For Better Lake Living

Earl Lake Riparian's

The Problem With Salt:

The introduction of salt to a lake has two major adverse effects:

Problem (1); Toxicity

   Salt constituents, sodium and chlorides, at some level of concentration are both toxic to fresh water plants and marine life, but almost all of our Michigan lakes have some salt in them.  The only question is at what level does the salt addition become a serious problem.   Because the information on Sodium is much more limited than it is for Chlorides I will limit this explanation to Chlorides for the time being. 

   So lets look at some benchmarks for starters, just to get oriented.  Table (1) below shows some common water sources that most of us are all familiar with, and their corresponding Chloride concentration Levels.

Source:

EPA  http://www.epa.gov/glnpo/monitoring/limnology/

Virginia Tech  http://ewr.cee.vt.edu/environmental/teach/gwprimer/group05/main.html

Aquatic Life

   The Michigan DEQ reports that the average Michigan lake Chloride concentration is about 10. mg/L.  This is based on about 650 Michigan lakes where data is available.  By comparison, Earl Lake is at least 40 times higher than the average Michigan lake, and according to the MDEQ has the highest lake Chloride concentration on record in the state of Michigan.

   Now getting back to “how much is too much”, the MDEQ has said that Michigan does not have a Chloride “standard” but that they are working on one.  There is however an EPA Water Quality  Criteria for Chlorides, which reports a significant number of tests which have been done with various types of salts containing Chlorides.

   You will notice that the EPA criteria has two separate sub-criteria; one based on a four day average (called Chronic) and one based on a one hour average (called Acute).  Notably, both are set in a frequency of occurrence limit of not more than once every three years.  However, in the case of Chloride data measured for large water bodies (lakes) the resultant measured values are pretty much 24/365 (24 hours a day and all year long).  Therefore, we have some “apples to oranges” kinds of comparisons issues.  i.e. is a four day average effect the same as a 365 day average effect?   In the case of Earl Lake, the Chlorides at 428-486 mg/L are about twice the chronic (4 day avg.) safe level of 230 mg/l outlined for a once every three years occurrence.   According to this EPA report some living things like specific algae types begin to die off with chronic Chloride exposures with concentrations as low as 200 mg/l. 

   Figure (1) compares some measured local lake Chloride levels with the EPA criteria and other reference points of interest.  The 3000. mg/l shown in this figure as “Earl #14”, is the “Latson County Drain” discharge which is the largest water source (storm drain outfall) to Earl Lake.  It was measured during the first major snow melt in December 2005.

Figure (1)  Relative Chloride Level Comparisons

 

Problem (2); Chemical Stratification

   Lakes are normally stratified (unless they are very shallow) based on water density differences created by variations in water temperature from top to bottom, and the “thermo cline” refers to the temperature-density interface for this condition.  In this normal condition the top, or surface layer holds a lot of Oxygen and it falls to the bottom of the lake twice a year when the surface water temperature becomes colder than the bottom layer water temperature.  I.e. the water density increases as the temperature decreases.  Keep in mind that all living things need oxygen.

   Now, the second problem generated by salt in a lake, and perhaps more serious than the toxicity problem, occurs when salt brine, which is heavier than even the cold water, sinks to the bottom of the lake below the normal cold water layer.  Over time the heavier salt water begins to fill the lake bottom and creates a heavy bottom layer.  You could think of this resultant lake condition as three different water density layers in the lake.  The top layer being the warm water, the middle layer being the cold water, and the bottom layer being the salt water.  The interface at the salt layer is sometimes referred to as the “chem-cline”.

   The problem that is created, occurs during the lake “turn over” events that normally happen twice a year during the early spring and late fall.  During these events the heavier salt layer is not displaced by the cold water layer as it descends toward the bottom of the lake.   Thus, the bottom layer is never displaced with oxygenated water.  Or, you could say that the oxygen never gets below the chem-cline, (middle layer).

   This is a very simplified explanation, and things in nature are rarely that simple but you get the Idea.  The evidence of this effect is in the actual measurement of water oxygen content vs. water depth, and the chloride concentration levels vs. depth.   In the case of Earl Lake the DEQ reported, after sampling oxygen levels, in April of 2005, not long after the spring lake turn-over, that most fish species would not live below a depth of ten feet.  In July of 2005 the Chloride concentration levels were at 426 mg/l and 482 mg/l for surface and at 22’ depths respectively.  This is also very problematic in as much as some fish species like Northern Pike, that require deeper cold water in the hot summer months, have all but disappeared from Earl Lake where they were once abundant along with other types of deep water fish.

   It is worth mentioning at this point that Thompson Lake, which receives water from Earl Lake, has been reported to be chemically stratified as well, (“A Comparison of Limnological Conditions in Thompson Lake: 1976 vs.2002, David J. Jude, Freshwater Physicians, Inc., Feb 19, 2003).  At the date of testing, the dissolved oxygen at this time was completely depleted below 18. feet, down to 49 ft water depth .

   Consider the last sentence of the forgoing Virginia Tech Quote.  This has to do with the reduction of  bio-mass.  i.e. the decomposition of the annual die off of aquatic weeds.  I call it the compost pile at the bottom of the lake.  Compost piles reduce (bio-degrade) through a process requiring bacteria and Oxygen.  Well, Chlorides kill bacteria and chemical stratification inhibits Oxygen down where we need it at the bottom where the compost pile is.  Thus an accelerated accumulation of bio-mass buildup occurs.

Is the salt Problem getting better or worse?

   If we look at the available data for Thompson Lake from 1973 to 2005, we see an increase in Chloride concentrations from 37 mg/l, to 99 mg/l.  That works out to about a 3% average annual increase.  Unfortunately the data history for Earl Lake only goes back one year so far, but for that year of record, the increase from April 2005 to April 2006 was 13.% (372. to 422. mg/l—surface).  Lake George Chloride levels increased by 28% (238 to 306 mg/l) for the same time period.

You can see the “Earl Lake Salt Intake Study” project for more details.

What can be done about it?

   Although there is a lot of work still remaining to quantify all the potential salt sources, and investigate more exactly what portions of the Earl Lake watershed are responsible for salt contributions, the final solution to salt concentration reduction eventually is a no brainer.  Either salt intake is reduced or more fresh water is retained (or added) or both.  The more difficult question is how much salt reduction is needed to start turning the situation around to achieve at least a gradual year to year lake salt concentration decline.  The answer to “how much” will require a “lake balance” and a good accounting of all the salt currently going into the lake’s watershed.  A complete understanding of both salt and water intake is essential.

   Nevertheless some ideas are worth consideration now. Click on the link below for a short discussion of the behavior of salt in our environment, and a description of some possible salt reduction action items.

Source:  http://www.epa.gov/storet/index.html

 The Complete 1988 EPA Water Quality Report   PDF File

The Problem

With Salt

 

 

 

· Page 1  The problem

· Page 2  Concentrations

· Page 3  What Can We Do