The best way to regulate metals in stormwater runoff has been widely discussed. Historically, instream quality criteria for metals, including EPA criteria (1986 and 1987 revisions), have addressed total metal concentrations. New EPA guidance on metals allows for the regulation of bioavailable metals, which make up a fraction of the total concentration. Predictive modeling of the form and associated toxicity of metals under various conditions would improve data transferability and supplement the proposed EPA approaches to metals regulation. EPA collaborated with the U.S. Geological Survey and local agencies to study stormwater runoff quality in 28 cities as part of the Nationwide Urban Runoff Program (NURP). Results showed that total metal concentrations frequently exceeded acute instream criteria set for aquatic life. These conclusions may be misleading because the metals that make up total concentrations are not always in bioavailable forms.