|Gossman Consulting, Inc|
GCI TECH NOTES
Volume 8, Number 4 A Gossman Consulting, Inc. Publication April 2003
Deposition Model vs. Measurements
Editor's Note: GCI
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,
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
rates in the midwestern and northeastern US the measurements show much
lower deposition rates throughout this region. The EPA has used
model results to target proposed mercury emission regulations toward
industrial and utility sources in the Midwest. Because of the
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
be widely disseminated.
Mercury Fate-and-Transport: How Good are the Models?
"All models are wrong, but some are useful." -
Multi-pathway risk assessments depend on numerous models to predict the
environmental path of pollutants released from an industrial
The models contain many assumptions and parameters that have generally
received only limited validation. Mercury modeling presents
unique and significant challenges as its fate and transport are highly
complex and only partially understood. The endpoint of interest
mercury modeling is usually its uptake by fish and the consumption of
those fish by humans and other animals. Modeling of mercury’s
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
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.
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.