Influences of Emergent Macrophytes on the Quality of Water Contaminated with Coal Ash Leachates

Abstract

Coal combustion currently remains as one of the leading sources of energy in the world. The potential environmental contamination from the storage of coal ash has been highlighted in recent years by large spills from power plant retention ponds in Tennessee and North Carolina. These incidents, as well as permitted coal ash leachate discharge, make it important to understand the influences of coal ash on the ecology and biogeochemistry of aquatic ecosystems. Wetland ecosystems are particularly of interest, as they serve as reservoirs and transformers of pollutants in the landscape, and constructed wetland treatment systems (CWTS) have been used in the past to treat coal combustion waste. Emergent macrophytes play a pivotal role in CWTS and are often a defining feature of natural wetland habitats that can contribute to the removal of pollutants from the water column through a range of biological and chemical processes. This study explored the explored the effects of two emergent macrophyte species, Juncus effusus and Eliocharis quadrangulata, on the physicochemical properties and trace element concentrations of water contaminated with coal ash. A greenhouse study was performed with wetland microcosms dosed with leachates of fly ash derived from high sulfur and low sulfur coal sources. Microcosms were planted with J. effusus, E. quadrangulata or were unplanted to control for the presence of plants. Both types of leachate increased the electric conductivity (Ec) of microcosm water relative to controls received reverse osmosis water. High sulfur leachates increased water pH while low sulfur leachates decreased water pH. Both leachates significantly elevated boron and lithium concentrations in microcosm water and high sulfur leachates also elevated molybdenum significantly. The highest boron concentrations measured in the study exceeded several aquatic toxicity thresholds. The macrophytes did not display any signs of toxicity, but did appear to exert an influence on the water chemistry. The presence of either species reduced the Ec of microcosm water significantly more than when plants were not presence. Both species also appeared to increase the removal efficiency of trace elements from the water column compared to microcosms with no macrophytes. The findings of this study indicate that emergent macrophytes are tolerant to aquatic coal ash pollution and could potentially reduce associated perturbations in water quality. Their presence in aquatic ecosystems downstream from coal ash discharges could help maintain ecosystem integrity or they may be effectively utilized in CWTS for coal power plant wastewater. Further studies are needed to evaluate the influence of higher volumes of leachate contamination, bioaccumulation of trace elements in macrophytes and the speciation of trace elements from coal ash leachates.