Comparison of deterministic and probabilistic calculation of ecological soil screening levels

The U.S. Environmental Protection Agency (U.S. EPA) is sponsoring development of ecological soil screening levels (Eco-SSLs) for terrestrial wildlife. These are intended to be used to identify chemicals of potential ecological concern at Superfund sites. Ecological soil screening levels represent concentrations of contaminants in soils that are believed to be protective of ecological receptors. An exposure model, based on soil- and food-ingestion rates and the relationship between the concentrations of contaminants in soil and food, has been developed for estimation of wildlife Eco-SSLs. It is important to understand how conservative and protective these values are, how parameterization of the model influences the resulting Eco-SSL, and how the treatment of uncertainty impacts results. The Eco-SSLs were calculated for meadow voles (Microtus pennsylvanicus) and northern short-tailed shrews (Blarina brevicauda) for lead and DDT using deterministic and probabilistic methods. Conclusions obtained include that use of central-tendency point estimates may result in hazard quotients much larger than one; that a Monte Carlo approach also leads to hazard quotients that can be substantially larger than one; that, if no hazard quotients larger than one are allowed, any probabilistic approach is identical to a worst-case approach; and that an improvement in the quality and amount of data is necessary to increase confidence that Eco-SSLs are protective at their intended levels of conservatism.