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140 Environmental Control in Petroleum Engineering

shoreline. If an onshore wind blows across the hydrocarbons, they
can be pushed to shorelines. Hydrocarbons spilled on water will
either evaporate, enter the water column, ground on the shore, or be
naturally degraded.

4.2 SUBSURFACE PATHS

Subsurface pathways of transport are those where released liquids
enter the pore structure of soil or sinks below the surface of open waters.

4.2.1 Releases on Land

There are two primary types of subsurface transport for onshore
releases that can impact the environment: the transport of fluids at or
above the water table from surface spills and the transport of fluids
from one geologic formation to another through improperly plugged
and abandoned wells.
When petroleum industry materials are discharged onto the ground,
the liquid fraction, including any dissolved chemicals, begins to enter
the pore network. These materials can travel through soil pore network
in four ways. First, a separate nonaqueous phase liquid (NAPL) can
flow through the pores. Second, contaminants can dissolve into ground-
water and be transported by it. Third, very small solids (colloids) can
also be transported with the water, although large particles will be
filtered by the porous media. Fourth, volatile contaminants can be
transported as a vapor through the vadose (air saturated) zone.
The transport of wastes through groundwater depends on a number
of factors, including the permeability of the soil, capillary pressure
between phases 'in the soil, solubility of the waste, partitioning coeffi-
cients, adsorption properties, and volatility. Adsorption, partitioning
and volatilization decrease the concentration of chemicals in water,
while leaching, desorption, and runoff increase the concentration. A
review of the mechanisms of hydrocarbon transport in groundwater
has been presented by Hunt et al. (1988a, 1988b).
Metals tend to form insoluble complexes in high-pH environments,
minimizing their ability to leach away from a site (American Petrol-
eum Institute, 1983b). The primary mechanisms for the fixation of
metals by soils are absorption, ion exchange, and chemical precipita-
tion. Ion exchange and adsorption are surface phenomena that are