While there were no recorded spills from offshore infrastructure in the Gulf of Mexico region despite widespread storm damage, onshore refinery and storage facilities appear to have been less well managed.
The Houston Chronicle newspaper reviewed data from the US Government's National Response Center which showed the hurricanes released 39.5 million litres of oil, comparable with the 48.5 million litres released by the Exxon Valdez oil tanker in Alaska in 1989.
The two storms also released other chemicals, and the oil spills they caused were not contained in one area but were spread across a region, occurring in difficult-to-reach wilderness areas, rural countryside and residential neighbourhoods.
In Meraux, Louisiana, south-east of New Orleans, Katrina's floodwaters swept through Murphy Oil's refinery, tearing a big storage tank free from its foundations and spilling 4.5 million litres of oil into the neighbouring town of Chalmette.
The spill spread over at least 2.5 square kilometres and stained 1700 homes, the Houston Chronicle reported.
It left dark-brown stains on every house and vehicle. Recent samples of sediment taken from the area show levels of organic chemicals that are high enough to cause skin rashes and respiratory problems, according to a report from the Agency for Toxic Substances and Disease.
There are doubts about when and even whether the community will be environmentally safe, US Environmental Protection Agency coordinator Wally Cooper told the newspaper.
"This is about the 10th disaster I have responded to, and this is the worst I have ever seen," Cooper said. "This is worse than the worst-case scenario."
The largest oil spill from either hurricane occurred at a remote site in Louisiana at Bass Enterprises Production Company's Cox Bay facility, according to the Advocate newspaper of Baton Rouge, Louisiana.
Katrina's storm surge picked up two partially full oil storage tanks that were almost 5m high and 59m across and carried them more than 30m, spilling 3.8 million barrels of oil.
Because of flooding, a retention pond designed to hold 130% of the contents of the two tanks was already full of water.
Almost 2 million litres of oil seeped into surrounding marshes. Remediation workers had to drive for two hours and then travel two hours by boat to reach the remote facility.
Past storms had indicated that the storage tanks were vulnerable.
In 1961, Hurricane Carla moved a tank in Hackberry, Louisiana, about 10km, and a five-year study released in 2003 by Louisiana State University's Center for the Study of the Public Health Impacts of Hurricanes found many storage tanks were not properly tied down and could be major sources of spills.
Many environmentalists and local activists consider faulty equipment and bad planning to be the cause of many of the spills.
Refinery Reform Campaign coordinator Denny Larson told the Houston Chronicle that energy infrastructure should not be sited in low-lying, hurricane-prone coastal areas.
"It's the poorest possible choice," he said.
