Environmentally significant shifts in trace element emissions from coal plants complying with the 1990 Clean Air Act Amendments

Highlights: • Acid rain standard compliance relies on coal switching or scrubber installation. • Both strategies affect trace element emission magnitude and phase at coal plants. • Concentrations of As, Br, Cl, Pb, Hg, Se in combusted coal have fallen since 1993. • New air pollution controls have reduced gas phase trace element emissions. • Aqueous phase discharges of unregulated trace metals in wastewater have increased. -- Abstract: The Clean Air Act Amendments of 1990 (CAAAs) reduced criteria air pollution from coal-fired power plants (CFPPs) in the United States, but compliance strategies also altered the magnitude and phase of environmentally significant trace element emissions. The present work quantitatively assesses changes in trace element emissions stemming from CAAAs compliance, attributes these changes to specific CAAAs compliance strategies, and discusses implications for future environmental regulatory design. We combine plant-level coal purchase and installed air pollution control device data with a trace element partitioning model to approximate changes in trace element mass entering and exiting CFPPs in the solid, liquid, and gas phase between 1993 and 2017. Though US coal combustion decreased by 2% during this period, the vast majority of trace element emissions reductions stem from the 18-64% reductions in the fleet-wide average concentrations of arsenic, bromine, chloride, lead, mercury, and selenium in the combusted coal. Combined with the installation of air pollution control technologies, these shifts resulted in nationwide reductions of gas phase trace element emissions by between 49 and 68% for the elements included in this study. In contrast, installation of flue gas desulfurization devices increased the mass of trace elements discharged in wastewater. This work reinforces the value of multi-phase emissions regulations in ensuring that contaminants are not shunted from regulated to unregulated phases.