Drainage Stone That Doesn’t Drain?

Drainage stone is supposed to drain. After all, typical 3/4″ to 1.5″ gravel used in septic trenches has a lot of void space. In my early days working at an engineering firm with a materials testing lab, I tested some typical, but fairly dirty, septic gravel and it had approximately 40% open space. So water should flow easily through that, right?

As part of our Septic SitterTM field validation program, we have been monitoring trench ponding levels in several leachfields located in PEI, Canada. We have observed some interesting results.

Septic Sitter installed in gravel trench.

Ultrasonic Septic Sitter sensors were installed in three locations in this Contour drainfield trench.

 System Description

Mr. & Mrs. Buns (the name is made up but the system and data is real) have a 15 year old raised-bed, C3 “Contour” drainfield. Contour drainfields were developed and popularized in Nova Scotia, and this system consists of a single gravel trench, 140 ft long by 4 feet wide and about 8″ deep.

The trench is constructed along a contour and the bottom of the trench is level throughout. The trench is trickle-fed at the centre with a Tee which should, in theory, cause the flow to split evenly in each direction.


Liquid Levels Throughout Trench

About a year ago we retrofitted inspection ports and installed Septic SitterTM sensors at three locations in the trench. As the data from the sensors show, the effluent distribution is not very even at all.

Graph of Septic Sitter liquid level data


During times of high flow, rainfall events or snowmelt, the middle of the trench (sensor #2) ponds to the top or even higher (see the 27″ high spike in the blue line on the graph). Otherwise, ponding levels fluctuate between 8″ and 12″.

At the location of sensor #3 (green line on graph) the trench is typically ponded about 10″ deep with very little variation. But near the end of the trench, at location #4 (black line on graph), the trench is always empty. We have only observed effluent ponding in this location once in the past year and that was only for a day after a major rainstorm.

What Is Going On..?

So why is this happening? The septic tank has had an effluent filter on it since the system was first installed, so I don’t believe that a lot of solids could have escaped from the tank and clogged the pipe or drainfield. However a video inspection of the perforated distribution pipe is recommended to ensure that a blocked pipe is not part of the problem.

Dirty Stone Theory…

The stone used in this system was quite “dirty” and contained a lot of fines (sand, silt, clay). If you saw it however, I doubt you would think it was dirty enough to prevent water from flowing freely through it.

However, perhaps over the years, biofilm developed in between the gravel particles, so that it started to bridge the void spaces between the stones. Could biofilm have developed to the point where it has started to interfere with the free movement of effluent throughout the trench? That is my current theory, however I would be interested to hear what you think.

Please comment below, and let us know how you would investigate or remediate this system so that the infiltration capacity of the entire leachfield can be restored.

Septic Sitter Launch This Week at WWETT 2017!

Dynamic Monitors is launching the Septic Sitter system this week at the WWETT show. Come by Booth 4312 and see a live feed of the data from this site. You can tell me in person what you think is going on with this system.

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7 Responses to Drainage Stone That Doesn’t Drain?

  1. Leah Boutilier March 14, 2017 at 1:40 pm #

    I suspect variability in biomat formation throughout the trench may be the primary factor. If gravity-fed, the biomat will develop more in the area receiving the majority of the effluent holding effluent for longer periods. As you move away from the inlet T, less septic effluent will make its way there during regular use resulting in less biomat formation, and therefore less ponding during peak use or heavy rain.

    Variability in sand hydraulic conductivity may also be a factor.

  2. Dave dewar February 25, 2017 at 6:41 pm #

    As one who has failed my first attempt at site Assesment ! Was the ditch perfectly level ? Personally I’m no fan of contour ! Any of them I see are to tthe point of leaking or the grass is heavy and wet that grows on top ! I don’t know just something I have seen ! Very wet at one point in the ditch and dry at another point in the ditch ! Pipe has to be level for even distribution ! I’m a thinking ! Anyway have to catch up to you ! Thanks be in touch !!

  3. George February 23, 2017 at 12:57 pm #

    We have done quite a bit of research on this issue and have verified what you have seen but not so dramatic a difference. I believe that the stone bridges/dams in areas so that the ponding levels across the bottom can be different. As the one area gets loaded (since in gravity fed systems there is ALWAYS differential loading in the trenches, areas of the bottom and sidewall thin and thicken causing occasional “breaks” in the dam and then you get some evening of the levels. We have only observed this with pipe-stone systems. I do not believe that it has any relationship with the fines in the stone, since in out experiments we had REALLY clean stone (<0.1% fine material).

  4. Garry-Lee Espinosa February 23, 2017 at 12:45 pm #

    I think you are correct with the biofilm theory. However, I’m not sure it’s biofilm plugging the void spaces between 1/2 – 3/4″ gravel particles. I think the biofilm growth is in the soil below the gravel. “Dirty Stone” would also exacerbate the failure of the soil with fines.

    We see this same thing with septic tanks that gravity feed to a d-box. The beginnings of the trench will be the first to experience ponding because it gets the highest loading rate as effluent is absorbed long before it gets to the end of the trench. As the beginning trench develops a biofilm, inhibiting absorption of the effluent, the effluent is pushed down the trench to other parts that don’t have the biofilm growth on it. I’ve always been told that this is called a “progressive failure” and common for gravity fed drainfields.

    One reason Orenco has always advocated pressure distribution as the only means of ensuring equal distribution along a drainfield trench.

  5. Bernie MIller February 23, 2017 at 11:04 am #

    I agree with the theory and have witnessed it myself on a few occasions. We have even seen this happen in a bed with pressure distribution. In most remedation cases we use a terralift and some pretreatment or a combination of both.

  6. Stw February 23, 2017 at 9:48 am #

    Are your retrofitted ports connected via a t to the perforated pipe, or are they sunk into the gravel trench

  7. John Roy February 23, 2017 at 9:46 am #

    I have seen in the past that if they used Lime Stone that the powder that washed off the stone will clog the underside of the stone area. Then the water can not get to the sand base.

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