Fuel contaminants are generally nonhalogenated. Information
presented for nonhalogenated VOCs (Subsection
2.3.1) and nonhalogenated SVOCs (Subsection
2.5.1) may also be appropriate for many of the fuel
contaminants presented in this subsection.
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
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.