Introduction to Basic Ground-Water Transport
 
By the earthDr!
 
The Source Recharge Area: mass loading of contamination to the ground water
 
When there has been a sufficient mass release of a product; such as TCE, PCE, or MTBE dissolved in gasoline; to the subsurface it may make its way to the water table and then, dissolve into it. Even if there is not sufficient product to make it the full way to the water table, there are various alternate ways for the product to contaminate the ground water. When it rains, some of this precipitation infiltrates the soil and if there is sufficient precipitation, this excess water will
percolate to the underlying water table as illustrated by the first figure. If the percolating water contacts contaminated soil, these contaminants may dissolve into the water and then, may be transported by the soil solution to the water table. Also, any product entrained in the soils, overlying the water table, may be remobilized by displacement of the percolating soil solution and thus be freed to flow to the water table. Often most flow of the soil solution or product that will make its way to recharge the underlying water table is limited to the larger soil pores, known as the macropores, and not the smaller sized pores. The plan view area where contaminants enter the ground-water system is known as the source-recharge area.

Remember that any soil is composed of a distribution of various pore sizes. Each soil typically has a somewhat unique pore-size distribution. This distribution of pore sizes typically will change within the soil column. These distributions of varied pore sizes and changing distributions with depth account for non-uniform flow to the water table. Therefore, rivulets of flow following the macropores can precede a uniform wetting front following the mesopores and micropores.

Obviously a faster rate of flow can occur in a macropore, but if there are few large-sized soil pores relative to the shear number of small-sized soil pores in a given soil, then flow will predominate in the small-sized soil pores and will be expressed as a uniform wetting front draining to the water table. Macropores don't typically follow a straight path whether or not it is expressed vertically. Therefore, macropore flow of water or product to the water table will follow a herky-jerky (tortuous) path as it drains
to the water table.

This tortuous-vertical flow tends to laterally distribute any contaminant discharged to water table over a greater area, than if vertical flow were plumb as indicated by this second figure. This second figure is an attempt to represent the earlier cross section view in plan view. Take note that the source-recharge area, where contaminant mass loading to the water table occurs, is typically larger than all areas of contaminated soil above that area. Even when macropore flow doesn't account for most recharge to the water table, there is often much mixing of any contamination as it flows in solution or as a separate phase through the smaller-size pores.This constant change in direction that individual water molecules
must undergo in traversing a distance results in much mixing of the soil solution along with any contaminants that may be transported as dissolved phase. Along with diffusion, mechanical mixing is responsible for dispersion of contaminants as the soil solution drains to the water table.
 
 
 
 
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