SOIL MORPHOLOGY AS AN INDICATOR OF SEASONAL HIGH WATER TABLES

Peter C. Fletcher
U.S. Department of Agiculture Soil Conservation Service
Middleboro, MA 02346
and
Peter L.M. Veneman
Department of Plant and Soil Sciences
University of Massachusetts
Amherst, MA 01003

Link to Glossary of Soil Terms

Introduction:

The study of soils has a long tradition in the field of agriculture, but only during the last 2 decades has it become familiar to people concerned with site-suitability assessment for onsite sewage disposal. For instance a number of states, including Maine and New Hampshire, have adopted onsite sewage regulations partly or entirely based on soils information.

Highest groundwater levels and water table fluctuations are routinely estimated by soil scientists from a soil's morphology, mainly the soil color. Gray colors are associated with saturated and chemically reducing soil environments, while yellowish-brown colors are related to generally aerobic and chemically oxidizing conditions. Soils without any excess water during the year usually are aerated and yellowish-brown-colored. Soils with high water tables during some part of the growing season, generally the early spring when snow melts, exhibit gray coloration at the depth of the high water mark and below. The grayer the soil, the more distinct the wet period is. Many soils in New England exhibit both gray and yellowish-brown colors, reflecting the presence of an elevated water table in spring and a drier, more aerated condition during late spring and summer when the water table subsides.

Soil classification provides a powerful tool for assessing the soil-water state throughout the year, particularly the Estimated Average Seasonal High Water Table (EASHWT) level. This paper describes the environmental conditions affecting soil color formation, the role of saturated soil conditions in the creation of gray colors, the application of the Munsell system for soil colors, and the limitations of soil morphological criteria in determining the EASHWT.

Soil Color Formation

Soil colors are commonly associated with the presence or absence of iron. Weathering of soil minerals is a slow process but with time causes the release of mineral constituents to the environment. Soluble weathering-products are removed from the soil profile, while more stable compounds will precipitate. Iron (Fe), released from a mineral, frequently coats soil particles with thin oxide-coatings. Well-aerated soils typically are yellowish-brown-colored from these iron-oxide (Fe_xO_z) coatings. Depending on the intensity of the weathering cycles, the iron-oxides may display different colors. In New England, the dominant soil color is brown to yellowish-brown, caused by the mineral goethite (FeO.OH). Very intense weathering under well-aerated conditions causes formation of the red-colored mineral hematite (Fe₂0₃), while the yellowish-colored mineral limonite (FeO.OH.nH₂0) forms in more humid environments.

Soil is not a sterile medium but contains large numbers of microorganisms, which, under conditions favorable for their particular species, flourish and multiply greatly. Most of these organisms generate energy from the oxidation of soil organic-matter, which enables them to perform basic life functions. When the soil becomes saturated from flooding events or high groundwater levels, the oxygen in the soil system exhausts within a few days; anaerobic conditions prevail. The longer the saturation is, the greater the oxygen deficiency. Organisms that depend on oxygen for sustenance perish or become dormant. Under anaerobic conditions certain microorganisms can derive energy from the chemical reduction of compounds like oxidized iron (Fe³⁺, called ferric iron) to reduced iron(Fe²⁺, called ferrous iron). The energy thus generated is used to create life-sustaining chemical compounds for the bacteria from soil organic-matter. Basic requirements for this reduction process are absence of oxygen, as induced by saturated soil conditions, temperatures above biological zero (41 degrees F), and presence of organic matter.

Prolonged soil-saturation results in anaerobiosis leading to the formation of mobile ferrous iron. The migrating groundwater redistributes the iron throughout the soil profile. Subsequent drainage restores aerobic conditions, but some iron coatings on the minerals may have been entirely removed, leaving the grayish surface of the mineral grains exposed. During drainage, some areas around pores, cracks, and root channels become dry and aerated more quickly than the rest of the soil. Ferric iron precipitates in these places, forming reddish-brown spots. During periods of alternating wetting and drying cycles, such as seasonal high groundwater, ferrous iron does not transfer out of the soil profile entirely but moves over short distances only and precipitates during the drying phase. Such conditions are characterized by blotches of gray and reddish-brown soil colors occurring at the same depth. The longer the saturation period, the more pronounced the reduction process, and the grayer the soil becomes. This pattern of spots or blotches of different color or shades of color interspersed with the dominant color is called soil mottling.

