An alternative screening model for the estimation of outdoor air concentration at large contaminated sites. (September 2017)

• Record Type: Journal Article
• Authors: Verginelli, Iason; Nocentini, Massimo; Baciocchi, Renato
• Is Part Of: Atmospheric environment. Volume 165(2017)
• Page Start: 349
• Page End: 358
• Publication Date: 2017-09
• Subjects: Contaminated sites -- Risk assessment -- Outdoor air volatilization -- Screening model -- Box model; Air -- Pollution -- Periodicals; Air -- Pollution -- Meteorological aspects -- Periodicals; 551.51
• DOI: 10.1016/j.atmosenv.2017.06.052
• Languages: English
• ISSNs: 1352-2310

Abstract

Abstract: Simplified analytical solutions of fate and transport models are often used to carry out risk assessment on contaminated sites, to evaluate the long-term air quality in relation to volatile organic compounds in either soil or groundwater. Among the different assumptions employed to develop these solutions, in this work we focus on those used in the ASTM-RBCA "box model" for the evaluation of contaminant dispersion in the atmosphere. In this simple model, it is assumed that the contaminant volatilized from the subsurface is dispersed in the atmosphere within a mixing height equal to two meters, i.e. the height of the breathing zone. In certain cases, this simplification could lead to an overestimation of the outdoor air concentration at the point of exposure. In this paper we first discuss the maximum source lengths (in the wind direction) for which the application of the "box model" can be considered acceptable. Specifically, by comparing the results of "box model" with the SCREEN3 model of U.S.EPA we found that under very stable atmospheric conditions (class F) the ASTM-RBCA approach provides acceptable results for source lengths up to 200 m while for very unstable atmospheric conditions (class A and B) the overestimation of the concentrations at the point of the exposure can be already observed for source lengths of only 10 m. In the latter case, the overestimation of the "box model" can be of more than one order of magnitude for source lengths above 500 m. To overcome this limitation, in this paper we introduce a simple analytical solution that can be used for the calculation of the concentration at the point of exposure for large contaminated sites. The method consists in the introduction of an equivalent mixing zone height that allows to account for the dispersion of the contaminants along the source length while keeping the simplistic "box model" approach that is implemented in most of risk assessment tools that are based on the ASTM-RBCA standard (e.g. RBCA toolkit). Based on our testing, we found that the developed model replicates very well the results of the more sophisticated dispersion SCREEN3 model with deviations always below 10%. The key advantage of this approach is that it can be very easily incorporated in the current risk assessment screening tools that are based on the ASTM standards while ensuring a more accurate evaluation of the concentration at the point of exposure. Highlights: A new model for assessing outdoor concentration at contaminated sites is proposed. The ASTM box model can overestimate the outdoor concentrations by a factor of 10. The new model well replicates the results of more sophisticated screening tools. An equivalent mixing height that allows the use of the ASTM box model is proposed.