Of 32 BNSF train cars that hurtled off the tracks near Alma, Wis., on Saturday, five of them broke open and spilled at least 18,000 gallons of ethanol into the Mississippi River, the railroad company said.

Four damaged tanker cars leaked an estimated five to 500 gallons of ethanol each and a fifth one released about 18,000 gallons, BNSF said in a statement. A full tank car holds about 30,000 gallons.

The effect of the spill on river habitat below Lake Pepin remains unknown, but reports after previous spills indicate that ethanol alone is less toxic than ethanol mixed with gasoline.

Meanwhile, another train derailed Sunday in Wisconsin. That incident sent at least 10 Canadian Pacific rail cars carrying crude oil off the tracks about 2 p.m. in Watertown, in the southeastern part of the state. No fires or injuries were reported, but the rail company said some of the tankers were leaking.

Saturday's derailment, about 2 miles north of Alma and near Wabasha, Minn., involved a train with both tank cars carrying ethanol and empty cars used to transport vehicles. It temporarily closed two state highways and prompted about a 2½-hour voluntary evacuation of nearby residents, the Buffalo County Sheriff's Office said.

Crews from BNSF stopped the leaks from all five cars, placed a containment boom along the shoreline and began removing ethanol from the cars, said railway spokeswoman Amy McBeth. Working with cranes and winches, crews hoped to clear the tracks and reopen service along that stretch of rail line by Monday, she added. On average, 45 to 50 trains travel that route in a 24-hour period.

The rail company will continue to monitor environmental effects and "will work with the EPA and state agencies on the best plan for mitigation and remediation efforts," McBeth said.

In July, Canadian Pacific Railroad said it was nearly done with the cleanup and environmental monitoring after a Feb. 4 derailment in Dubuque County, Iowa, spilled ethanol into the Mississippi River. In that incident, three tank cars carrying ethanol started on fire and three others slipped onto the frozen river.

Railroad officials removed ethanol from 15 derailed cars but some of the fuel drained into the nearby soil and water. Ethanol degrades quickly in water, unlike oil. But ethanol can deplete oxygen in the water, hurting fish and other aquatic life. No fish kills were reported from that incident, the Iowa Department of Natural Resources said.

However, another spill of ethanol mixed with gasoline in June 2009 caused a significant fish kill in the Rock River near Rockford, Ill. In that case, an explosion and fire caused 75,000 gallons of the mixture to spill into a creek that flowed into the Kishwaukee River and emptied into the Rock River.

Two days after that derailment, residents began to notice large numbers of dead fish washing up onto a 54-mile stretch of the Rock River.

The Chicago, Central and Pacific Railroad ultimately paid a $150,000 fine to the Illinois Environmental Protection Agency and Winnebago County.