The Spills and Accidents database contains data on toxic chemical spills and other accidents reported to the National Response Center (NRC). This database used to be called ERNS, the Emergency Response Notification System, and is still referred to as ERNS in many situations.
Spills and accident data was last updated on RTK NET as of January 5, 2016, and includes incidents from 1982 through 2015.
Incidents reported to NRC range from minor to serious, from an oil-sheen on water to a release of thousands of gallons.
NRC reports are extensive, but also known to be incomplete, as many incidents are never reported, and those that are reported generally are not subject to verification.
More information about the data is available here.