Introduction to Basic Ground-Water Transport
By the earthDr!
Genesis of Separate Phase Contaminated Soils and Dissolved Phase Contamination of Water
In this first figure a product, such as, gasoline, TCE, or PCE is moving somewhat laterally, but overall downward, through soils above the water table. Product coating of soil is indicated by the light brown layer surrounding a soil particle. Layering of water is indicated by the light blue layer surrounding a soil particle. Even in a hot, desert soil there will be layers of water surrounding the individual soil particles. Most times, these water layers that surround an individual soil particle will coalesce with other nearby water layers to completely fill some small run of a soil pore. Obviously, the smaller is the size of the soil-pore diameter, the more likely that water will occlude that run of the soil pore. Occlusion of the soil pores with water limits product movement through soils.

This second figure illustrates a latter stage of product
contamination of the soil. It is apparent from this
figure that not all of the soil becomes saturated with product, as a separate phase. The surface tension of the water trapped by capillary forces in the smaller pore sizes prevents the movement of free product into many of the soil pores. Free product tends to migrate through the larger soil-pore sizes. The product will coat the walls of the soil pores and eventually coat the surface of nearby soil particles. Product layers may coalesce with other product layers and occlude short runs of soil pores just as does water. These discrete, disjunct runs of product trapped in the capillary pores are often referred to as ganglia of product. Product contaminated soil is much more difficult to remedy than are solusols (dissolved product contaminated soil).

The third figure illustrates that point in time when only clean ground water enters the system. Since only clean water is entering the soil pore system, then the flushing of contamination from the soil is now
possible. As clean water, indicated by the light blue arrows, flows through the system two things can happen. Product as a separate phase may be mobilized and driven by the total head of the soil solution out of the system. Another, possibly is that the product will dissolve into the water and be transported out of the system as a dissolved phase.

The product can be mixture of many different chemicals such as gasoline. There may also be a single compound in a product. TCE (trichloroethylene) is one such sole chemical in a product. The MTBE in many gasolines will dissolve into the soil solution or ground water more readily than will benzene or especially any of the xylene isomers. Therefore, MTBE is more easily flushed from the soi
l than is benzene and the xylene isomers. Product may remain trapped as ganglia in the soil even after hundreds of flushes with clean water. Unlike produsols, it is much easier to exhaust soils of contamination if it is only contaminated by dissolved phase.