Testing the USA EPA's ISCST-Version 3 model on dioxins: a comparison of predicted and observed air and soil concentrations

The central purpose of our study was to examine the performance of the Unit ed States Environmental Protection Agency's (EPA) nonreactive Gaussian air quality dispersion model, the Industrial Source Complex Short-Term Model (I SCST3) Version 98226, in predicting polychlorinated dibenzodioxins and poly chlorinated dibenzofurans concentrations (subsequently referred to as dioxi ns and furans, or CDD/Fs) in both air and soil near the Columbus Municipal Solid Waste-to-Energy Facility (CMSWTE) in Columbus, OH. During its 11 yr o peration, the CMSWTE was estimated to be emitting nearly 1 kg of CDD/F Toxi c Equivalents (TEQs) per year, making it one of the highest single emitters of dioxin in the United States during its operation. An ambient air-monito ring study conducted in 1994, prior to its shutdown in December of 1994, cl early identified high dioxin air concentration in the downwind direction du ring two sampling events. In one of the events, the CMSWTE stack was concur rently monitored for dioxins, A soil sampling study conducted in 1995/1996 was similarly able to identify an area of impacted soil extending mainly in the predominant downwind direction up to 3 km from the CMSWTE, Site-specif ic information, including meteorological data, stack parameters and emissio n rates, and terrain descriptions, were input into ISCST3 to predict ground -level 48-h concentrations which could be compared with the 48-h measured a ir concentrations. Predicted annual average dry and wet deposition of parti cle-bound dioxins were input into a simple soil reservoir model to predict soil concentrations that would be present after 11.5 yr of emissions, which were compared to measured concentrations. Both soil- and air-predicted con centrations were generally within a factor of 10 of observations, and judge d to be reasonable given the small number of observations and the uncertain ties of the exercise. Principal uncertainties identified and discussed incl ude: source characterization (stack emission rates of dioxins), meteorologi cal data, and atmospheric and soil fate and transformation processes of the dioxins (C) 2000 Elsevier Science Ltd, All rights reserved.