Fremont State Lake #20: A Case Study

The Problem:

Water Quality at Fremont State Lake #20

  • By 2005, this 50 acre sand pit lake had very high phosphorus concentrations and significant toxic cyanobacteria (blue-green algae) blooms
  • Algal toxins lead to lake closures during the summer recreation season
  • The average summer total phosphorus concentration was 139 ppb
  • The average summer chlorophyll a concentration was 100 ppb
  • The average summer water clarity was only 14 inches

The Cause:

Phosphorus!  Excessive amounts of phosphorus were causing nuisance algal blooms and the associated symptoms of poor water quality.

The Solution:

An environmentally-safe alum formulation was added to the lake in May, 2007 to control internal phosphorus loading, resulting in less algae and better water quality. This approach was not toxic to the algae, but rather reduced the amount of phosphorus available for algal growth by retarding phosphorus inputs.

The Results:

In the three summers after the alum treatment (2007-09):

  • Total phosphorus was reduced by 85% to 21 ppb
  • Chlorophyll a (algal abundance) was reduced by 92% to 8 ppb
  • Water clarity increased by over 8 feet
  • Algal toxins were eliminated and the lake was re-opened for recreation