|Hydrocarbon Contaminated Soils
Many advances have been made in recent years concerning remote imaging of
hydrocarbon contaminated soils. Electrical resistivity and electromagnetic
surveys are able to map conductive plumes associated with petroleum releases
into soils and shallow groundwater. Natural bioremediation of the
hydrocarbon increases the soil conductivity and resistivity cross-sections or isoconductivity maps can show the extent of contamination prior to excavation.
Non-invasive geophysical site characterizations conducted prior to removal of
the contamination can assist in designing an appropriate remedial action.
Jet Fuel Refinery
The Mosby Jet Fuel Refinery in Mosby, Montana discontinued operations in
1976 after 23 years of producing leaded gasoline, diesel,
fuel oil and JP-4 jet fuel. Remediation of the site was initiated in the early
1990's. The final stages of removing the infrastructure was completed in
2002 and involved digging
up and removing the underground steel piping.
ECHOTECH collected soil samples for petrochemical analysis during the
removal of the buried piping. It became obvious from the condition of the
removed piping, soil staining, and the presence of free product, that a
considerable volume of petrochemicals of many types had been released into the
subsurface and onto the surface over a prolonged period of time.
In an effort to reduce the assessment cost of this 10-acre site, ECHOTECH
designed and conducted a geophysical investigation to assess the extent of
contamination. An EM61 survey revealed
that over 10,000 linear feet of underground piping was present instead of the
estimated 3,000 linear feet. EM34 and
electrical resistivity surveys were
then conducted using a grid of 20 parallel and perpendicular profiles.
The profiles were analyzed individually and the entire data set was combined
to allow for quasi-3D processing and interpretation. The geophysical
results were correlated to surface and subsurface soil samples and provided a
more continuous interpretation of the contamination than would be determined
from drilling and soil sampling alone.
The following figure shows 1.5-3.0 meter thick horizontal slices of the 3D
model taken from the center of the facility. The blue areas correspond to
zones of hydrocarbon contamination based on the comparisons with the
petrochemical analysis of the soil samples.
These vertical cross-sections are east-west slices through the middle and
northern portion (top) of the model. Most of the contamination appears to
be concentrated in the upper horizontal strata at the site.
The subsurface study also showed the presence of vertical zones of
conductivity as seen below which suggests that some vertical migration of
petrochemicals may be occurring. Near-surface southwest trending faults
and fractures exist in the horizontally bedded confining layers on the site and
could act as primary corridors of contaminant migration. The blue
"bull's-eye" at 60 meters
in this cross-section corresponds to a leaking underground pipe, as does the
surface injection point at 165 meters.
In all, the geophysical investigation showed that some 137,800 cubic yards
of hydrocarbon contaminated material was present in the near-surface soils at
the refinery site.
ECHOTECH has designed a remediation plan for the Mosby site which involves
landfarming the contaminated soils on-site. This is an effective and economical
approach that allows natural biodegradation of the hydrocarbons and minimizes
the risks to health and human safety.