Development and Application of Health-Based Screening Levels for Use in Water-Quality Assessments

Many water resources sampled by the U.S. Geological Survey (USGS) are used as drinking-water sources. Water-quality conditions in these assessments historically have been evaluated, where appropriate, by comparing measured contaminant concentrations to drinking-water standards or guidelines. Federal drinking-water standards and guidelines have not been established for about half of the organic contaminants most routinely analyzed in water by the USGS (Toccalino and others, 2005). Thus, in May 2001, interagency consensus was reached on a methodology for developing Health-Based Screening Level (HBSL) values for unregulated contaminants (sidebar 1) to be used as benchmarks for interpreting water-quality data in a human-health context (Toccalino and others, 2003). HBSLs supplement established Federal drinking-water standards and guidelines, thereby providing a basis for a more comprehensive evaluation of contaminant-occurrence data in the context of human health (Toccalino and others, 2005).

Sidebar 1. Health-Based Screening Levels (HBSLs)

HBSLs are benchmark concentrations of contaminants in water that, if exceeded, may be of potential concern for human health. HBSLs are non-enforceable benchmarks that were developed by the USGS in collaboration with the USEPA and others using: (1) USEPA methodologies for establishing drinking-water guidelines, and (2) the most recent, USEPA peer-reviewed, publicly available human-health toxicity information (Toccalino and others, 2003; Toccalino and others, 2006b). The USGS and its cooperators continue to refine the HBSL methodology.

HBSLs are based on health effects alone and do not consider cost or technical limitations. For carcinogens, the HBSL range represents the contaminant concentration in drinking water that corresponds to an excess estimated lifetime cancer risk of 1 chance in 1 million to 1 chance in 10 thousand. For noncarcinogens, the HBSL represents the maximum contaminant concentration in drinking water that is not expected to cause any adverse effects over a lifetime of exposure. HBSL calculations adopt USEPA assumptions for establishing drinking-water guidelines, specifically, lifetime ingestion of 2 liters of water per day by a 70-kilogram adult. For noncarcinogens, it also typically is assumed that 20 percent of the total contaminant exposure comes from drinking-water sources and that 80 percent comes from other sources (for example, food and air).

The original methodology for calculating HBSLs was published in 2003 with co-authors from the USGS, the U.S. Environmental Protection Agency (USEPA), the New Jersey Department of Environmental Protection (NJDEP), and the Oregon Health & Science University (Toccalino and others, 2003). The consensus HBSL methodology for unregulated contaminants uses existing USEPA Office of Water (OW) equations for establishing drinking-water guidelines. As referred to in this report, unregulated contaminants are those without Federal drinking-water standards established under the Safe Drinking Water Act.

Since 2003, revisions have been made to the HBSL methodology for unregulated contaminants in order to reflect updates to relevant USEPA policies, and this report summarizes these revisions. These revisions resulted from meetings between the USGS and the USEPA OW in August and September 2005 and continuing discussions with USEPA OW throughout the remainder of 2005. These meetings and discussions took place as part of the review process for USGS Circular 1292 (Zogorski and others, 2006), in which the USGS National Water-Quality Assessment (NAWQA) Program completed a national assessment of volatile organic compounds (VOCs) in ground water. HBSLs were used in Circular 1292 in a screening-level assessment to provide an initial national-scale perspective on the potential significance of unregulated VOC concentrations to human health. HBSLs used in Circular 1292 and provided on the HBSL website (Toccalino and others, 2006a) were calculated using the methodology presented in this report.

The purpose of this report is to summarize the revisions to the HBSL methodology for unregulated contaminants and to update guidance on the use of HBSLs for interpreting water-quality data in the context of human health.

Introduction