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
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).
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.