health and safety plan solid waste management unit assessment

AUTOMOTIVE GASOLINE 65-13
hydrocarbons was selected from the dominant hydrocarbon classes, i.e.,
alkanes, cycloalkanes, and aromatics. These specific compounds were
chosen on the basis of their relative concentrations, and were intended
to span the boiling point average of the gasoline hydrocarbons.
Table 65-3 identifies the selected hydrocarbons and presents the
predicted partitioning of low soil concentrations of those hydrocarbons
among soil particles, soil water, and soil air. The portions
associated with the water and air phases of the soil are expected to
have higher mobility than the adsorbed portion.
Estimates for the unsaturated topsoil indicate that sorption is
expected to be an important process for all the dominant hydrocarbon
categories. Partitioning to the soil-vapor phase is also expected to
be important for the lower molecular weight aliphatic hydrocarbons
(C,-C,) which are characterized by high vapor pressure and low water
solubility. The alkyl benzenes have higher water solubilities and
transport with infiltrating water may be important for these compounds;
volatilization, on the other hand, may be less, important. In
saturated, deep soils (containing no soil air and negligible soil
organic carbon), a significant percent of both aliphatic (particularly
less than C,) and aromatic hydrocarbons is predicted to be present in
the soil-water phase and available for transport with flowing ground
water.
In interpreting these results, it must be remembered that this
model is valid only for low soil concentrations (below aqueous
solubility) of the components. Large releases of gasoline (spills,
leaking underground storage tanks) may exceed the sorptive capacity of
the soil, thereby filling the pore spaces of the soil with the fuel.
In this situation, the hydrocarbon mixture would move as a bulk fluid
and the equilibrium partitioning model would not be applicable.
Overall, ground water underlying soil contaminated with gasoline
hydrocarbons is expected to be vulnerable to contamination by at least
some of these components. The type of spill (surface VQ. sub-surface)
is of particular importance, since volatilization from the surface is
expected to be a significant removal process for low molecular weight
aliphatics. At this point, it should be mentioned that environmental
fate/exposure/toxicology chapters for several of the components in
Table 65-3 were included in Volume 1 of the IRP Toxicology Guide. The
gasoline components addressed in Volume 1 include: benzene, toluene,
xylenes, ethyl' benzene, and naphthalene. Three major gasoline
additives - TOCP, tetraethyl lead and ethylene dibromide were addressed
in VoSume 2 of the IRP Toxicology Guide, while ethylene dichloride was
addressed in Volume 1.
65.2.1.2 Transport Studies
Hundreds of thousands of underground gasoline storage tbks are
currently used at service statians, commercial locations, residences,
and petroleum depots; and almost all the gasoline used for
transportation purposes in the U.S. is stored underground at least once
6/87