An official with the Department of Natural Resources says a plume of spilled fertilizer on the Floyd River appears to be dissipating. D-N-R spokesman Kevin Baskins says the “slug” of nitrogen fertilizer reached the Alton dam at 1 p-m today.They measured the plume at 15 miles long yesterday, while today it measured only five miles. He says the readings of ammonia have dropped from around 250-parts-per-million to about 75-parts-per-million. He says the dam is helping spread out the plume as is the river itself.Baskins says the volume of water in the river increases as it flows downstream and that helps dilute the plume. He says they won’t know until tomorrow if the flow of water over the dam helps dissipate the fertilizer. Baskins says ice has made it difficult to tell how many fish have been killed.The fertilizer spilled out Monday when a pipe on a bulk tank broke at the Midwest Farmer’s Co-op near the northwest Iowa town of Sheldon. The City of Alton pumped some water into storage before the plume reached the city, to ensure they have water until the plume passes.