Mapping DNAPL transport contamination in sedimentary and fractured rock aquifers with high resolution borehole seismic imaging Project No. SF11SS13 FY01 Annual Report Page: 2 of 44
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Executive Summary
The objectives of this project are to develop, demonstrate and evaluate, at appropriate field sites, the utility
of high frequency seismic imaging methods to detect and characterize non-aqueous phase liquid (NAPL)
contamination in sedimentary and fractured rock aquifers. Seismic methods may significantly reduce the
final cost of remediation by accurately targeting source areas. Our field work for FY01 was performed at
the Northeast Site of the Pinellas Science, Technology, and Research (STAR) Center, in Largo, Pinellas
County, Florida. The STAR Center is a former DOE facility that manufactured neutron generators and
other electronic and mechanical components for nuclear weapons from 1956 to 1994, and is now owned by
Pinellas County, with DOE maintaining responsibility for site remediation. Waste and construction debris
were disposed of at Northeast site in the 1950s through 1960s. The site is contaminated by volatile organic
chemicals (VOCs), including NAPL tricholorethylene (TCE), toluene, methylene chloride, and weathered
oils and perhaps resins.
The Northeast Site is underlain by a surficial aquifer composed of a heterogeneous distribution of beach-
type deposits, including fine sands with variable shell content, and silty and clayey sands. The water table
depth is about two meters below ground surface and the surficial aquifer is approximately 9 m deep. The
Hawthorn Group, a 18 to 21 m thick confining layer, lies below the surficial aquifer. The Hawthorn
consists of silty-clay with gravel, silty-sandy phosphatic clay and carbonaceous clay with friable carbonate
layers. Below the Hawthorn Group is the Floridian Aquifer, a limestone formation, which is an important
regional groundwater resource. VOC contamination occurs within the surficial aquifer and at its contact
with the Hawthorn.
Laboratory granulametric and geophysical analyses were performed on grab samples donated by the site
that were taken from various depths within the surficial aquifer. All of the samples were fine-grained or
smaller, having an occasional pebble; one sample had significant shell content. Clay fractions of six
samples from the surficial aquifer and from the Hawthorn unit were analyzed. In five of the aquifer
samples, the clay fraction was mostly kaolinite. One sample from 3 m depth had a significant amount of
smectite (an expandable clay) and Hawthorne unit sample was smectite, possibly combined with chlorite.
One sample from about 6 m deep, having a median grain diameter 150 m, was used to measure P-wave
transmission as a function of water/TCE saturation. P-wave velocities declined as a function of TCE
saturation, consistent with previous studies, however the amplitude change was much less. This is
significant, because P-wave attenuation could be a helpful diagnostic for NAPL detection. The minimal
attenuation observed in this experiment may be due to either the lower reflectivity between TCE and water
compared to other NAPL-water pairs, and/or the relatively small grain size of the sample. Of the NAPLs
presumed to be present at the site, the reflectivity between toluene, resins and water is almost an order of
magnitude greater than for TCE and water, which may mean that attenuation through these NAPLs would
be greater. Testing in FY02 will investigate the relationships between seismic attributes and various NAPLs
in intact core from the site.
High-frequency seismic and radar cross-hole surveys were performed in several locations at the site. Eleven
2" diameter boreholes were installed to a depth of approximately 10 m (extending into the Hawthorn unit)
with a cone penetration test (CPT) rig and cased with blank PVC. Four lines of two or three boreholes
were installed. Within each line, the boreholes were spaced about 2.3 m apart. One line of three boreholes
was located in the northern area of the site, where NAPL contamination has been detected. Two lines of
three boreholes each were located in the southern area of the site where NAPL has been detected, and one
line of two boreholes was located in the southern part of the site where no NAPL had been detected. A
CPT log was collected in one borehole in each line. Electrical conductivity logs were measured in all
boreholes. Cross-hole seismic data were collected for six borehole pairs, to yield at least one tomogram at
each transect location. The dominant frequency range for the seismic data was 3-5 kHz, but 1-2 kHz was
also visible; velocities ranged from 1800 to 2200 m/s. These transmission characteristics suggest that
wavelengths on the order of 30 to 70 cm are achievable at the site. Cross-hole radar data were collected for
three borehole pairs, and zero-offset (horizontal transmission only) data were acquired for another three
pairs. In the first two boreholes, radar tomograms were acquired using borehole antennas having a central
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Geller, J. T.; Majer, E. L.; Peterson, J. E.; Williams, K. H. & Flexser, S. Mapping DNAPL transport contamination in sedimentary and fractured rock aquifers with high resolution borehole seismic imaging Project No. SF11SS13 FY01 Annual Report, report, December 1, 2001; Berkeley, California. (https://digital.library.unt.edu/ark:/67531/metadc741207/m1/2/: accessed April 25, 2024), University of North Texas Libraries, UNT Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.