Summary
The results of this study demonstrate the usefulness of imaging
contamination plumes in three-dimensional space from geoelectrical data.
Introduction
In this study, the land owner and regulators assumed that a given land
area was contaminated from a source that was an old abandoned multi-purpose
pipeline. Using a 120-channel multi-mode geoelectrical data acquisition and imaging
system, contamination plumes were easily identified and more precisely
localized than with any other known method.
Theory/Method
Geoelectrical measurements have been widely used in near-surface
investigation and assessment of petroleum hydrocarbon contamination. However,
unambiguous detection of hydrocarbon contamination is the most difficult task
for geoelectrical method at suspiciously hazardous sites. The difficulty is two-fold:
1) the low level of resistivity contrasts that the contaminants may provide
against the background soils, 2) precise identification and assessment of
sources of contamination plumes.
Due to costs estimations, the operator wanted to use an approach that
would have less environmental impact and be less costly than extensive excavation
along the pipeline and across the entire site.
The data was collected in an agricultural field north of Edmonton using a
120-channel multi-mode geoelectrical data acquisition and imaging system. The
imaging in 3D of the contamination plumes allowed for the precise spatial
delineation of the plumes and the calculation of the volume of earth to be
remediated. Some contamination plumes extended to depth of 12 metres below
ground level. Groundwater flow in the vicinity of the contamination plumes was
also interpreted and mapped from the geoelectrical data. Results were compared
to monitoring well samples.
Existing drill hole data was used to calculate the geoelectrical
thresholds of faint variations of conductivity related to hydrocarbon
contamination. Those thresholds were
used to constrain the three dimensional imaging of the geometry of anomalies.
Groundtruthing was conducted with drill holes. The magnitude and spatial
location of the contamination plumes identified by the geoelectrical survey and
imaging method correlated with the results provided by the drill holes.
Conclusion
This study revealed that the sources of the contamination
plumes were not related to the pipeline. The virtual scan of the site by the
geoelectrical survey showed a clear picture of old abandoned oil wells and buried
mud pools from which leaked contaminants were transported across the site following
groundwater and geological pathways.
Geoelectrical measurements using a 120-channel multi-mode imaging is an
environmentally conscious and financially feasible method to obtain accurate
details of a near-surface assessment of petroleum hydrocarbon contamination.
