Minnesota winters confound mound septics

Just a couple weeks ago it was thirty below zero. Now teenagers are wearing shorts and the dirt roads are breaking up. I saw forecasted highs in the 60s coming down the pike. That’s how fast the weather turns in Northern Minnesota.

A few weeks ago I warned about the possibility of frozen septic mounds this year due to the deep penetrating frost and lack of insulating snow cover. Well, with a few exceptions I’ve heard about, the major regional septic freeze didn’t happen. Now, with overnight lows near or above freezing, it won’t. We dodged a nasty, stinky bullet.

But last week veteran Iron Range septic installer and op-ed commentator Joseph Legueri contacted me with some independent research he had conducted on septic mounds over by Palo. Yes, these are the kinds of e-mails I get. Yes, I do answer them.

In essence, Legueri explains how the fact that septic mounds in Northern Minnesota typically spend half the year very near or even below freezing essentially depletes them of their sterilizing, detoxifying abilities for those months.

Legueri writes:

The mounds we use up here were developed in Madison, Wisconsin, which is over 400 miles south of Ely. Thus they are called “Wisconsin Pressure Dosed Mounds.” Although the mounds were tested extensively in Madison, the Minnesota Pollution Control Agency (MPCA) mandated their use in far Northern Minnesota without constructing a mound up here and testing it before they mandated it.

That’s a problem. Legueri found that for six months out of the year, mound septics don’t do anything. The waste does not break down; it waits for spring. And when spring arrives (like it will this month or next) the sudden warming of the dormant bacteria and toxins prove too much for the system, and bad stuff likely escapes into the groundwater.

Essentially, Legueri argues, the septic mounds mandated on so many rural Minnesota residential properties aren’t the most environmentally responsible option after all.

Is there more? Yes, so if you want to know how Legueri reached this conclusion, see the following. If not, you are a champ for reading this far on a beautiful day (in which metastasizing toxins are bubbling beneath the earth).

Here’s what Legueri did (the chart he references is found here):

In September of 2012, I asked six rural Palo, MN, homeowners with pressurized, mound-type septic systems if I could stick three soil thermometers into their mound. A soil thermometer is like a meat thermometer, only it has a three foot long stem; and a mound-type drain field sticks up four feet above the surface of the ground. Because I worked for a licensed septic system installer since the seventies, I was present as a laborer at the installation of dozens of mounds, so I knew exactly where to place the thermometers. We placed one thermometer where the wastewater entered the mound, one in the middle of the mound, and one at the end of the mound. The homeowners were kind enough to read the thermometers once per week (on Saturday) and report their readings to me. I kept track of those weekly readings until June of 2013.

You will see “In,” “Middle,” and “End” at the top of the chart’s columns. The word “Control” heads the fourth column. I placed a soil thermometer at the far end of my own underground, gravity-flow septic system. I read the thermometer each week and listed the readings under the “Control” Column. Please also note the average monthly temperatures and the snowfall figures listed at the right of the temperature chart.

Knowing that our refrigerators are factory-set at 40 degrees to reduce bacterial action, and knowing that septic systems use bacterial action to treat sewage, I sent the Mound Temperature Chart to more than a dozen bacteriologists, engineers, and biologists. Following are three of their comments about what happens when the mounds try to treat sewage at such low temperatures for six months:

Dr. Brian Lanoil, University of Alberta (Edmonton)-“When (mound) temperatures drop, biochemical reaction rates drop as well. So the treatment of wastewater will slow down at lower temperatures. Furthermore, pathogens (poisonous materials) such as viruses and bacterial pathogens will be preserved by the cold, making the end product more dangerous.”

Dr. Scott Meschke, University of Washington (Seattle)- “In general, as temps in the mound system approach freezing, the biological activity in the mound will slow reducing the degree of biological stabilization and pathogen reduction. Pathogen survival will likely be enhanced by colder temps (particularly for viruses; some bacteria and Protozoa may be slightly reduced by freeze-thaw damage if temps actually reach freezing).

Craig Mains, Engineer, National Environmental Services Center, West Virginia University (Morgantown)- “The worst case scenario would seem to me to be that in extremely cold weather there is little or no treatment taking place in the mound itself and treatment is taking place in the native soil underneath the mound…You might make the case that in those circumstances the mound is providing no better treatment than a conventional, in-ground drainfield…”

I could quote many more scientists and engineers, but all whom I have contacted say that at these temperatures, for six months out of the year, our northern mounds are manufacturing pathogens and putting them either into the soil or into the water table.

Legueri’s data and notes may be found here.


  1. Interesting. So is there a suggestion as to a system that would be more effective and safer? What does Mr. Legueri suggest as an approach that would avoid the high levels of dangerous contamination of lakes and wells due to the poorly functioning systems of the past and the pitfalls of the mound approach? Does the state have any alternative suggestions?

    Sounds like a good project for in depth journalism.

  2. I believed that the mound system was assumed to be safe and possibly the only good option where surface soils are relatively impermeable, surface water is distant, and population density is low.

    Absent those conditions, problems will most likely occurr, but there is also a political aspect: Imagine demanding a developer to install a rural sanitary treatment system? Me niether. Now demand that a group of existing homeowners pitch in on one when the first neighbor’s system starts to fail!

  3. What Legueri says is convincing as far as it goes. But it needs to be investigated further with actual measures of pathogens in the system and groundwater impacts.

    And I, also, would be interested to know his thoughts on preferable alternatives to mound systems.

  4. What I don’t like about the mound system is how much dirt they haul in and how much space it takes up. Plus you have to pump it out and put that waste somewhere. There must be a better way. For one thing, there should be more done on using gray water. Also, composting systems can work quite well year round if in a heated environment. It just seems that we resist making these kinds of changes.

  5. I have been fighting against this, but I have to say it:

    The evidence seems to suggest that only bears should be allowed to s*** in the woods.

  6. Mound systems work well if properly constructed and maintained. What other septic can be used where soils are thin and water tables are high. They started in North Dakota, and are widely used across the north. Where i live on the range, I prefer a conventional underground cement tank and drain field, because we have deep clay and sand. Pump them every couple years and they are effective for years.
    Mound systems have a hot bio film layer which breaks down wastes and kills pathogens, kept warm by microbial actions. If you worry about freezing and the normal snow is lacking, then add a layer of mulch, straw, or wood chips. Running hot water daily for baths, dish washer, or laundry, will also help.
    Mr. Legueri is not listed on the states Web site for licensed installers.

    • Joe Legueri says

      This is Joe Legueri responding to Mike’s comment that I am not a licensed septic system installer. In a way, Mike is right. I have never owned a company which installed septic systems, but as I have stated “I worked for a licensed septic system installer” for many years. Further, “I was present as a laborer at the installation of dozens of mounds.” I’m sure that the Minnesota web site which lists licensed septic system installers does not list laborers . However, the company which I worked for is listed as a licensed installer.

  7. Way to go , Joe. I’ve found that the laborer has more knowledge of what actually takes place out in the field than the licensed contractors. They are there doing the work. And have a better visual memory of what was done, also. They are not just paper shufflers like the big dogs.

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