Several hundred gallons of residual No. 6 fuel oil contaminated several miles of a river that was previously contaminated by oil from different urban sources. ERA provided investigatory, remedial design, and monitoring services. After notifying the regulatory agency, ERA planned and monitored an Immediate Response Action (IRA) to remediate surface water, tainted vegetation and the river banks. Booms placed at strategic locations up to five miles downstream removed product and protected a major municipality's water supply intake on the river. Following completion of the IRA, ERA conducted an extensive forensic evaluation of the oil in surface water and sediment to distinguish between the spilled oil and large amounts of oils existing as background from urban sources.