2 Evaluation Criteria
2.4.1 Non-Halogenated Volatile Organic Compounds (VOCs)
Volatile Organic Compounds (VOCs) are a group of organic compounds found in products
such as gasolines, paints, paint thinners, and solvents used for dry cleaning and metal
degreasing. These compounds are typically used in liquid form and are defined as volatile
because many can readily evaporate. Substances containing VOCs can find their way into the
ground water through point sources such as leaking storage tanks or direct spills. VOCs
also can enter ground water from non-point sources such as storm water runoff from roads
and parking lots. Some airborne compounds can mix with rain and rainfall containing VOCs
also may recharge aquifers as a non-point source of contamination. Once in the ground
water, VOCs can degrade the quality of water supplies.
Subsurface contamination by VOCs potentially exists in four phases:
- Gaseous phase: Contaminants present as vapors in unsaturated zone.
- Solid phase: Contaminants are adsorbed on soil particles in both saturated and
unsaturated zones.
- Aqueous phase: Contaminants are dissolved into pore water according to their solubility
in both saturated and unsaturated zones.
- Immiscible phase: Contaminants are present as non-aqueous phase liquids (NAPLs)
primarily in unsaturated zone.
Movement of contaminants under the surface occurs primarily through two mechanisms:
- Volatilization into the unsaturated pore spaces produces a vapor plume. Lateral
migration of this vapor plume is independent of ground water movement and may occur as a
result of both advection and diffusion. Advection is the process by which the vapor plume
contaminants are transported by the movement of air and may result from gas pressure or
gas density gradients. Diffusion is the movement of contaminants from areas of high vapor
concentrations to areas of lower vapor concentrations. Volatilization from contaminated
ground water also may produce a vapor plume of compounds with high vapor pressures and
high aqueous solubilities.
- Dissolution into water may occur in either the unsaturated or saturated portions
of the subsurface with the contamination then moving with the water. Insoluble organic
contaminants may be present as Non-Aqueous Phase Liquids (NAPLs). Dense NAPLs (DNAPLs)
have a specific gravity greater than 1 and will tend to sink to the bottom of surface
waters and ground water aquifers. Light NAPLs (LNAPLs) will float on top of surface water
and ground water. In addition, DNAPLs and LNAPLs may adhere to the soil through the
capillary fringe and may be found on top of water in temporary or perched aquifers in the
vadose zone.
The following is a list of common non-halogenated VOCs:
| Acetone |
Acrolein |
| Acrylonitrile |
n-Butyl alcohol |
| Carbon disulfide |
Cyclohexanone |
| Ethyl acetate |
Ethyl ether |
| Isobutanol |
Methanol |
| Methyl ethyl ketone (MEK) |
Methyl isobutyl ketone |
| 4-Methyl-2-pentanone |
Styrene |
| Tetrahydrofuran |
Vinyl acetate |