Nebraska Department of Environmental Quality
Summer 2009

Treatment at Fremont Lake #20 Shows Impressive Results

A treatment project on Fremont Lake #20, conducted in 2007, has shown some impressive results in reducing levels of toxic algae and phosphorus at the lake.

The treatment project was a cooperative effort of the NDEQ, the Nebraska Game and Parks Commission and the University of Nebraska-Lincoln. Fremont Lake #20, one of a chain of sandpit lakes known as Fremont State Lakes, was the chosen site for this treatment project due to a number of unique characteristics.

The lake had historically tested high in phosphorus. High levels of phosphorus in a lake can lead to the growth of toxic algae, which had been a persistent problem at Fremont Lake #20. In fact, from June of 2004 through September of 2007, 68 of the 209 samples collected from the lake exceeded the toxic algae health alert threshold of 20 parts per billion of the toxin microcystin. This resulted in the beach being closed for 35 weeks, making this one of the most impacted public lakes in the state for toxic blue green algae toxins.

The other reasons the lake was chosen for alum treatment was due to its size (relatively small – 50 surface acres) and the fact that it is considered a “closed” system for phosphorus. Being a “closed” system means that it was determined that there were no major sources of external nutrient loading. Therefore, after treatment, there would not be a substantial new flow of nutrients into the lake.

The treatment process conducted in 2007 involved applying large amounts of alum into the lake. This alum binds with the phosphorus and pulls it down to the bottom of the lake, removing it from the water and burying it into the lake bed. State and University officials predicted that by binding the phosphorus, this source of nutrition for toxic algae would be effectively removed. The expectation was that levels of both phosphorus and toxic algae would be greatly reduced.

Followup lake testing after the treatment was applied appears to prove those expectations to be true. As the alum was applied, there was an almost immediate and dramatic improvement in water clarity, according to NDEQ Environmental Assistance Coordinator Paul Brakhage. Phosphorus was reduced five-fold – from levels around 110 parts per billion in the summer of 2007, to around 21 parts per billion after the treatment was completed. There was also a dramatic reduction in toxic algae readings. Prior to treatment, weekly readings were above the health alert threshold of 20 parts per billion for microcystin about 33 percent of the time. In 2008, there were no health alerts, and the highest reading was 0.23 ppb – nearly a hundred-fold below the health alert level.

Although this appears to be a successful treatment method at this lake, Brakhage said that alum treatment would only be feasible in limited applications. Due to cost, such treatment would not be feasible at large lakes and reservoirs, Brakhage said. In addition, if there are sources of additional nutrients flowing into a lake, such as an incoming stream, or farmland runoff, then the treatment may not be a long-term solution.

But, since Fremont Lake #20 is a groundwater-fed sandpit lake that has few sources of surface runoff, it is expected that the levels of phosphorus and toxic algae will continue to stay low for years to come.

Nebraska Department of Environmental Quality
1200 "N" Street, Suite 400
P.O. Box 98922
Lincoln, Nebraska 68509
(402) 471-2186