Survey objective
To monitor
a contamination situation at a small gas station at which the source of
the contamination has been removed. The location has been mapped just
before removing the source and 1 year after the source had been
removed.
Fieldwork
During fieldwork
a 6 feet x 6 feet grid was outlined. Experience shows that this is the
optimum choice for a grid to detect small 'spots' of contamination if
present. After establishing the local grid, a radar survey with 200 MHz
antenna (approx. 20 feet deep) Normally a location with the size of on
average gas station can be measured within half a day's fieldwork. The
data processing and preparation of a report takes a few days.
Results
The field data are
converted into a 3-dimensional model using EnviScan software. In this
model the relative contamination levels are visualized in different
colors. If the relative contaminant levels of the model are calibrated
with results of a limited number of actual analytical data obtained from
available boring data, the relative contaminant levels can be converted to
'absolute' contaminant levels.
Comparison of survey data from 1997 and 1998 (see figure) shows the
difference in contaminant levels at this site between 1997 and 1998.
Survey Objective
The
purpose of the first survey performed in 1999 was to determine the extent
of the soil contamination present under three feet thick concrete runway.
Conventional borehole drilling combined with laboratory analysis did not
produce sufficient information to determine the extent of the subsurface
soil contamination. This was mainly due to large area involved and limited
access for collection of samples underneath the concrete.
A second survey was performed in 2001 survey was to verify the
efficiency of the in-situ remediation through natural attenuation.
Fieldwork
For this large site, a 30 feet x 30 feet
grid was outlined. The measurements were carried out with a 200 MHz
antenna pulled behind a vehicle. The computer and peripherals were placed
inside the vehicle. The fieldwork took approximately one week using GPS
for positioning during the survey.
Results
The radar data were converted on location with
an adapted version of our EnviScan software in order to process the large
quantity of data. The obtained relative contamination values were
converted with the aid of limited analytical data into absolute values and
classified in two categories: light contamination (mineral oil <1000
mg/l) and heavy contamination (mineral oil >1000 mg/l). In comparing
the two surveys, it was shown that contamination was removed only slowly
through natural attenuation. Based on the results of the second
radar survey a new remediation plan was set up, in which selected heavily
contaminated soils were excavated and the light contaminated soil were
treated using in-situ bioremediation.
Repeated (yearly) surveys can be performed to monitor progress and
effectiveness of the remediation. The results of each survey can be used
to optimize remediation efforts, and consequently reduce total operating
time and remediation costs.
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