Contamination by fuel contaminants in the unsaturated zone exists in four phases: vapor in the pore spaces; sorbed to subsurface solids; dissolved in water; or as NAPL. The nature and extent of transport are determined by the interactions among contaminant transport properties (e.g., density, vapor pressure, viscosity, and hydrophobicity) and the subsurface environment (e.g., geology, aquifer mineralogy, and ground water hydrology). Most fuel-derived contaminants are less dense than water and can be detected as floating pools (LNAPLs) on the water table.
Typically, after a spill occurs, LNAPLs migrate vertically in the subsurface until residual saturation depletes the liquid or until the capillary fringe above the water table is reached. Some spreading of the bulk liquid occurs until pressure from the infiltrating liquid develops sufficiently to penetrate to the water table. The pressure of the infiltrating liquid pushes the spill below the surface of the water table. Bulk liquids less dense than water spread laterally and float on the surface of the water table, forming a mound that becomes compressed into a spreading lens.
As the plume of dissolved constituents moves away from the floating bulk liquid, interactions with the soil particles affect dissolved concentrations. Compounds more attracted to the aquifer material move at a slower rate than the ground water and are found closer to the source; compounds less attracted to the soil particles move most rapidly and are found in the leading edge of a contaminant plume.
More volatile LNAPL compounds readily partition into the air phase. A soil gas sample collected from an area contaminated by vapor-phase transport typically contains relatively greater concentrations of the more volatile compounds than one contaminated by ground water transport. Vapor-phase transport can be followed by subsequent dissolution in ground water. Alternatively, aqueous-phase contaminants with high Henry's law constants can be expected to volatilize into the pore spaces.
For compounds with vapor densities greater than air, density-driven flow of the vapor plume may occur as a result of gas density gradients. Toluene, ethylbenzene, xylenes and naphthalene are less dense than water and unlikely to move by density-driven flow. However, they may be capable of diffusive transport, causing vapor plumes to move away from residual saturation in the unsaturated zone. Residual saturation is the portion of the liquid contaminant that remains in the pore spaces as a result of capillary attraction after the NAPL moves through the soil. Volatilization from contaminated ground water also may produce a vapor plume of compounds with high vapor pressures and high aqueous solubilities. Dissolution of contaminants from residual saturation, or bulk liquid, into water may occur in either the unsaturated or saturated portions of the subsurface with the contamination then moving with the water. Because the solubility of fuels is relatively low, contaminant dissolution from NAPL under laminar flow conditions typical of aquifers is mass-transfer limited, requiring decades for dissolution and producing a dilute wastestream of massive volume.