FARGO, N.D. -- As the Red River slowly started to recede Monday and fears of major flooding diminished, Fargo residents were left to deal with another problem: environmental hazards that linger long after the waters decline.

Floodwaters can be noxious brews of pesticides, sewage, garbage and animal carcasses that foul drinking water, spread disease and damage fish habitat. Although the Red River didn't do nearly as much damage this year as during record-breaking floods in 2009, authorities say danger could persist.

"Fuels, chemicals, all kinds of things find their way into the water system and it's a huge environmental risk," said Keith Berndt, engineer for Cass County, which includes Fargo and West Fargo.

Towering mounds of sand on the outskirts of Fargo are a vivid symbol of those hazards.

The 70,000 tons of moist sand had been packed into bags that were stacked along the Red River a year ago, when flooding damaged hundreds of homes, forced thousands to evacuate and caused $100 million in damage.