Laura Ruhl

The Impact of Coal Combustion Residue Effluent on Water Resources: A North Carolina Example

The combustion of coal to generate electricity produces about 130 million tons of coal combustion residues (CCRs) each year in the United States; yet their environmental implications are not well constrained. This study systematically documents the quality of effluents discharged from CCR settling ponds or cooling water at ten sites and the impact on associated waterways in North Carolina, compared to a reference lake. We measured the concentrations of major and trace elements in over 300 samples from CCR effluents, surface water from lakes and rivers at different downstream and upstream points, and pore water extracted from lake sediments. The data show that CCR effluents contain high levels of contaminants that in several cases exceed the U.S. EPA guidelines for drinking water and ecological effects. This investigation demonstrates the quality of receiving waters in North Carolina depends on (1) the ratio between effluent flux and freshwater resource volumes and (2) recycling of trace elements through adsorption on suspended particles and release to deep surface water or pore water in bottom sediments during periods of thermal water stratification and anoxic conditions. The impact of CCRs is long-term, which influences contaminant accumulation and the health of aquatic life in water associated with coal-fired power plants.

Boron and strontium isotopic characterization of coal combustion residuals: validation of new environmental tracers.

In the U.S., coal fired power plants produce over 136 million tons of coal combustion residuals (CCRs) annually. CCRs are enriched in toxic elements, and their leachates can have significant impacts on water quality. Here we report the boron and strontium isotopic ratios of leaching experiments on CCRs from a variety of coal sources (Appalachian, Illinois, and Powder River Basins). CCR leachates had a mostly negative δ(11)B, ranging from -17.6 to +6.3‰, and (87)Sr/(86)Sr ranging from 0.70975 to 0.71251. Additionally, we utilized these isotopic ratios for tracing CCR contaminants in different environments: (1) the 2008 Tennessee Valley Authority (TVA) coal ash spill affected waters; (2) CCR effluents from power plants in Tennessee and North Carolina; (3) lakes and rivers affected by CCR effluents in North Carolina; and (4) porewater extracted from sediments in lakes affected by CCRs. The boron isotopes measured in these environments had a distinctive negative δ(11)B signature relative to background waters. In contrast (87)Sr/(86)Sr ratios in CCRs were not always exclusively different from background, limiting their use as a CCR tracer. This investigation demonstrates the validity of the combined geochemical and isotopic approach as a unique and practical identification method for delineating and evaluating the environmental impact of CCRs.

Isotopic imprints of mountaintop mining contaminants.

Mountaintop mining (MTM) is the primary procedure for surface coal exploration within the central Appalachian region of the eastern United States, and it is known to contaminate streams in local watersheds. In this study, we measured the chemical and isotopic compositions of water samples from MTM-impacted tributaries and streams in the Mud River watershed in West Virginia. We systematically document the isotopic compositions of three major constituents: sulfur isotopes in sulfate (δ(34)SSO4), carbon isotopes in dissolved inorganic carbon (δ(13)CDIC), and strontium isotopes ((87)Sr/(86)Sr). The data show that δ(34)SSO4, δ(13)CDIC, Sr/Ca, and (87)Sr/(86)Sr measured in saline- and selenium-rich MTM impacted tributaries are distinguishable from those of the surface water upstream of mining impacts. These tracers can therefore be used to delineate and quantify the impact of MTM in watersheds. High Sr/Ca and low (87)Sr/(86)Sr characterize tributaries that originated from active MTM areas, while tributaries from reclaimed MTM areas had low Sr/Ca and high (87)Sr/(86)Sr. Leaching experiments of rocks from the watershed show that pyrite oxidation and carbonate dissolution control the solute chemistry with distinct (87)Sr/(86)Sr ratios characterizing different rock sources. We propose that MTM operations that access the deeper Kanawha Formation generate residual mined rocks in valley fills from which effluents with distinctive (87)Sr/(86)Sr and Sr/Ca imprints affect the quality of the Appalachian watersheds.

Selenium Speciation in Coal Ash Spilled at the Tennessee Valley Authority Kingston Site

Selenium (Se) in coal ash spills poses a threat to adjacent ecosystems because of its potential to mobilize and bioaccumulate in aquatic organisms. Given that the mobility and bioavailability of Se is controlled by its valence states, we aimed to define Se speciation in coal ash solids and examine the relationships between Se speciation and the magnitude of its mobilization from coal ash. We used coal ash samples from the Tennessee Valley Authority (TVA)-Kingston fossil plant and the site of a coal ash spill that occurred in 2008 in Tennessee. Results of X-ray absorption spectroscopic analyses showed that Se in coal ash samples was a mixture of elemental Se(0) and Se oxyanions. The amount of leachable Se increased with an increase of pH from 3 to 13. At the natural pH of coal ash samples (from pH 7.6 to 9.5), the leachable Se was comprised of Se oxyanions, mainly selenite. This was observed by both direct quantification of Se oxyanions in the leachate and the corresponding loss of Se oxyanions in the solid phase. At pH 12, however, the Se release appeared to derive from both desorption of Se oxyanions and oxidative dissolution of elemental Se(0). Our results indicate that Se oxyanions are the most labile species; however, the magnitude of Se mobilization will increase if the waste material is subjected to alkaline conditions.

Impacts of coal ash on methylmercury production and the methylating microbial community in anaerobic sediment slurries

Mercury (Hg) associated with coal ash is an environmental concern, particularly if the release of coal ash to the environment is associated with the conversion of inorganic Hg to methylmercury (MeHg), a bioaccumulative form of Hg that is produced by anaerobic microorganisms. In this study, sediment slurry microcosm experiments were performed to understand how spilled coal ash might influence MeHg production in anaerobic sediments of an aquatic ecosystem. Two coal ash types were used: (1) a weathered coal ash; and (2) a freshly collected, unweathered fly ash that was relatively enriched in sulfate and Hg compared to the weathered ash. These ash samples were added to anaerobic sediment slurries constructed with a relatively pristine sediment (containing 0.03 mg kg-1 Hg) and a Hg-contaminated sediment (containing 0.29 mg kg-1 Hg). The results of these experiments showed negligible net production of MeHg in microcosms with no ash and in microcosms amended with the low sulfate/low Hg ash. In contrast, slurry microcosms amended with high sulfate/high Hg ash showed increases in total MeHg content that was 2 to 3 times greater than control microcosms without ash (p < 0.001). 16S amplicon sequencing of microbial communities in the slurries indicated that the coal ash addition generally increased the relative abundance of the methylating microbial community, including sulfate-reducing bacteria and iron-reducing bacteria species that are known to be efficient methylators of Hg. The stimulation of these microorganisms was likely caused by the release of substrates (sulfate and Fe) originating from the ash. Overall, the results highlight the need to incorporate both environmental parameters and coal ash characteristics into risk assessments that guide coal ash management and disposal.