Volume 8, Number 4
A Gossman Consulting, Inc. Publication
April 2003

Mercury Deposition Model vs. Measurements

Editor's Note: GCI shared a booth with Cambridge Environmental at A&WM 's HWC Combustors Specialty Conference in Charleston this year.  Cambridge Environmental Inc. is a consulting and research firm that assesses and helps to minimize risks to health and the environment.  They provide objective, expert, and timely analysis of problems related to chemicals in the environment, workplace, and home.  At the conference Cambridge Environmental handed out one page fliers on mercury, dioxin, and PM2.5 issues that they have recently addressed.  Their mercury flier commented on the significant differences between EPA's regional model estimates of mercury wet deposition and recent measurements of mercury deposition.  While the model predicts the highest regional deposition rates in the midwestern and northeastern US the measurements show much lower deposition rates throughout this region.  The EPA has used the model results to target proposed mercury emission regulations toward industrial and utility sources in the Midwest.  Because of the many difficulties in modeling the transport and fate of atmospheric mercury emissions, it is vital that regulators and others consider the discrepancies between the model estimates and measured deposition data in determining how, where, and to what extent mercury emissions should be limited.  Consequently, GCI believes it is important that this data be widely disseminated.

Mercury Fate-and-Transport: How Good are the Models?

"All models are wrong, but some are useful." - George Box

Multi-pathway risk assessments depend on numerous models to predict the environmental path of pollutants released from an industrial facility.  The models contain many assumptions and parameters that have generally received only limited validation.  Mercury modeling presents several unique and significant challenges as its fate and transport are highly complex and only partially understood.  The endpoint of interest for mercury modeling is usually its uptake by fish and the consumption of those fish by humans and other animals.  Modeling of mercury’s behavior in the atmosphere and in aquatic systems is sensitive to many factors and variables.  Even the estimation of how much mercury enters a watershed from the atmosphere is a difficult task.  Models typically consider different species of mercury (elemental, divalent/gaseous, divalent/particulate), transformations between species, and wet and dry removal processes for each species.  The first difficulty involves characterizing the distribution of mercury species, which can vary considerably both within and between source categories.  Atmospheric transformation rates are rarely considered for individual sources, although long-range transport models depend on mercury oxidation to effect sufficient removal to maintain mercury cycling.

The RELMAP study, central to the U.S. Environmental Protection Agency’s 1997 Mercury Report to Congress, modeled mercury emission and deposition nationwide.  The model’s predictions, however, do not appear to agree well with recent mercury deposition measurements.  The figures below illustrate this point.  Both figures depict annual wet deposition rates of mercury.  The top figure, extracted from the Mercury Report to Congress, depicts the RELMAP predictions, while the bottom figure presents measurements of mercury deposition in precipitation.  The first notable difference is the magnitude of deposition, which measurements suggest is about half as large as the RELMAP predictions.  Perhaps more striking, however, are differences in the geographic patterns.  Although limited in coverage, the measurements do not seem to corroborate RELMAP’s concentrated pattern of predicted deposition over the midwestern and northeastern U.S., and the modeling does not successfully predict the notably elevated measured deposition rates in the southern states, especially Florida and Louisiana.

Many factors lend uncertainty to the prediction of mercury deposition.  In assessing mercury deposition from individual sources, Cambridge Environmental has found it important to carefully estimate the speciation of stack emissions and consider the deposition characteristics of individual species.  The modeling of mercury transport in watersheds and its bioaccumulation in the food chain should be performed using site-specific data.  Paradoxically, recent suggestions to enhance mercury removal by promoting the pre-emission formation of oxidized species to enhance the mercury removal efficiencies of existing pollution control devices, could result in greater deposition of mercury to local watersheds.