14    EVALUATING TRANSPORT OF ORGANIC CHEMICALS IN
SOIL RESULTING FROM UNDERGROUND
FUEL TANK LEAKS
Dar-Yuan Lee, Assistant Professor,
National Taiwan University
Taiwan Republic of China
A. C. Chang, Professor
University of California, Riverside
Riverside, California 92521
INTRODUCTION
Leaking underground fuel storage tanks (LUFT) contaminate the surrounding soil with potentially
harmful organic chemicals which, migrating through the vadose zone, may be a threat to the groundwater quality. Approximately 78% of the nation's 3 million underground storage tanks are used to
store petroleum fuel products.1,2 A survey of underground fuel storage tanks conducted by the U.S.
Environmental Protection Agency found leaks in 35% of those tested.3 The gasoline storage tanks
accounted for the majority of the leaking underground storage tanks in California.4 Once a leak is
detected, the situation must be appraised whether the LUFT is causing or will cause groundwater
pollution. A decision can then be made on the appropriate cleanup or remedial actions.
Petroleum-based fuels are complex mixtures of hydrocarbons. For example, gasoline is a mixture of
over 200 petroleum-derived chemicals and several synthetic performance enhancement additives. The
transport behavior of these petroleum hydrocarbons in soils is not well understood. Tracking the
movement of escaped petroleum fuel through both the unsaturated zone and the saturated zone are
difficult,5 because the chemical's possible reactions with various phases in the soil are complex and
experimental data is inadequate. The solute transport in the presence of a nonaqueous phase organic
liquid (NAPL) phase, such as the gasoline leaking out of a storage tank, requires further articulation
as the ordinary solute transport equations are derived only for chemicals partitioned between an
aqueous liquid and a solid phase.6 In this paper, we illustrate the limitations in using the current solute
transport modeling approaches to estimate mobility of selected petroleum fuel components of gasoline in soils.
General Approaches of Estimating the Movement of Fuel Components in Soil
Many solute transport models have been developed to simulate the movement of organic compounds in the unsaturated zone. These models considered, however, only the migration of a single
solute in an aqueous phase which is absent of other reacting components. In the case of a LUFT, there
exists, in addition to the aqueous phase, a NAPL which consists of a large number of petroleum-
related hydrocarbons. Because of the added nonaqueous phase, the solute transport process involves
two immiscible liquid phases and is more complicated to describe mathematically than that of the
traditional single-solute aqueous phase-only system. There are two complications. First, one must
consider the mass flow of the nonaqueous liquid phase (gasoline in this case) as well as the aqueous
liquid phase (water) in the soil matrix. Second, the extent to which an organic chemical present in the
NAPL may partition into the air and the water phases. With more phases acting as the vehicles of
transport, chemicals often move at a more rapid rate than what occurs in the aqueous phase alone.5
Two approaches have been suggested to describe the transport of hydrocarbons of the petroleum
fuel in the vadose zone. One approach assumes the fuel constituents are dissolved in water and are
transported by advection flow of water and the presence of a NAPL phase may be ignored. The
second approach considers that the fuel constituents are partitioned between a water phase and a
NAPL phase and they are transported by two streams of immiscible flows.8 This approach is realistic,
but the governing equation is mathematically complicated and many parameters in the equation are
46th Purdue Industrial Waste Conference Proceedings, 1992 Lewis Publishers, Inc., Chelsea, Michigan 48118.
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