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, the