Multimedia Mass Balance Modelling of Two Phthalate Esters by the Regional Population-Based Model (RPM)
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Achieving an adequate understanding of the fate of commercial chemicals in the environment is best demonstrated by assembling a comprehensive multimedia mass balance model of the chemical's behaviour in a specified region. This approach is demonstrated for two phthalate esters, di-n-butyl phthalate (DBP) and di-2-ethylhexyl phthalate (DEHP), in an industrialized region with similar characteristics (including population density) to the Netherlands and the Eastern United States. To accomplish this task the relevant physical-chemical properties of these substances are compiled and emission rates are estimated on a per capita basis. The "regional population-based model" (RPM) is described and concentrations of the phthalate esters are estimated and compared with available monitoring data from Europe. A sensitivity and uncertainty analysis is also included. Comparison of model predictions and monitoring data suggests that the major uncertainties in the multimedia fate assessment are the degradation half-lives and emission rates, but despite this, most reported concentration ranges are within a factor of four of the median predicted concentrations. Improved agreement between predicted and observed water, sediment and fish concentrations of DEHP is obtained by using a measured value of KOC for the suspended particles in the water column. The overall persistence or residence time of DBP and DEHP attributable to reaction only (not including advection) are estimated to be 25 and 47 days, respectively; thus historical accumulation of these chemicals over periods of years or decades is unlikely. It is concluded that, in general, the model captures the key processes that control the behaviour of these substances and predicts environmental concentrations that are of the correct order of magnitude. Consequently, this analysis appears to provide a reasonable quantitative description of the environmental fate of these chemicals.
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