Water from our rivers is “hard,” meaning it has a high mineral content, especially calcium and magnesium. This is because 100 million years ago, what is now Iowa, was under the surface of the ocean, and large limestone deposits are the result. As rain water and snow melt trickles through the limestone, toward shallow groundwater and eventually into our streams, it dissolves mineral deposits. Hardness is reduced in DMWW’s treatment plants using a process called lime softening.
The river water has a high nutrient content, especially nitrate-nitrogen and phosphorous. Some of the fertilizer used for crops is washed away or enters the streams through agricultural drainage tiles. Our super-rich soils also contain a lot of natural nitrogen and phosphorous, which is liberated from the soil during cultivation. The Raccoon and Des Moines Rivers have the highest and second-highest nitrate concentrations of the 42 largest Mississippi River tributaries. DMWW operates the world’s largest nitrate removal facility at its Fleur Drive Plant.
Our rivers have a relatively high level of sediment. Erosion is a natural process that has been greatly increased by both rural and urban land uses. It’s likely that most Iowa streams ran much clearer prior to settlement than in the present day. Loss of native perennial vegetation and increased streamflows because of human modification of natural hydrology, along with changes in land use and climate, are the primary culprits for increased erosion. In addition, levees have divorced streams from their floodplains, increasing river energy during high flows. This helps the stream scour sediment from the stream bed and banks. DMWW removes about 20,000 pounds of sediment (1,400 pounds per million gallons) from the treated river water every day, so the water will be clear and safe to drink.
Although DMWW’s source waters are considered impaired, DMWW is continuously adapting its treatment processes to deliver Water You Can Trust for Life.