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VENTING
OF THE SOIL ATMOSPHERE INTO A BASEMENT: causing vapors in a home
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A third mechanism for entry of a product, such as gasoline, into a basement, into a crawl space, or through a slab is the most interesting. Venting of the soil atmosphere into a living space is the final mechanism for the entry of a product, such as gasoline, into a structure from the subsurface. The soil atmosphere can vent into the basement whether or not it is contaminated with cancerous or toxic substances. When the soil atmosphere is contaminated by a vapor cloud, such as gaseous gasoline, it can only enter into a basement when the proper condition or conditions exist. The soil atmosphere can vent into a basement when certain conditions are attained. Movement of soil air can be via bulk flow (also known as mass flow) or via gaseous diffusion.
Let's discuss gaseous diffusion first. If I placed 1,000 white marbles into a jar and then topped off these marbles with 50 red marbles, the jar of marbles would look stratified. If I then fitted a cap on this jar, assuming it has ample volume to allow the marbles to move such as twice the volume occupied by these 1,050 marbles, and then, shook it hard enough (sufficient energy): the white and red marbles would eventually look rather uniformly mixed. Gaseous diffusion is like this. Initially, there is not much mixing of the product vapor cloud with the soil atmosphere. Given sufficient energy, time, and space: individual contaminant component molecules can mix with gaseous molecules in the clean soil atmosphere. The soil atmosphere is continually mixed by this action of gaseous diffusion. The respiration of plant roots consumes O2 and generates CO2. Oxygen is re-introduced to the soil atmosphere and mixed into soil pores with depleted O2 levels through this process of gaseous diffusion: establishing some sort of equilibrium between higher levels of O2/lower levels of CO2 and lower levels of O2/higher levels of CO2 within the soil atmosphere.
Movement of the soil atmosphere into a basement can be facilitated by pressure differences, between the atmosphere in a home basement and the soil atmosphere, just through the operation of a furnace. Barometric pressure changes may also result in pressure differences between the atmosphere in a home basement and the soil atmosphere, especially when the soil pores of surface soils become occluded (sort of like clogged) by ice during the winter. Another such cause for the movement of the soil atmosphere into a basement is by the displacement of the soil atmosphere by a wetting front. The figure
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Again, take note of the elliptical area to left of the text depicted as "Soil" in this figure. This area depicts a blowup of a section of soil. Soil particles are depicted in orange with voids in between them. There will always be voids (known as pore spaces) between the soil particles, no matter how tightly (densely) the soil is packed. Voids in soils, such as these, are known as pore spaces. Soil pore spaces above the water table are filled with varied amounts of both water and air.
When it rains some water is lost to sheet flow, eventually discharging to local water bodies. Some portion of the rain will infiltrate the soil and a somewhat smaller portion of this water may percolate down to the water table, the location where the soil pores are completely filled with water. Water that infiltrates into the surface soils can't instantly percolate to the water table.
It can take considerable time for this water to percolate from the surface soil to the water table. Much of the water percolating to the water table flows somewhat en mass. It is this mass of percolating waters that can displace the soil atmosphere into a structure, such as a basement. The mass of water, in the developed wetting front, can cause the soil atmospheric pressure to be greater than both the basement atmospheric pressure and the ambient atmospheric pressure. The soil atmosphere will then vent into the basement. If the local soil atmosphere is contaminated by a cloud of a gaseous product, such as gasoline, then the basement air can become contaminated by such vapors. Transport of vapors into a basement is analogous to radon gas transport into basements.
The displacement of the soil atmosphere is effected by the areal and vertical extent of the wetting front, the distribution of the pore sizes (macropores-mesopores-micropores), the matric potential of the soil (how tightly is water held by the various soil pore sizes), and the vadose-zone (unsaturated zone) thickness relative to the thickness of the wetting front.
Local
clouds of contaminated vapors can arise from product sorbed to the soil; dissolved in water;
and, from product, as a separate (liquid) phase. Product that is
dissolved in water can off gas or volatilize to the soil atmosphere.
Liquid, separate phase can vaporize into the soil atmosphere. Once
the soil atmosphere is contaminated, all that is needed to contaminate
your home is the right condition or conditions for the soil atmosphere
to vent into your living space. Please note that I referred to water and not
ground water; and, as separate phase product without any differentiaton. Product
in water can be dissolved in either the ground water or the soil solution.
Separate phase product can be entrained in the soil as films of product within
the soil pores, known as ganglia, and a continous pool of product at or below
the water table. Please note that soils can also sorb vapors contained in the
soil atmosphere and these same contaminants dissolved in the soil solution or
ground water.
REVIEW AND FURTHER QUESTIONS
Just to re-iterate: the three mechanisms for entry of any product, including gasoline, into a basement, into a crawl space, or through a slab are the following:
- Seeps of water containing dissolved product (such as gasoline);
- Seeps of product; and,
- Venting of the soil atmosphere
Would you think that there might be a different remedy for each of the differing mechanisms?