Contaminant Properties

Organic contaminants in the subsurface can be present as a separate nonaqueous phase liquid, dissolved in the aqueous phase, in the vapor phase in the soil gas, partitioned into the soil organic matter, or adsorbed onto the solid mineral phase. The relative amount of the contaminant in each of these phases is determined largely by the properties of the contaminant. Generally, the most important property of the soil in determining the distribution of contaminants is the soil organic matter, which normally controls the absorption of hydrophobic compounds. Table 9 is a compendium of common chemicals found to have contaminated groundwaters. The table lists some of the relevant physical properties of volatile and semi-volatile organic chemicals that have been, or have the potential to be, recovered using carbon adsorption and other techniques. Many of these are commonly found at Superfund sites and other sites where ground-water contamination has occurred. Most of these organic chemicals are essentially immiscible with water; acetone and methanol are the exceptions. Many of these compounds have low viscosities and, thus, have the potential to flow readily in the liquid phase. Approximately half of these compounds are less dense than water; the other half are more dense than water. The density of an organic liquid relative to that of water is important in determining the vertical mobility of the contaminant. Those that are less dense than water will tend to float on the groundwater table, while those that are more dense than water may move downward through the aquifer if the pressure in the organic liquid is greater than the displacement pressure of the aquifer materials. Low permeability clay layers in the aquifer may restrict the vertical movement and allow the liquids to accumulate on top of the layer. The properties listed in Table 9 are taken from various literature sources and should be considered average values.

In tackling a groundwater remediation problem, careful attention should be given to the properties of the contaminant relative to water. Let's take for example the two chlorinated hydrocarbons - trichloroethylene (TCE) and perchloroethylene (PCE, tetrachloroethylene). Both of these solvents were often used interchangeably in industrial applications involving the degreasing of metal surfaces, and significant numbers of sites throughout the United Sates have been found to have groundwaters contaminated by these solvents. They both represent genuine health risks (TCE is a suspected carinogen, and both TCEand PCE have low MCLs).

Table 9. Properties of Organic Chemicals Found at Contaminated Sites

Chemical

Boiling Point

°C

Desnity gm/cm3

Viscosity CP

@ 25° C

Water Solubility mg/L

Vapor Pressure mm Hg

Diffusion Coefficient, cmVday

in h2o

in Air

Methylene Chloride (Dichloro-methane)

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