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.
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.
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.