Potential environmental impact of mercury emissions from Portland cement kilns

The reduction of mercury (Hg) releases to the environment, particularly airborne mercury emissions, is currently a major focus of both US state and federal regulatory agencies. While mercury emissions from hazardous waste incinerators and fossil-fuel power plants have been and continue to be regulated under the Resource Conservation and Recovery Act (RCRA) and the Clean Air Act (CAA), non-hazardous waste cement kilns are currently excluded from regularly controls. However, the US Environmental Protection Agency (EPA) continues to assess the need for possible mercury emission controls nationwide, under the CAA, and for specific facilities, through the Total Maximum Daily Load (TMDL) Program of the Clean Water Act. The EPA's major concern appears to be the potential impacts the bioaccumulation of mercury in fish and other aquatic organisms may have on humans and wildlife that consume them. This paper uses fate and transport modeling to evaluate whether mercury emissions from cement kilns could pose unacceptable risks to fish-consuming populations in the area of a source because of high levels of methyl mercury that could accumulate in fish. Emphasis is placed on assessing the effects that parameter variability and the lack of the parameter specificity for environmental conditions have on risk assessment results. The key parameters that are evaluated include emission rates, mercury speciation, methylation rates, and watershed and water body configurations. The results of this assessment indicate that mercury emissions from cement kilns pose much less of a risk to fish consumers than mercury emissions from other types of combustion sources; however, there is substantial uncertainty in the risk estimates. These uncertainties in the EPA model for mercury emissions indicate that this model is only refined enough currently to screen risks from combustion facilities (to identify facilities that pose no significant risk of mercury exposure), but is insufficient to characterize risks for populations at any specific site. Finally, prior to any attempt at regulating Portland cement kiln mercury emissions, the EPA should refine and validate its models based on mercury emission and fish concentration data for actual sites.