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.

The figure to the left depicts two chemicals dissolved in the ground water: benzene and MTBE. This figure assumes that equal numbers of both benzene and MTBE molecules (for our example the same concentration means the same number of molecules) are initially dissolved in the water and that benzene has a greater affinity for sorption to the soil than does MTBE (which it does). Since twice as many benzene molecules than MTBE molecules are sorbed to the soil particles (count them), then, it is obvious that MTBE will be transported farther by the ground water (as it flows from left to right) when there were equal numbers of both molecules dissolved in the flowing ground water at the beginning. It will take twice as many soil particles to sorb all the MTBE molecules than it does for benzen molecules.

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.

Assuming that this replacement water again contains the initial concentrations, where there are equal numbers of MTBE and benzene molecules; and, the bonding sites on all the soil are now occupied, then and only then, the velocity of contaminant mass transport finally equals the velocity of the ground-water flow rate as indicated in this second figure. Don't worry yourself with the meaning of this last statement - you'll appreciate it later. Eventually after there are sufficient flushes of water through the soil pores, the concentrations of both contaminants, whether in the ground water or soil solution, are re-established at the initial, startup concentrations, because the bonding sites on the soil are occupied and no more contaminant removal from the water to the soil occurs as the contaminated ground water or soil solution flows through the soil.

The third figure illustrates that point in time when only clean ground water enters the system. Since benzene is retained by the soil to a greater extent than is MTBE, MTBE is more easily flushed from the soil than is benzene. MTBE is more stable in the soil solution or the ground water than is benzene. Benzene is more stable sorbed to the soil particles than is the MTBE. When clean ground water or the soil solution flows through dissolved product contaminated soil (the solusols) MTBE tends to go into solution more than does the benzene. Eventually, successive flushes with clean water will finally leave only the benzene sorbed to the soil particle. Additional flushes with clean ground water or the soil solution are required to remove any remaining benzene from the surfaces of the soil particles.