EPA recently revised the Waste Minimization Prioritization Tool (WMPT) in response to public comments on the beta version (1). The revised version was used as the foundation for developing EPA's Draft RCRA PBT List (2), a list of chemicals that, when final, will serve as the focus of voluntary national waste minimization program activities. The WMPT has also been proposed as a way to identify additional candidate chemicals for the Agency's PBT (Persistent, Bioaccumulative and Toxic Pollutants) Initiative (3). 

The WMPT is a joint product of EPA's Office of Solid Waste (OSW) and EPA's Office of Pollution Prevention and Toxics (OPPT). It provides a screening-level assessment of a chemical's inherent ability to persist, bioaccumulate, and result in chronic toxic effects to humans or ecological systems. One purpose of the system is to screen across the thousands of chemicals that may potentially appear in hazardous waste streams. To conduct this broad-based screening, the WMPT makes use of data from a variety of EPA and outside sources and follows established "data preference hierarchies." For example, Reference Dose (RfD) and Reference Concentration (RfC) data from the Agency's Integrated Risk Information System were considered to be "highest preference" for the scoring of human noncancer effects for a chemical; if these data were unavailable, data from other, lower-preference sources were used. These data preference hierarchies assign higher preference to measured data than to estimated data, based on the public comments received on the beta version. 

Data from the various sources used in the WMPT were placed on similar scales by comparing data values with established fencelines (i.e., cutoffs) to place the values into low, medium, and high bins (i.e., categories). For example, if a chemical had a bioconcentration factor greater than or equal to 1,000, it was placed in the "high" bin and assigned a bioaccumulation score of 3. Similarly, values from 250 to 999 were placed in the medium bin (score of 2), and values below 250 were placed in the low bin (score of 1). Where possible, these fencelines were made consistent with the Agency's protocols for screening new and existing chemicals under the Toxic Substances Control Act. Where Agency precedents were unavailable for establishing fencelines, the fencelines were set so that 25 percent of the values would be considered high, 50 percent medium, and 25 percent low. There are a number of pro's and con's of this type of simple binning approach, but on balance the Agency felt that a three-bin system was the best approach to place disparate types of data on comparable scales and was generally consistent with the degree of uncertainty in the underlying data.

The scores for persistence, bioaccumulation, human toxicity, and ecological toxicity were then aggregated to obtain overall chemical scores, ranging from a low of 3 to a high of 9, as shown in Exhibit 1. 

Important assumptions are interwoven in this scoring approach. For example, it is assumed that human cancer and non-cancer effects can be placed on comparable scales and that human and ecological effects should receive equal weight in overall chemical scoring. 

One of the most significant revisions to the WMPT was the incorporation of a level 3 multimedia fate model to estimate environmental persistence. Public comments had pointed out that the previous WMPT approach, which focused on degradation in surface water alone, would mis-characterize some chemicals that were persistent in surface water but could partition to air and rapidly degrade. To address this and similar concerns, EPA's Office of Research and Development provided support in developing a spreadsheet version of the evaluative multimedia fate model described in Mackay et al. (4).The spreadsheet format offered the advantages of bulk processing of thousands of chemicals and an ability to alter some of the default parameters of the evaluative model, particularly advection rates.

For the general screening applications supported by the WMPT, it was desired to evaluate persistence as the ability of a chemical to remain in the environment unless removed through degradation. Therefore, since advection is not a degradation process and only results in the re-distribution of a chemical in the environment (removal from one modeled region to another), a closed system or zero advective removal approach was adopted to calculate persistence in the multimedia fate model.

The Revised WMPT was prepared in spreadsheet format for the explicit purpose of supporting development of the Draft RCRA PBT List. Background information and technical documentation for the Revised WMPT can be obtained from www.epa.gov/wastemin/ or by calling the RCRA Hotline at 1-800-424-9346. Since some of the public comments on the Draft RCRA PBT List address the WMPT, additional revisions may be made in the near future.
 

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