Billions of dollars have been spent over decades to reduce harmful runoff into the Gulf of Mexico, where fish and other species die off each year. But the longstanding efforts have failed to slow down the problem.

The source of the trouble originates upstream along the Mississippi and other rivers, where fertilizers and manure from Midwestern farms plus urban sewage carry nitrates into these waterways. The pollution eventually flows into the Gulf, feeding algae blooms that eventually draw the oxygen out of the water.

The result: a dead zone off the coast of Louisiana and Texas that either drives species away or kills them. And according to the U.S. Environmental Protection Agency, it creates the costliest and most challenging environmental problem for the nation.

Biologists first discovered the dead zone four decades ago. Since then, the federal government has allocated billions of dollars to figure out a solution. But the flow of nitrates into the Gulf has only increased—by 10%—since 1980, according to an investigation by the Des Moines Register.

The newspaper found that nine states are responsible for three quarters of the nitrates flowing into the Gulf. The two biggest culprits are Iowa and Illinois.

-Noel Brinkerhoff

To Learn More:

Runoff From Iowa Farms Growing Concern In Gulf (by Perry Beeman, Des Moines Regiser)

Dead Zone Pollutant Grows Despite Decades Of Work (The Times-Picayune)

U.S. Coastal Dead Zones Grow (by Noel Brinkerhoff, AllGov)