A photo of soil mottling (Redoximorphic features) - click for larger image.

During the last 100 years, soil scientists used soil colors to predict the drainage status of a soil. Occurrence of gray colors at a particular depth marks the presence of elevated groundwater levels at that depth during part of the year. The grayish colors do not form rapidly but result from many cycles of soil saturation. Those colors are not easily obliterated; they serve as an almost permanent marker of the mean groundwater elevation at a site. Gray colors by themselves do not, however, indicate the duration of the anaerobic event, only that a water table exists there part of the year.

Munsell Color System

Soil colors are expressed in terms of hue, value, and chroma, in accordance with the standardized Munsell color system (Soil Survey Staff, 1951). Hue refers to the wavelength, value expresses the degree of lightness or darkness, whereas chroma notes the purity or strength of a particular color (Munsell, 1974). This system is used to describe the matrix or dominant soil color and the blotches of less-prevalent colors commonly called mottles.

10YR Munsell Page

Determining the Estimated Average Seasonal High Water Table

The presence of low chroma colors covering over 5% of the surface area exposed in a soil pit marks the level of the mean seasonal groundwater elevation in most New England soils-the Estimated Average Seasonal High Water Table (EASHWT). Low chroma colors result from reduction / oxidation cycles occurring over many years (generally in terms of centuries), which makes this estimation method a reliable and conservative indicator of maximum seasonal groundwater elevation. Occasionally, the groundwater may be found at shallower soil depths, but there is no scientifically sound method to assess accurately this highest-ever level. If that interval is short, aerobic soil conditions persist; consequently, no low chroma colors are formed.

Soil mottling within coarse textured sandy soils is typically less distinct and not as gray in color as those associated with finer textured soils. Interpreting the soil morphology within these areas is often more difficult. A soil feature unique to some coarse textured soils is a dark reddish or yellow layer often referred to as a rust line. This feature can form within the soil through two contrasting processes which may or may not be the result of a high water table. Those not associated with a water table may develop when percolating water is momentarily interrupted as it passes through different soil strata. This brief pause may cause dissolved iron within the water to precipitate out and over many years develop a bright red or yellow streak. This soil feature is common within some stratified sand and gravel deposits, and can often be observed on the sides of gravel pits high above any water table. This streaked or blotchy pattern of bright colors formed under aerobic soil conditions also is referred to as mottling.

Only in a few unique situations does a rust line result from a fluctuating water table. Rust lines associated with a water table are the result of the fluctuating water table and dissolved iron in the ground water. As the water level fluctuates, dissolved iron precipitates, forming a coating on the surface of soil particles and with time develops a bright red and yellow line in the soil. For a rust line to be interpreted as an indicator of the EASHWT elevation it should meet some or all of the following criteria: the rust line should appear as a nearly continuous band on all sides of the deep observation hole; it should be on a nearly level plane within the hole; soil mottling should be observed below the rust line; and in some situations dark nodules or layers (iron pans) of hardened or cemented soil material are present within the rust line. In situations where gray color mottles occur above a rust line, the elevation of the gray mottles not that of the underlying rust line, should be interpreted as the height of the EASHWT.

Low chroma colors are not always present, even though the soil has distinct periods of saturation. For instance, reddish-colored soils in Worcester County and in the Connecticut River Valley have such high iron contents that low chroma mottles are masked. Also, in soils developed in stratified deposits consisting of alternating layers of fine-grained and coarse- grained materials, the more clayey layers may be quite gray-even though the soil is never totally saturated and the maximum water table may actually be much deeper than the low chroma colors indicate. In such situations, the presence of the yellowish-brown-colored sandy layers shows that the drainage of that soil is much better.

