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Water Quality

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A Summary Of Ten Mile Lake Trends

Based on Volunteer & Consultant 2005 E&E Reports

Prepared by Marty McCleery

General Comment by Del Hogen,
President, Instrumental Research, Inc.

 “The water quality of Ten Mile Lake has improved substantially over the past 20 years and the Main Basin continues to improve from year to year.  Nitrogen concentrations are decreasing steadily in four of the basins; Main Lake, Long Bay, Kenfield Bay, and Lundstrom Bay. Total nitrogen is increasing substantially in Flower Pot Bay and Robinson’s bay.  Nutrient phosphorus concentrations in the water column continue to drop, as does chlorophyll-a.  Water clarity as measured by Secchi disc readings is improving as well.”

The reports are a continuation of the process that began in 1994 which involves composite surface sampling of Ten Mile Lake, at six sites, three primary and three secondary.  The primary sites are sampled three times every year, while the secondary sites rotate so that each is sampled every three years.  Four sites are sampled on the lake and these sites are sampled three times each summer season.  Sampling dates for the season are as close as possible to May 15, July 15, and September 15.

The Ten Mile Lake Association has gathered 12 years of lake related, water quality, information.  Water clarity data is in the form of Secchi disc readings.  Chemical macro nutrient concentrations are in the form of total phosphorus and total nitrogen. Solar energy conversion to carbohydrates is dependent on Chlorophyll-a.

At this time the collected information can be converted to an easily understood water quality index, commonly referred to as the Carlson Trophic State Index (CTSI).   The Carlson Trophic State Index (CTSI) formulas have been selected as the mechanism for conversion of Ten Mile Lake analytical information to a recognizable scale.  This data conversion allows a direct comparison of the water quality of Ten Mile Lake to other lakes in the Minnesota Pollution Control Agency (MPCA) Citizen Lake Monitoring Program (CLMP).

Secchi Disc Readings

 

Warren Goss started the Ten Mile Lake Secchi disc program and was one of the first strong advocates for improved lake water quality in Ten Mile Lake.  These efforts continue today through the efforts of Jim Schwartz and Tom Moore and a host of volunteers on the Environmental and Ecology Committee.

According to the MPCA Secchi disc data, as analyzed by David Wright PhD, MN Department of Natural Resources, Ten Mile Lake has the second highest positive change in Secchi disc readings from the two time periods 1988-1991 to 1992-1995. 

Ten Mile Lake water quality has been managed with an aggressive onsite septic system upgrade program, and an intensive effort to analyze trends and changes in water quality and efforts to educate and inform the lakeshore residence about best management practices.

Carlson Trophic State Index (CTSI)

Robinson’s Bay, and Flower Pot Bay have increasing CTSI units (Carlson Trophic State Index), meaning poorer water quality.   While Flower Pot Bay only increased two units, Robinson’s Bay increased a significant eighteen units over the twelve-year sampling period.  The Main Basin and Long Bay are virtually unchanged.  Kenfield Bay and Lundstroms Bay have improved by dropping four and three units respectively.

Total Nitrogen

The total nitrogen does not fit the CTSI parameters but is important because it has its own set of environmental consequences.   Total nitrogen levels have been see sawing for many years and will likely continue.  The input levels are likely to be steady, but the process of denitrification is concentration dependent.  The effluent from onsite septic systems is a significant source of nitrogen for the bays while airborne precipitation input affects the lake overall.

The problem caused by nitrogen as nitrate or ammonia is a substantial lowering of the disease immune response of the trees and shrubs in the flow path from the septic system.   There is normally a symbiotic relationship between the trees or shrubs and the native soil fungi.  Symbiosis is a working relationship between partners where each contributes something to the relationship that the other party needs but cannot produce without the association. This relationship between the fungi and the tree is lost when nitrogen soil levels rise.

