Hyperlink 3. Contaminant Behavior in Soil

Contaminants occur in many forms, including gases and vapors, liquids, single atoms or molecules, microscopic particles, and relatively large pellets or fibers. Some chemicals evaporate easily; others stay in the same place for a long time. The longer contaminants have been in the environment, the more likely they are to exist in forms that interact with soil particles. Some contaminants (e.g., metals) can be present in the environment as cations or anions. Ions with an overall positive charge (cations) adhere strongly to soil particles, especially particles made of clay minerals that carry negative charges. Polyatomic ions such as chromium and arsenic, on the other hand, have an overall negative charge (anions) and so may adhere weakly or experience electrostatic repulsion from clay minerals. When repulsive forces outweigh attractive forces, polyatomic ions become mobile in soil and groundwater, especially under conditions of changing acidity/alkalinity (pH) and oxidation-reduction potential (Eh). There are also weak atomic forces (i.e., Van der Waals interactions) that cause contaminants to adhere to the surface of soil particles even if no ionic charge attraction is involved. Van der Waals attraction is an important adsorption mechanism for organic contaminants.

Further complicating the picture, contaminants may be present as polar/nonpolar molecules (e.g., organic contaminants). “Polar” means that part of a molecule carries a partial negative charge, while another part of the same molecule has a partial positive charge. Many organic contaminants are hydrophobic (nonpolar), meaning that they do not dissolve well in water. Organic contaminants may readily infiltrate the interstitial space (i.e., nooks and crannies) of organic soil matter and bind within the large hydrophobic surface area it provides. These organic compounds will tend to stay bound in the soil organic material rather than migrate in aqueous phase such as rainwater infiltration. Depending on the geochemical makeup of the soil, the organic matter content, the ambient conditions, and the physical and chemical properties of a particular contaminant, its molecules may bond loosely or tightly to soil particles.