Some of the coarser textured soils in Cape Cod and southeastern Massachusetts, and loamy in the Berkshires at elevations above 330 m (1100 ft) have a gray-colored layer directly below the topsoil. Incomplete breakdown of soil organic matter in the topsoil results in the formation of low molecular-weight organic acids, which causes extensive leaching in underlying soil layers unrelated to anaerobic soil conditions. The iron is stripped from the sand gains by the process of chemical complexation resulting in gray colors. Soil scientists call this process podzolization, and the brownish-colored underlying soil layers constitute a better indicator of the actual hydrology than the grayish-colored, leached layer. Other situations, where wet soil conditions do not necessarily cause distinct low chroma colors, occur in soils with organic matter distributed throughout the soil profile, such as in frequently flooded fluvial soils.

Sometimes soils exhibit low chroma colors that do not result from seasonal anaerobic conditions. Soils high in dark-colored phyllitic minerals inherited low chroma colors from their geologic parent materials; over 10,000 years of soil formation have not released sufficient iron to give the soil a uniform brown appearance. Also, some well-drained soils developed in glacial till are distinctly yellowish-brown-colored in the subsoil but become grayer with increasing depth because of the greater degree of weathering in the upper part of the profile. This does not necessarily indicate the presence of a elevated groundwater level

Soil morphology is not a reliable indicator in drained soils. Soil colors and patterns develop as a result of a fluctuating seasonal high water table and leave an almost-permanent marker within a soil. Even when these soils have been drained for many years, this morphology persists and the altered hydrology may be misinterpreted.

Areas of fill also present a problem. The time needed to develop soil wetness morphology within fill material is variable. Depending upon the kind of fill soil texture, presence of organic matter, kind of vegetative cover and chemistry of the ground water, the time needed to develop soil colors in fill material may vary from a few years to several decades or longer.

Soil science provides a powerful tool for interpreting the Estimated Average Seasonal High Water Table level throughout-the-year, even during dry seasons when water tables may not be present. In many situations, trained personnel without a formal soil science education can make reliable interpretations. However, in areas of unique and complex soil conditions or disturbed sites careful consideration of the soil's morphology, the kind of parent materials, the position of the soil in the landscape, and topography need to be assessed by a qualified soil scientist before a determination can be made.

Natural Soil Drainage Classes:

Soils are divided into drainage classes depending on the frequency and duration of periods of saturation or partial saturation during soil formation.

Excessively and somewhat excessively drained soils typically have sandy and gravelly textures within the subsoil and substratum. They have bright colors (strong brown to yellowish brown) in the subsoil that fade gradually with depth. Seldom are there mottles within the upper 5 feet of soil material and water tables are generally below 6 feet.

Well-drained soils typically have bright colors in the subsoil that fade gradually with depth. These soils are free of mottling to depths greater than 40 inches. Water tables are generally greater than 6 feet.

Moderately well drained soils generally have bright colors in the upper portion of the subsoil and are mottled between 15 and 40 inches beneath the soil surface. These soils are wet for a short but significant part of the year. Moderately well drained soils commonly have a restrictive layer, seepage water, or a seasonal high ground water table at a depth of 24 to 40 inches.

Somewhat poorly drained soils generally have grayish (chroma 3 or 4) subsoils underlain within 24 inches by horizons with low chroma dominant colors. The upper subsoil may or may not have mottles. Seasonal water table is between 12 to 24 inches.

Poorly drained soils typically have a blackish surface layer that is underlain by a gray subsoil that is mottled. These soils have a water table at or near the surface for a significant portion of the year.

Very poorly drained soils typically have ponded water on the surface or a water table at or near (<6" below) the surface for most of the year. Very poorly drained soils can be divided into those that developed in mineral deposits and those that developed in organic material. Very poorly drained mineral soils have a dark (black) organic surface layer underlain by a gray (gleyed) subsoil and substratum. Very poorly drained organic soils developed in thick, dark (often black) deposits of partially or well-decomposed organic matter, and are black in the topsoil, subsoil, and substratum.

REFERENCES

Munsell Color. 1990. Munsell soil color charts. Munsell Color, 2441 North Calvert Street, Baltimore, MD 21218.

Soil Survey Staff. 1951. Soil survey manual. U.S. Dept. Agric. Handbook No. 18, U.S. Government Printing Office, Washington, DC.

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