Estimated Nutrient Budget

 

With the physical data related to Ten Mile Lake a nutrient budget can be formulated. The following list of parameters is required to construct such a formula:

  1. Direct liquid precipitation to the lake surface.
  2. Surface water runoff into the lake.
  3. Groundwater input.
  4. Discharge of nutrients from septic systems into the lake.
  5. Groundwater discharge from the lake.
  6. Discharge via the Boy River.
  7. Evaporation from the lake water surface.
  8. Transpiration from the terrestrial vegetation in the watershed.

The chemical components of each item in the above list are used to build the yearly nutrient budget. As the nutrient content of each constituent changes, which it will, a response plan can be formulated to counteract undesirable reactions.

From the sources mentioned, the project was able to construct a fairly accurate nutrient budget for Ten Mile Lake. The majority of the chemical nutrients that support the biomass of Ten Mile Lake are internally recycled nutrients.  That is these nutrients are precipitated to the sediments in the fall and recycled into the plants, epiphytic algae, and the water column at turnover each year. The majority of the nutrients accompanying the water effluent from the onsite septic systems are still being retained in the intervening soils between the septic system and the lake shoreline. About 15 % of the nutrient budget for Ten Mile Lake comes from Onsite septic systems.

The nutrient budget load of total phosphorus was 4,505 pounds and total nitrogen was 42,400 pounds for the year.

Outflow of phosphorus via the Boy River is 125 pounds and nitrogen is 3,750 pounds per year, which is 3 % of the total load for both phosphorus and nitrogen.

Currently the internal cycling nutrients, the portion of phosphorus and nitrogen that is redissolved again each year from the lake sediments, is phosphorus about 6,150 pounds, about 60 % of the total load of phosphorus, and about 115,600 pounds of nitrogen per year.  Retained phosphorus was 4,380 pounds and retained nitrogen was 38,650 pounds in 2005.

What can we learn from these numbers?  In the not too distant past, about 1970, the phosphorus and nitrogen internal loading was higher than it is today. The average septic system did not meet Minnesota standards for onsite wastewater treatment.  This means that almost 50% of the internal cycling nutrients are currently buried in the bottom sediments. During 2005, about 40 % of both the phosphorus and nitrogen were precipitated into and added to the bottom sediments.  As long as the sediment interface remains oxygenated this nutrient load will stay put.  

Precipitation

 

A 200-foot dissolved oxygen (DO) profile was done in February of 2005 and shows   7.48 milligram per liter of DO at the 200-foot depth (60 meters).  This is an excellent result because even though the bacteria, algae, invertebrates, and fish have been breathing from the ice covered lake, they have not been able to deplete the DO more than 5.6 milligrams per liter.

A short period of oxygen removal from any of the deep lake basins will generate a large influx of internal cycling nutrients from the bottom sediments.  This will in turn generate higher algae populations.  Secchi disc readings in the main basin of Ten Mile Lake in 1974 were about 12 feet.  Today we get maximum readings of 24 to 26 feet, which means that in 1974 there was twice the mass of nutrients in the water column that there is today. 

Eurasian Watermilfoil

Boat launch sites are the most likely points for Eurasian water milfoil to be inoculated into Ten Mile Lake.  For this reason, a fall survey of the aquatic vegetation patches is done at as many of the boat launch sites on Ten Mile Lake as possible.  There are eight potential launching sites that could become inoculation areas for Eurasian water milfoil and other exotic species.  No Eurasian Watermilfoil has been found through 2005.

Drinking Water Well Sampling

Sampling your drinking water well water is important for protection of your health.  This is especially true for seasonal residents that shut off and turn on their water each year.  There is a greater chance of contamination during shut down and start up.  The nitrogen and bacteriological data collected tells you if you have a problem with your in home distribution and storage system and can also indicate a poorly functioning septic system that may be contaminating your well.

Several members took advantage of the TMLA annual well water testing system.  The 2005 well samples have been about the same number of tests from the North Shore and the South Shore.  The number of positives is also the same from year to year.  The following data is for the past three years.