health and safety plan solid waste management unit assessment

65-18
Although the microbiota of most non-contaminated soils include
-nY naturally occurring hydrocarbon-degrading populations, the
addition of petroleum selectively enriches that sector able to adapt
and utilize the new substrate. Other environmental factors shown to
have a major effect on biodegradability are availability of oxygen and
moderate temperatures.
In hoary. biodegradation of the petroleum hydrocarbons
comprising automotive gasolines is expected to be rapid under
conditions favorable for microbial activity and when fuel components
are freely available to the microorganisms. Degradation may be limited
and/or slow in environments with few degrading organisms, low pH, low
temperatures, and high salinity (e.g., arctic environments). It should
be mentioned that Walker gg al;. (2257) state that even under optimum
conditions, total and complete biodegradation is not expected to occur
except possibly over an extremely long time period.
65.2.3 Primary Routes of Exposure from Soil/Ground-water Systems
The above discussion of fate pathways suggests that the major
components of gasoline are highly volatile but vary in their potential
for bioaccumulation and tendency to sorb to soil. They range from
moderately to strongly sorbed to soil, and their bioaccumulation
potential 'ranges from low to high. The variability in the properties
of the components suggests they may have somewhat different potential
exposure pathways.
Spills of gasoline would result in the evaporative loss of the
more highly volatile components leaving those of lesser volatility in
the soil. The fraction remaining in the soil is expected to be relatively
mobile and will be carried by gravity to the saturated zone of
the soil. There, the more soluble components will dissolve into the
ground water or form emulsions with it. These components are primarily
aromatic and lower molecular weight aliphatic compounds; in one study
using unleaded gasoline, approximately 95% of the water soluble fraction
was benzene and substituted benxenes (2318). The insoluble
fraction of gasoline floats as a separate phase on top of the water
table. The movemmnt of gasoline dissolved in ground water is
especially important because of its relatively high soluhility (173-200
mg/L (2287,2297)). Furthermore, the movement of dissolved hydrocarbons
in ground water is much greater than that of the separate liquid phase,
reaching distances of hundreds to thousands of meters compared to tens
of meters for the movement of the separate phase. In the presence of
cracks and fissures, however, the flow of the separate hydrocarbon
phase is greatly enhanced.
The movement of gasoline in ground water may contaminate drinking
water supplies, resulting in ingestion exposures. Ground-water discharges
to surface water or the movement of contaminated soil particles
to surface water drinking water supplies may also result in ingestion
exposure and in dermal exposures from the recreational use of these
waters. The potential also exists for the uptake of some gasoline
6/87