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.

No abstract

The unintended release of coal ash to the environment is a concern due to the enrichment of contaminants such as arsenic (As) and selenium (Se) in this solid waste material. Current risk assessments of coal ash disposal focus on pH as the primary driver of leaching from coal ash. However, redox speciation of As and Se is a major factor for their mobilization potential and has received much less attention for risk assessments, particularly in disposal scenarios where coal ash will likely be exposed to microbially-driven redox gradients. The aim of this study was to demonstrate the differences of aerobic and anaerobic conditions for the leaching of As and Se from coal ash. Batch sediment-ash slurry microcosms were performed to mimic an ash spill scenario and were monitored for changes in As and Se speciation and mobilization potential. The results showed that the dissolved As concentrations were up to 50 times greater in the anaerobic microcosms relative to the aerobic microcosms during the two week incubation. This trend was consistent with As redox speciation determined by X-ray absorption spectroscopy, which indicated that 55% of the As in the solid phase at the end of the experiment was present as As(III) (a more leachable form of arsenic relative to As(V)). In the aerobic microcosms, only 13% of the As was As(III) and the rest was As(V). More than half of the Se was present as Se(IV) in the original fly ash and in the aerobic microcosms, while in the anaerobic microcosms Se was gradually transformed to less soluble Se(0) species. Likewise, dissolved Se concentrations were up to 25 times greater in the aerobic microcosms relative to anaerobic conditions. While the overall observations of As and Se mobilization potential from coal ash were consistent with expectations for aqueous and solid phase speciation of these elements, the findings directly show the relevance of these processes for coal ash disposal. These results highlight the need to select appropriate environmental parameters to include in risk assessments as well as provide potential geochemical monitoring tools through the use of dissolved Se/As ratios to determine the redox conditions of ash storage and spill sites.

The chemical composition of coal ash is highly heterogeneous and dependent on the origin of the source coal, combustion parameters, and type and configuration of air pollution control devices. This heterogeneity results in uncertainty in the evaluation of leaching potential of contaminants from coal ash. The goal of this work was to identify whether a single leaching protocol could roughly group high-leaching potential coal ash from low-leaching potential coal ash, with respect to arsenic (As) and selenium (Se). We used four different leaching tests, including the Toxicity Characteristic Leaching Protocol (TCLP), natural pH, aerobic sediment microcosms, and anaerobic sediment microcosms on 10 different coal ash materials, including fly ash, lime-treated ash, and flue gas desulfurization materials. Leaching tests showed promise in categorizing high and low-leaching potential ash materials, indicating that a single point test could act as a first screening measure to identify high-risk ash materials. However, the amount of contaminant leached varied widely across tests, reflecting the importance of ambient conditions (pH, redox state) on leaching. These results demonstrate that on-site geochemical conditions play a critical role in As and Se mobilization from coal ash, underscoring the need to develop a situation-based risk assessment framework for contamination by coal ash pollutants.