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earthDr! "the
environmental ombudsman" earthDRx.org - prescriptions (Rx) for |
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Floating
Product Sinks!
Sinking Non-Aqueous Phase Liquid: for liquids with specific gravities less than water |
Most people know that gasoline floats on water. However, that is not always the case in the subsurface environment. Typically two immiscible liquids (meaning that neither liquid appreciably dissolves into the other liquid) of differing specific gravities (specific gravity is the ratio of the mass/volume of a given liquid to the mass/volume of water) will separate into two distinct layers due to bouyancy. We know that a given volume of gasoline contains less mass and therefore weighs less than does an equal volume of water. A beach ball contains less mass and therefore weighing less than the same volume of water will float. The first figure
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The first figure on the webpage entitled "Capillary Fringe and Soil Pore-Size Distribution in Relation to the Water Table and Vadose Zone," (please click on this hypertext to refresh your memory) illustrates that soils are composed of variously-sized soil particles and packed in a variety of ways resulting in a wide distribution of pore-space sizes between the soil particles. The figure on this page also illustrates variously-sized soil particles and pores. Additional refreshment of your memory can be gotten on the webpage entitled "Genesis of Separate Phase Contaminated Soils and Dissolved Phase Contamination of Water" that details the interaction of trapped water in soil pores occluding the movement of free product through the soil pores, particularly limiting the movement of free product through the smaller soil pores relative to the larger soil pores. Gravity drainage of both water and non-aqueous phase liquids will be greatest in the largest soil pores.
The figure on this page illustrates the movement of product from a leaking underground storage tank through the larger soil pores. Remember, from these earlier webpages, the difficulty of free product movement from a larger soil pore to a smaller soil pore that is occluded by water. Free product just won't flow into an interconnected soil pore unless the product head in the one soil pore can overcome the matric potential (pressure potential, if it is below the water table) of the water in the other soil pore, should that soil pore be occluded by water.
Many investigators have been attributing the presence of separate phase contamination below the water table solely to a fluctuating water table. The depth to the water table can and does change. A prolonged droughty period can result in a dropping water table. A rainy spell can result in a rising water table. The fluctuating water table can raise or lower any free product that may be floating within the upper two-thirds of the capillary fringe and therefore, smear the product through the soil. This process is known as smearing.
Note the position of the water table. Also, note the position of the capillary fringe. The large red arrow with an arrowhead at each end illustrates the range over which the water table moves in response to wet-dry cycling.
The large, navy-blue arrow extends from the elevation of the top of product in the leaking tank to below the gravel-cobble layer showing the vertical range that the separate phase product occupies. Please note that the larger soil pores, the macropores extend from above the water table to below the gravel-cobble layer sandwiched within the water table. When the macropores are fully developed from above and into the water table, these larger pores can behave just like a pipe in your home plumbing system. Assuming that the length of the portion of this arrow above the water table is 10-feet, then this soil pore containing gasoline would be exerting a pressure comparable to a 9-foot head of water at the water table because the specific gravity of gasoline is approximately 0.9 (0.9 X 10 feet). Assuming that the soil pore is infinitely developed vertically, then, if sufficient product is released, the product could penetrate to a depth of 90 feet below the water table. Penetration will cease at this depth since 90 feet of water is required to buoy 100 feet of gasoline (10 feet above plus another 100 feet of product below the water table).
The
pore diameter of the gravel-cobble layer is much greater than in the soils
above or below and therefore, the matric potential of the water is much
reduced in these larger soil pores.
Therefore, transport of product encounters less resistance to flow through
the gravel/cobble layer than through the macropores of the finer textured
soils below the gravel/cobble layer and therefore, it doesn't penetrate
90 feet into the water table. At the interface of the gravel-cobble layer
there is a dramatic reduction of the pressure potential of water in these
larger size soil pores. The product head is now sufficient to overcome
the pressure potential of the water injecting free product into
the soil pores of the gravel/cobble layer. Free product now gravity drains
into the gravel/cobble layer displacing more water from this layer.
The question to ask is why
would the gasoline or other product, that was injected below the water
table by the product head exceeding the pore pressure, eventually not
rise to the surface of the water table when it has a specific gravity
less than water and product injection has been exhausted due to the termination
of the release and subsequent drainage to the elevation of the water table.
If gasoline is poured on free water (unrestricted), the gasoline will
float on water because an equal volume of gasoline weighs less than the
same volume of water. (The density of gasoline is less than that of water,
because the mass of a given volume of water is always greater than the
mass of gasoline of the same volume). If water is poured into a 55-gallon
drum that is already half full with gasoline, the less dense gasoline
will float to the top displacing the water from the upper position. What
happens when the 55-gallon drum is first filled about a quarter full with
gasoline; then, sufficient sand is poured into the drum to fill it to
the top; and, finally water is misted, not poured, onto the surface of
the sand to fill the remaining voids of the soil pores that remain with
water? In other words, will the gasoline float up toward the surface and
the water sink to the bottom of the drum? No it won't because the gasoline
becomes trapped below the upper mass of the recently introduced water.
The skeleton of the soil (the matrix composed of the soil particles) and
the water potential within the soil pores prevent the re-distribution
of the gasoline to the upper position and the water to the lower position
due to the overlying pore pressures of the water filled soil pores resulting
from the water potential of the water in overlying soils.
Remember that free product
contaminated soil (produsols) is two to four orders of magnitude more
contaminated than dissolved product contaminated soil (solusols). Please
also remember that the source-recharge area is that area where mass loading
to the ground water occurs: mass loading is not just at the phreatic surface
(top surface of the water table), but also includes the mass loading of
contaminants below the phreatic surface contributed from the gravel/cobble
layer. The footprint area of the source-recharge area (contaminant mass
loading zone) is much larger than if defined solely by the area at the
phreatic surface where contaminants rain down upon (recharge) the water
table.
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