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The dominant surficial geology in my region of Prince Edward Island, Canada is glacial till soil, rich in iron, so our mineral soil horizons are typically very reddish-brown in colour. This can make it hard to detect redoximorphic features in the soil profile. Where the soil is seasonally saturated, often all we get for redox are alternating duller and darker shades of red-brown.

I am not a soil scientist, I am a civil engineer who practices in the area of on-site sewage treatment and disposal. So over the years, I have had to fill in the gaps in my soil science knowledge through independent study, short courses, and asking questions from soil science professionals when I have the opportunity. I thought I would reach out to some of my LinkedIn groups to see if you might be able to provide some insight on what I am seeing in the following photographs of a soil test pit I recently examined. Note: The photos are of the same area in the test pit. The only difference is that for one I had the flash on (truer to the natural colours) and for the other I did not.

Photo of test pit – flash on. How the soil colours looked to my eye.












Same area of test pit – flash off. Mottling more obvious with less light.












The land at this site sloped gently at about 5% to 8% toward a coastal bay. At the time of the test pits (August), the vegetation consisted of a grassy field with some small shrubs.

The test pit photos show one of several large (12″x12″ roughly) zones of loose, very sandy soil (sand to loamy sand) which were  mixed in with the dominant, loose to compact, sandy loam soil. The area shown in the photos was fairly close to the ground surface, about 40cm (16″) below the rootmat. This sandy zone seemed to have dark brown to almost black mottles contrasting against the reddish-brown colour which I think are quite evident in the photos. Do you think the colour pattern shown in these photos is indicative of redox reactions (concentrations and depletions) and a seasonal high water table? Or could this pattern be the result of something else (eg. translocation of minerals, manganese, carbon through this soil)?

At a depth of about 60cm (2ft), these shallow horizons were underlain by the parent material. Parent material generally consists of dense, finer textured, sandy loam soil, increasing in silt and clay content with depth. The parent material could act as a limiting layer to the downward movement of water/effluent and could cause a seasonal perched water table above it.

The water table was encountered in the deeper parent material, and/or in the underlying sandstone bedrock at a depth of approximately 4 feet to 6 feet below ground surface in all the test pits. There were other indicators of a seasonal high water table. In most test pits, the soil was very moist to wet (ie. at field capacity) at a depth of 60cm (2ft) plus or minus, with some free water visible in the macropores. The soil mapping indicated a soil type that is poorly drained.

Photo of the site


To be conservative, I am assuming these colour patterns are further evidence of a seasonal high water table, which will have implications for the Planning Act lot category, and the type of septic system required.  But let me know what you think, and feel free to comment below, or on LinkedIn.

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13 Responses

  1. Gordon Rogerson (via LinkedIn, Onsite Wastewater Professionals Group)

    Knowing P.E.I. and all the farming there, the redox features could be caused by several factors. In the 50s and 60s they used DDT before it was outlawed on plants to control pests. The DDT was tilled into the soil after the growing season to a depth of 30cm to 40cm. Also at that depth, there was created a plow layer which would retain snow melt and rain water in the spring. You should see an increase in redox features as you go deeper in the soil pit for a true ground water.

    1. Thanks for your comment Gordon. Re: “You should see an increase in redox features as you go deeper in the soil for a true ground water [table]”. I assume when you say that, you mean “somewhat deeper than 30 to 40 cm”, but not really deep (like 1.5m or more), as you are less likely to get alternating periods of re-aeration necessary for the redox reactions at those depths.

      Jeff Brewbaker, (Soils Scientist from Wisconsin) offered the following comments in a private email communication:

      The soil needs organic carbon to feed the redox process. The carbon feeds the micro organisms that remove the oxygen leading to ultimately redox features. The top 40 cm usually has the most organic matter. Deep rooted plants also supply some carbon at depths. Once you go deeper than approximately 200 cm you would normally see little to no redox features in the soils of central WI. The plants around here just do not root to that depth. So, [in] my opinion, not always more redox features at greater depth.

      Lack of organic carbon in a soil would produce few, slight or faint redox features. This is true even with significantly saturated soils.

  2. Kelly —

    Interesting soils. My take is that almost certainly the greens splotches are caused from lack of oxygen and reduction processes. I come to this conclusion as much from your description of the site and deeper soil formation as I do from the mottling itself. I feel we sometimes get too focused on mottling or lack thereof, and lose the larger picture of site and soil drainage. It is so important that as designers we look at the whole picture.

