Soil and Sediment

by Alfred R. Conklin, Jr.

Sediment could be considered soil under water. However, there are some significant differences between soil and sediment. Soil has constantly changing water contents; sediments are always saturated with water. Sediments are less variable in particle size and often contain more organic matter than soil. Soils tend to be aerobic and oxidizing with fluctuating oxygen content whereas sediments are anaerobic and reducing. Also terminology used to describe sediments are different from those used by soil scientist.

In soil the water content varies between saturated and air dry. Under saturated conditions all void space is taken up by water. Under air dry conditions the soil may contain between 1 and 15 % water on a dry weight basis. The cloud of soil behind your car as you drive down a sandy dirt road may contain 1% moisture (1 g of water per 100 grams of dry soil) in spite of the fact that it looks, feels and taste dry.  On the other hand the cloud of soil behind your car on a clayey dirt road may contain 15% moisture (15 g of water per 100 grams of dry soil.

Sediments by their very nature are always saturated with water. The pores between the solids are filled with water and thus sediments are always anaerobic and reducing. The water between solid particles is called interstitial water.

Soil is composed of mixtures of sand, silt and clay. They are intimately mixed so that they are not readily identified as individual particles. Such a mixture will be identified as a textural class such as saying a soil is a sandy clay. Often there are large areas of soils with the same or similar textures. Sand, silt and clay particles in soil do not act independently from each other but form secondary particles called peds. These are made up of sand, silt, and clay cemented together.

In addition to the original material at the bottom of bodies of water sediments form as soil particles washed off the land settle out of water. In streams and rivers water turbulence and velocity keep soil particles in suspension. When flowing water enters a pond or lake, the velocity and turbulence decreases quickly and sand size particles are the first to fall out of suspension. Thus the edges of a lake or pond tend to be sandy while the middle of the bottom has the finer material. Also the sediments do not have the same structure as soil and most frequently have no secondary structure at all.

The amount of organic matter added to soil is generally limited to the plants growing on that soil. When organic matter falls on the soil it is subject to aerobic decomposition. During the decomposition process organic matter is oxidized to carbon dioxide and water. The material remaining after decomposition is called humus and represents only about 10% of the original organic matter added.

Organic matter is added to sediments by two mechanisms. Plants and animals living in and around the body of water, including algae die and fall to its bottom. There is plenty of water at the edges of bodies of water and thus they tend to be highly productive of plants. Plant residues and other organic matter washed off the catchment area are added with water running into the lake or pond. Even when drought decreases the amount of plant material added to soil in a field the amount added to bodies of water will be large and significant.

Organic matter added to bodies of water undergo anaerobic decomposition. During this process organic matter is broken down to carbon dioxide, methane and water. Carbon dioxide and methane bubble to the surface of the water and are dissipated in the air. The anaerobic decomposition process is neither as rapid nor as complete as aerobic decomposition. However, the material remaining after complete decomposition is still considered humus.

Click image to enlarge

Whether decomposition is aerobic or anaerobic plant nutrients are released during the decomposition process. The most important nutrients released are nitrogen, phosphorus and potassium. However, all the other nutrients – sulfur, magnesium, calcium, zinc, chlorine, iron, copper, etc. are also released. These nutrients are in forms, which are available to plants. At the end of the decomposition process humus which is a complex organic compound having high absorptive and complexing capacity, remains.

In undisturbed soils and sediments molecules and ions from decomposition of the organic and inorganic components are in “equilibrium” between the liquid and solid phases. The word equilibrium should always be used cautiously when applied to soil or sediments. Both are dynamic and are constantly gaining and releasing components. Also it is important to remember that in soils the liquid phase is also in “equilibrium” with the gaseous phase. 

In sediments the liquid phase is constant while in soil it is constantly varying. Soil components can become soluble when water content is high and precipitate when it is low. When the water content increases again the components may again go back into solution. This cycle can continue until some other environmental change takes place. It also sometimes happens that the precipitated components do not go back into solution when the water content again increases.

Because sediments are saturated they are anaerobic and reducing. Thus, as noted before we find methane being produced. Also we would expect to find reduced forms of metals in this environment. We would find ferrous (Fe⁺⁺) as opposed to ferric (Fe⁺⁺⁺) iron. Ferrous iron is soluble while ferric iron is not. The reducing condition of sediments thus affects the solubility and mobility of all the components present.

All soils contain aerobic and anaerobic zones. In a well-aerated soil there will be some anaerobic areas. Pores filled with water are examples of such areas. In aerobic soils we expect to find the oxidized forms of elements. Indeed most of the elements are found in their oxidized form including carbon in the form of carbon dioxide. However, it will also be possible to identify reduced forms in low concentration.

Although it would seem that soil and sediment are the same they tend to be very different in their physical and chemical make up. Because of this their interaction with contamination will be different. In sampling a sediment it is important to keep it anaerobic if knowledge of the true nature of its component parts including contaminants is desired. Likewise methods of clean up must be different. Not only will one be dealing with a great deal more water than would be the case with soil but also the oxidation state of the components present will change during the clean up process. This must be kept in mind when designing and carrying out a sediment cleanup. 

When sampling, handling, storing and analyzing sediments it is important to remember that they are significantly different from soil. In terms of composition sediments and soil may be similar. However, in terms of environment, chemistry and biology the two are very different.

Top