Coal Combustion Residuals in Receiving Lake Ecosystems: Trophic Transfer, Toxicity, and Tracers
Modern ecotoxicology draws considerable criticism for its lack of ecological relevance despite its history of interdisciplinary research overlap with the fields of conservation biology and ecology. The overarching aim of this dissertation was to unite the goals of these fields in the context of freshwater pollution by coal-fired power plant (CFPP) effluents, which contribute to the largest point source of environmental pollution in the United States and comprise a substantial threat to receiving ecosystems. CFPPs discharge the by-products of coal combustion, collectively referred to as coal combustion residuals (CCRs), into freshwater rivers and lakes through permitted discharges overseen by the National Pollutant Discharge Elimination System. CCRs are characterized by high concentrations of numerous inorganic elements, many of which are toxic to aquatic organisms. For decades, research on this waste stream has focused on selenium (Se) as an especially toxic trace element for oviparous vertebrates, historically causing severe deformities and species extirpations from which affected ecosystems took decades to recover.
The majority of work described here began with a 2015 field survey of three CFPP-associated lakes in North Carolina from which surface waters, sediments, sediment pore waters, biofilm, zooplankton, and resident fish species were collected and analyzed for their trace element profiles by inductively coupled plasma mass spectrometry (ICP-MS). Initial analyses focusing on Se alone revealed significantly higher tissue liver, muscle, and gonad tissue concentrations in fish from lakes receiving CCR inputs than those from reference systems. At two sites, Mayo Lake and Sutton Lake, water samples and fish tissue concentrations additionally exceeded the US Environmental Protection Agency’s recently revised aquatic life criteria. These analyses were subsequently expanded to consider a more comprehensive suite of ten CCR-associated elements. The results of paired univariate and multivariate analyses of abiotic and biotic compartments for each of the three pairs of lakes showed that CCR-receiving lake sediment pore waters are consistently enriched in manganese, arsenic, selenium, strontium, cadmium, and nickel. From this abiotic compartment, preferential uptake by biological compartments differed among species and among lakes such that only Se was consistently enriched in fish across the three systems. Across all lake pairs, the three fish species differed in their aggregate CCR tissue burdens, with bluegill having the highest burden and largemouth bass having the lowest. A lab-based trophic transfer study of field-collected biofilm and zooplankton to the model fish species, fathead minnow (Pimephalas promelas), supported the hypothesis that diet and trophic position are important mediators of fish tissue trace element composition. CCR loading history, lake size, and water residence time influence the magnitude of trace element accumulation in these systems, with important implications for the legacy of this waste stream as CFPPs are retired and their effluent streams to adjacent water bodies are terminated.
An additional lab-based dietary study of organic selenomethionine exposure sought to describe the bioenergetic consequences of low-level Se exposure to adult zebrafish (Danio rerio) and their offspring. Sex-specific metabolic effects were observed in whole organisms and individual tissue types and metabolic partitioning using the Seahorse Extracellular Flux Analyzer indicated that mitochondrial dysfunction could underlie altered metabolic rates, with implications for the ecological fitness of Se-impacted fish communities. This study also explored maternal and paternal exposure routes of Se exposure to F1 generation embryos and found that both routes of exposure resulted in reduced reserve respiratory capacity relative to control fish.
Inorganic trace elements are incorporated into the lattice of metabolically-stable fish otoliths, providing unique records of fishes’ lifetime exposure history. Two distinct otolith applications are explored here. First, time series otolith concentrations of a subset of CCRs were compared to overlapping time series of CFPP loading data to determine whether changes in system inputs were reflected in otolith uptake. Significant time lags, intra-species variation, and differences among species for Se suggested that otolith chemistry could be reflecting Se input legacies and complex biogeochemical cycling through the food webs from which fish are exposed. In contrast, fish otolith 87Sr/86Sr isotopic ratios did distinguish fish from a CCR-receiving lake in agreement with ratios measured in surface and pore water samples. If this initial proof-of-principle result holds for fish collected from larger reservoirs in which water chemistry is less uniform, otoliths could be powerful biological tracers of freshwater CCR impacts.
In tandem with the decreasing reliance on coal in the US, CFPPs are retiring, excavating their coal ash ponds, and otherwise terminating their CCR effluent streams. As these processes unfold, actively-receiving freshwater ecosystems are undergoing a transition to legacy status. An especially important avenue for future research is how CCR stoichiometry will change over time to reflect the relative transformation, sequestration, and transport of individual elements within and through these recovering systems. The ecotoxicological implications of the legacy CCR waste stream will likely evolve as exposure mixtures change.
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