    One further comment. Sometimes I have observed redox features in the soil that I’m convinced are caused by pockets of finer texture soils that hold capillary moisture for long periods of time, but surrounded by macro-channels that can pass a lot of water. You may or may not have the presence of a “water table” during wet periods, as the term implies. Such conditions still present some design challenges.


  3. One more question, Kelly.

    Your high iron content soils must have a lot of capacity to tie up phosphorus, right? Do you have any evidence of that?


  4. I would agree with Larry. My further comment would be redox features only form when soil temperatures are above biologic zero (5 degrees C). Based on what Kelly has described I would say that the soil temperatures stay cool for a significant period while saturated. I would also say the black color is manganese (redox concentration). In central and northern Wisconsin we have similiar redox patterns on coarse textured stream terraces (central) and finer textured soils on side slopes and toe slopes of drumlins in finer textured soils (northern).

    Jeff Brewbaker

  5. Ian Ralston Eng.L. (via LinkedIn, Onsite Wastewater Professionals Group)

    The lack of roots may be indicative of seasonal saturation, given the soil consistence does not sound as if it is limiting?
    It is likely the sand lenses are acting as drainage paths and also are re aerating more readily than the surrounding Sandy Loam soil.
    Do the regulators there allow you to pre install an interception drain and re assess over the winter?

    1. Thanks Deb.

      Ian, there were actually quite a few rootlets in the surrounding sandy loam down to a depth of 80cm in this pit. There were none in this particular sandy zone, but I am zoomed in fairly close with the camera. I agree that the sand lenses are very likely acting as drainage paths.

      Our regulations are pretty wishy washy about water table issues and also vague when it comes to what sort of water table the vertical separations apply to. Here in PEI they tend to to treat all water tables the same, and don’t differentiate between a perched and non-perched (true groundwater table). There was moderate water seepage to inflow in all the pits at this site at a depth ranging from 120cm to 160cm (4 to 6ft) BGS.

      In Nova Scotia (origin of the Contour disposal field concept), they are fine with installing a permanent upslope interceptor drain which would extend a short distance down into the underlying limiting layer. The intent is that this interceptor would divert shallow perched groundwater around and away from the drainfield in the spring, or whenever a perched water table existed.

  6. Ian Ralston Eng.L. (via LinkedIn, Onsite Wastewater Professionals Group)

    So, they would allow the interceptor to justify an increase in the claimed vertical separation? My thought on the sand lenses was not just that they were acting as drainage, perhaps keeping the SL soil from being completely saturated, but also, when not saturated, more rapid re oxygenation of the lens and so perhaps more obvious redox. features?

  7. William Sledjeski (via LinkedIn, Onsite Wastewater Professionals Group) Try page 26, figure 8.

    The US system refer to these as spodosols. The wet ones are Cryods. Your site appears to have redox features that satisfy this classification. We monitor SWT soils during the wet season for confirmation. New regs in VA allow direct discharge to a water table with tertiatry treatment and disinfection, love or hate it.

    1. Thanks for that link William! Nova Scotia is the province right next to me, but I was not aware of that publication. There are some great photos of redox and gleyed soils. The photos on page 26 are much more prominent than what I saw at my site.

      The photo I took with the flash on is more representative of how the soil colours looked to my eye. But the contrast of the mottling is more prominent in the second photo with the flash off.

      Regardless, would it be fair to presume that the darker patches of soil (brown to almost greenish/black) would likely be accumulations/concentrations of Fe or Mn? Or could these darker patches be “organic staining” as referred to in the Nova Scotia publication?

  8. Jeff Brewbaker (via LinkedIn, Onsite Wastewater Professionals Group)

    I would like to point out that redox features will not be forming when soil temperatures are below biologic zero (5 degrees C). These soils as you describe are most likely seasonally saturated long enough with low temps to have an impact on on-site waste water system performance. The dark colors are most likely manganese concentrations. You could test this by adding small amount diluted hydrogen peroxide. I have observed similar color patterns in drumlins of northern Wisconsin and coarse textured stream terraces in central WI.

    1. Thanks for your insights Jeff. Our spring temperatures are indeed very cool. The ground is generally thawed with perched groundwater flowing, but still cold, starting around mid-March. Some years the frost doesn’t fully come out of the ground until late April, although areas with grass cover which tend to trap a lot of snow generally don’t freeze that deep.

      Our “red” soil is so well known for its high iron content, that I figured it was iron which was concentrating, but the colour didn’t seem to match up. I will have to add a bottle of hydrogen peroxide to our field kit!

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