Three Tier Approach to Long Range Transport Screening

Pennington D.W.
ORISE Research Fellow, Systems Analysis Branch, NRMRL, US EPA, Cincinnati, OH

The ability of a chemical to be transported over a long-distance is an important criterion in many national and international efforts for determining whether concern is warranted. These efforts can involve the screening of thousands of chemicals for which data is not always readily available. A three tiered screening methodology is therefore proposed, which was evaluated for 318 chemicals. The approach facilitates estimation of the relative distance that a chemical can travel using increasing levels of degradation data. Multimedia transport, discharge medium and the concentration in the remote media of concern are all taken into account.

A novel steady-state multimedia model facilitates estimation of the relative distance to a remote concentration associated with a given mass release rate (Tier 3). Six alternatives for the basis of the remote concentration can be evaluated (multimedia average, concentration in air, water, soil or sediment and the maximum irrespective of media) and four discharge scenarios (to air, to water, to soil and equally to all three). A simplified version of the proposed model is demonstrated to generally provide conservative estimates of the distance to a specified remote concentration suitable for Tier 1 screening. This model requires the specification of only one physical-chemical parameter, the degradation rate in air, and yields results within an order of magnitude of the full model predictions for 75% of chemicals. Guidelines are presented to help identify when this model is appropriate to avoid significant error. The second screening tier uses these same guidelines to identify which degradation data will be pertinent in the full transport model. Using Tier 2, the degradation data requirements of the full model are reduced by over 50 percent. The error introduced is less than an order of magnitude for over 95% of chemicals.