MERCURY AIR EMISSION IMPACT ASSESSMENTAND APPROVAL CONSIDERATIONS IN THE MDEQNEW SOURCE REVIEW PROGRAM

摘要

The Michigan Department of Environmental Quality Air Quality Division (MDEQAQD)reviews Permit to Install applications under the New Source Review (NSR) permitting program.Modeled ambient air impacts of toxic air contaminants are compared to AQD’s inhalation healthriskbasedscreening levels (SLs) to determine if they are approvable. However, a SL has notbeen established for mercury (Hg), because deposition, persistence and bioaccumulation maypose significant sitespecificconcerns, which precludes the establishment of a single appropriateambient air SL. Therefore, the approvability of proposed mercury air emissions is evaluated anddetermined on a casebycasebasis. Models are available which can provide estimates ofmercury deposition rates near air emission sources, bioaccumulation in fish in local surfacewaters, and the incremental exposure of sport anglers who eat their catch from those waters.Those models require certain sourceand water bodyspecificinformation or reasonable defaultassumptions, such as the speciation profile of mercury emissions, water body characteristics, andthe preexistingfish mercury levels. The resulting characterization of the incremental sourceimpacts to local environmental mercury levels is valuable information for AQD Staffconsideration, and for public review and comment, and it informs risk management decisionmaking(I.e., permit approval, denial, or approval with modification). Those comments andconsiderations may appropriately take into account other relevant sitespecificinformation, suchas the preexistingfish mercury levels relative to fish consumption advisory trigger levels orother benchmarks, and the relative magnitude of the proposed incremental impacts. There isonly limited EPA risk management guidance available on the approvability of a proposedincremental mercury deposition impact when preexistingenvironmental levels may alreadyapproach or exceed benchmarks of acceptability.