A review of common approaches to determining allocation factors and relative source contribution factors for drinking water contaminants: caveats and areas for improvement

Abstract

Relative Source Contribution (RSC) factors, or Allocation Factors (AF), in the 0–100 % range are commonly applied for calculating non-cancer health-based guidance values (HBGVs) for drinking water. The use of an RSC/AF allows consideration of non-water exposures when developing HBGVs. An RSC/AF value can be calculated from chemical-specific exposure data or assigned a default value based upon qualitative information or a lack of data. In this review, we analyzed RSC/AF procedures and outcomes from six agencies and 30 published scientific papers. For agency-derived RSC/AF values, default values with no rationale provided were most common, followed by default values informed qualitatively by data, non-default values informed qualitatively by data, and non-default values calculated from data. Data-based non-default RSC/AF values were uncommon due to insufficient exposure data for critical population groups. Furthermore, we found that the bases for RSC outcomes are poorly documented, making analysis of the decision process sometimes impossible. For RSC/AF values from the literature, we observed proportionally more data-calculated RSC/AF values compared to agency results. Our findings indicate a need to make better use of available exposure data, including allowing a wider range of default options and/or the use of modeling approaches. A decision matrix is proposed in this regard.