Introduction to Basic Ground-Water Transport
By the earthDr!
Separation of Contaminants as they are Transported, Dissolved in Water, through Soils or Rock
There are a number of genetic processes that can contaminate soils. Soils can become contaminated by contact with chemicals in a solid state, a liquid state, and a gaseous state. There can be mixtures of chemicals or a single chemical that contaminates the soil. The following examples illustrate a process where a mixture of chemicals that are dissolved in water contaminates the soil. This process illustrates a mechanism known as partitioning.
Many liquids are mixtures
of various chemicals. One common mixture with which most people are familiar
is gasoline. Gasoline is composed of hundreds of chemicals. There are
many additives that are added to the basic mix of gasoline. Methyl tertiary
butyl ether (MTBE) is just one of these additives and tertiary butyl alcohol
(TBA) is another. Benzene is just one of the hundreds of chemicals present
in the refined gasoline product prior to the addition of additives. The
chemicals in gasoline will dissolve in water when gasoline, as a separate
phase, contacts water. Some of these chemicals will dissolve more in water
than others. Gasoline components are soluble in water; however, most do
not dissolve appreciably in water. Though these concentrations are not
elevated, it is not healthy to drink such contaminated water even at these
lower concentrations. Even when it may not be unhealthy to drink such
low levels of contamination in the water, the water still would not be
palatable. The odor and taste of water contaminated by dissolution of
gasoline, even at low concentrations, would make it undrinkable. Many
of these contaminants when dissolved in water, at the fairly low parts
per billion range, would make many people gag if they attempted to drink
it. A part per billion is one pound of any particular chemical in a billion
pounds of water. A part per million is one pound of any particular chemical
in a million pounds of water. Ten parts per billion is 10 pounds of a
particular chemical in a billion pounds of water.
I tried to depict that for every one molecule of MTBE sorbed to the soil that there are two molecules of benzene sorbed (This is only for illustration - it could be a ratio of 1 to 2 or 1.4 to 2 or whatever). Soil surface adsorption does tend to have a greater affinity for benzene than for MTBE. Soil particulates tend to pull more benzene out of the soil solution or ground water than it does MTBE because of preferential sorption. Remember that soil particles still adsorb and absorb (adsorb and absorb equals sorbed) the MTBE molecules, but just in lesser quantities to that of benzene. Sorption of a dissolved constituent decreases the concentration of that contaminant in the water. The ground water or soil solution, that has been depleted of contaminant mass by sorption to the soil (partitioning), is replaced or flushed out by water, containing the contaminants at the initial concentrations, moving from left to right.