, a periodic function can be used to fit a sinusoidal model to the salinity pattern. The model framework used here is broadly applicable in systems with streamflow-dependent chronic salinity stress.

Modeling Patterns of Total Dissolved Solids Release from Central Appalachia, USA, Mine Spoils

Surface mining in the central Appalachian coalfields (USA) influences water quality because the interaction of infiltrated waters and O with freshly exposed mine spoils releases elevated levels of total dissolved solids (TDS) to streams. Modeling and predicting the short- and long-term TDS release potentials of mine spoils can aid in the management of current and future mining-influenced watersheds and landscapes. In this study, the specific conductance (SC, a proxy variable for TDS) patterns of 39 mine spoils during a sequence of 40 leaching events were modeled using a five-parameter nonlinear regression. Estimated parameter values were compared to six rapid spoil assessment techniques (RSATs) to assess predictive relationships between model parameters and RSATs. Spoil leachates reached maximum values, 1108 ± 161 μS cm on average, within the first three leaching events, then declined exponentially to a breakpoint at the 16th leaching event on average. After the breakpoint, SC release remained linear, with most spoil samples exhibiting declines in SC release with successive leaching events. The SC asymptote averaged 276 ± 25 μS cm. Only three samples had SCs >500 μS cm at the end of the 40 leaching events. Model parameters varied with mine spoil rock and weathering type, and RSATs were predictive of four model parameters. Unweathered samples released higher SCs throughout the leaching period relative to weathered samples, and rock type influenced the rate of SC release. The RSATs for SC, total S, and neutralization potential may best predict certain phases of mine spoil TDS release.