How are HBSLs Used?
Concentrations of contaminants in water are compared to human-health benchmarks in screening-level assessments to provide an initial perspective on the potential relevance of detected contaminants to human health and to help prioritize further investigations. Two human-health benchmarks are used in USGS screening-level assessments: U.S. Environmental Protection Agency's (USEPA) Maximum Contaminant Levels (MCLs) and USGS's Health-Based Screening Levels (HBSLs). Concentrations of regulated contaminants (those with MCLs) are compared to their MCLs and concentrations of unregulated contaminants (those without MCLs) are compared to their HBSLs, when available. See "Guidance
on Use of Benchmarks in Screening-Level Assessments" and SIR 2007-5106 for more information.
Comparisons of measured contaminant concentrations in water to MCLs and HBSLs are useful for local, State, and Federal water-resource managers and others charged with protecting and managing drinking-water resources. For example, these comparisons can indicate when measured concentrations may be of potential human-health concern and can provide an early indication of when contaminant concentrations in ambient water resources may warrant further study or monitoring.