Floating Product Sinks! Sinking Non-Aqueous Phase Liquid: for liquids with specific gravities less than water
	By the earthDr!
	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 displays a leaking underground storage tank. Separate phase is depicted both floating on the water table and sinking some depth into the water table. We all know that gasoline, fuel oil, and diesel fuel float on water because each mixture has a lower specific gravity than does water. This figure seems to contradict this logic as it depicts gasoline as a separate phase (in this figure, I haven't illustrated dissolved phase in water so that it is less congested) at some depth below the water table in addition to floating just above the water table. 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 appreciably flow into an interconnected soil pore of smaller diameter if there are available larger diameter soil pores that can relieve the pore pressures of the free product. 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. 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. The large, navy-blue arrow extends from the elevation of the tank leak to below the gravel-cobble layer. Please note that the larger soil pores, the macropores extend both above and below the water table with 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 that portion of this largest arrow above the water table is 10-feet long, 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). However, the pore diameter of the gravel-cobble layer is much greater than in the soils below and therefore, the product encounters less resistance to flow through these larger diameter soil pores of the gravel-cobble layer than through the smaller diameter, macropores of the underlying, finer-textured soil. Note 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 contaminant-mass loading to the ground water occurs: contaminant-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 separate phase product in the gravel/cobble layer. Therefore, the footprint area of the source-recharge area 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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