The cars went off the tracks Wednesday morning in a steep, remote area along the river about 10 miles north of Dubuque. Canadian Pacific said 14 of the derailed cars were carrying ethanol, and eight of them appeared to be leaking.
"We have verified some ethanol has reached the water but we do not have an estimate of how much," said CP spokesman Andy Cummings, who was at the scene Thursday.
Ethanol mixes with water and, in high concentrations, can deplete the oxygen in water and kill fish, said Iowa Department of Natural Resources spokesman Kevin Baskins. He noted the impacted segment of the river was within the Upper Mississippi National Fish and Wildlife Refuge.
Baskins said the primary concern is the threat to fish and other aquatic life, such as mussels, which can't easily move away when oxygen levels dip. The DNR plans to sample fish collected from fishermen and monitor open-water areas in the largely iced-over river for signs of dead fish.
The train cars were carrying denatured ethanol, the grain-alcohol product made from corn before it's mixed with gasoline to make the automotive fuel. Baskins said about a half-acre of ethanol pooled on the ground and froze, and ethanol covered about an acre of ice on the river. Workers were trying to thaw the ethanol for removal.
The 81-car train originated in northwest Iowa and was heading to New Jersey. Each tanker car can carry up to 30,000 gallons. Some ethanol burned off in the three cars that caught fire.
The derailed cars are DOT-111 models, which the National Transportation Safety Board has been urging the industry to replace or retrofit since 1991. The NTSB calls the original DOT-111 cars still in use an "unacceptable safety risk."
The federal government is finalizing new standards for tank cars, but they aren't expected to be issued until this summer.
The AP contributed to this report.