What is the problem?
In 2002, the Minnesota Pollution Control Agency listed Lake Independence as impaired for aquatic recreation under Section 303(d) of the Clean Waters Act. LICA, Three Rivers Park, Hennepin County Environmental Services, the DNR and many other patrons of the lake have been working since then to reduce the amount of phosphorus in the lake to improve water quality.
The main cause of the impairment is excessive nutrients in the lake. The TMDL (Total Maximum Daily Load) document assesses the nutrient load reductions needed for Lake Independence to comply with Minnesota water quality standards. The specific sources of nutrients, target reductions from each source, and strategies to achieve the reductions are discussed in the document.
What is the solution?
Reduce the amount of phosporus, entering the lake from all sources in order to reduce the growth of algae bloom (that is where the TMDL-Total Maximum Daily Load-comes in, that's the total amount of phosphorus that can enter the lake per day without overfilling) and other aquatic invasive species.
After all sources of phosphorus loading have been controlled, the lake can begin to heal and a whole lake alum treatment to boost clarity can be considered.
Secchi Depth Measurements on Lake Independence
by LICA members Jim and Kim Klancke
Kim and I are volunteer lake quality monitors for the MN Pollution Control Agency (MPCA) Citizens Lake Monitoring Program. We collect water quality data using a Secchi disk - an 8-inch circular black and white plate attached to a calibrated rope. The disk is slowly lowered into the lake and a reading is taken at the point the white disk is no longer visible. Then the disk is lowered further and slowly raised, noting the reading when the disk re-appears. We submit the readings to the MPCA at the end of each monitoring season.
The Secchi disk measures transparency, which is an excellent indicator of water quality. Water transparency indicates the amount of light penetration and indirectly shows the amount of suspended material in the water, which may include sediment and plant nutrients such as algae. Plant nutrient sources include septic tanks and lawn and farm fertilizer. Sources of sediment include shore erosion, farm fields and storm runoff. By measuring Secchi depth throughout the season and from year-to-year, transparency trends (and by extension water quality trends) can be observed.Over time, the collected data helps detect water degradation.
This graph combines data from Secchi measurements taken on Lake Independence during spring and summer from May, 2009 to October, 2019. The data are plotted as 7-day moving average measurements to smooth the data to better see the trend in water clarity throughout the spring and summer collectively, as individual readings have some variability based on a number of factors such as sunlight, waves, recent rainfall, etc. For example, the data point on Day 32 is the average of readings from 2009-2019 in the seven day period surrounding June 1. Typically, water quality gradually decreases beginning in June, stabilizing around the 4th of July and holding through August.
Because the water quality fluctuates throughout the spring and summer, taking an average of one year’s measurements is not particularly useful. Trends only emerge when data is collected and compared over a number of years. To date, the MPCA is reporting no detectable trend in water quality. The summary for Lake Independence can be found on the MPCA website.