Published August 4, 2015
WINTER 1984
13
Part II. Soil Mottling, An Indicator of Saturation
Dr. James L. Anderson l
In the first article of this two-part series, the concept of soil mottling
as an indicator of saturated conditions and the physical and chemical
environment necessary for the formation of soil mottles was discussed.
Following that article, the editors received letters questioning the use
of soil mottling to determine water table levels. These letters point to a
problem often encountered when discussing soil saturation. The common
connotation brought to mind by the word water table is that this would
be a source of domestic water supply. Actually, when we look at a soil
profile to a depth of 5 feet, a more descriptive term for the occurrence of
saturated conditions is a zone of soil saturation. Most of the time these
zones are very transient in nature. They are present during times of the
year when precipitation has been high and evapotranspiration low.
The effects of these zones of saturation on the functioning of on-site
sewage treatment systems are considerable. The presence of saturated
conditions near or within the area of the seepage trenches results in slower
movement of effluent through the biomat and reduced treatment efficiency. Sewage surfacing or backing up into the house are results that
are immediately noted by the homeowner. It is very important that we
have a field method of determining where these saturated zones occur in
soil profiles, even during periods when saturated conditions are not
present. Soil mottling, although imperfect, provides that opportunity.
The standard mottling notation using abundance, size, and contrast
allows too great a latitude for interpretation, leading to the wrong
evaluation of the soil wetness state under certain circumstances. Considering the wide range in background experience and soil training of persons evaluating building sites, it is easy to visualize the potential for discrepancies in relating observed gray mottle patterns to seasonal wetness at
a site.
A case study was made at the Univ. of Wisconsin regarding a legal
problem following routine application of mottling criteria on a parcel of
land with Grays silt loam (fine-silty, mixed, mesic Mollic Hapludalfs)
which formed in about 36 inches of loess overlaying glacial outwash sand
(Bouma, 1977). The lower 12 inches of loess was mottled with chromas of
2 which implies the occurrence of "saturation during some part of the
year" (Boersma et aI., 1972; Simonson and Boersma, 1972; Soil Survey
Staff, 1975). A soil scientist made the above analysis during an on-site
investigation and because the two chroma mottling was well within the
critical depth defined by the Wisconsin health code, the site was turned
down as a home construction site with an on-site sewage treatment system.
Not accepting this conclusion, the owner placed a tube to a depth of
32 inches, well within the mottled layer, and did not observe free water in
the pipe even during wet periods of the year.
I
Extension soil scientist, Dep. of Soil Science, Univ. of Minnesota, St. Paul, MN 55108.