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Section 2 - Field Investigation
As outlined in the Environmental Encyclopedia site-specific work plan (Volume 1-2, SWMU B-9), project RFI requirements were to conduct a
geophysical survey and to collect surface and subsurface soil samples.
SWMU B-9 is an area where miscellaneous solid waste was reportedly
disposed. All field activities
were conducted in accordance with the Field Sampling and Analysis Plan (Volume
1-4, Field Sampling Plan and Quality Assurance Project Plan)
Electromagnetic (EM) and ground penetrating radar (GPR) geophysical surveys
were conducted at SWMU B-9 in November 1999 under DO5068.
Prior to collecting EM or GPR data, a grid system was established to
encompass the areas of suspected ground disturbance. This grid consisted of staked locations separated by
intervals of 50 feet. Figure B9-4 illustrates the layout of the geophysical survey grid at
SWMU B-9.
EM data were collected at 2-foot intervals along transects that were separated
by 10 feet using the established geophysical survey grid. EM measurements were taken using a Geonics EM31-DL ground
conductivity meter and recorded with a Polycorder data logger.
The conductivity meter consists of transmitter and receiver coils that
are separated by 12 feet. The
instrument has a nominal depth of penetration of approximately 16 feet when
operated in the vertical-dipole mode. The
instrument measures both quadrature- and in-phase components of an induced
magnetic field. The quadrature-phase
component is a measure of apparent ground conductivity while the in-phase
component is more sensitive to the presence of ferromagnetic metal.
A lateral variation in apparent ground conductivity indicates a lateral
change in subsurface physical properties (i.e., related to degree of
disturbance). Apparent ground
conductivity is measured with a precision of approximately ±2 percent of the
full-scale meter reading, which corresponds to approximately two milliSiemens
per meter (mS/m). The in-phase
component of the EM-31 is the response of the secondary to primary magnetic
field measured in units of parts per thousand (ppt). The primary magnetic field is due to the current source from
the EM-31. The secondary magnetic
field is due to induced currents within conductive material in the subsurface.
Data was collected by setting the instrument to record in an automatic
vertical dipole mode. Readings
were taken at 0.5 second intervals which corresponded to a reading every two
feet along a given transect. Both
apparent ground conductivity (i.e., quadrature phase) and in-phase data were
recorded. The operator aligned
himself along a transect and, with the instrument parallel to the transect,
paced between marked or staked stations separated by 10 feet.
The EM-31 survey was completed according to the procedures described in Volume
1-4, Sampling and Analysis Plan, Section 1.1.2.
Prior to the survey, an area near SWMU B-9 that was determined to be
free of disturbances and anomalies was selected and marked to perform
background checks and calibration. The
background checks were also performed after the survey.
All calibration data along with before and after background readings
were recorded in the field logbook.
GPR is a surface geophysical technique that uses high-frequency EM energy.
Pulses of short-duration EM energy are transmitted into the subsurface
from the radar antenna that is moved across the ground surface at a slow and
uniform pace. The radiated energy
encounters heterogeneities or anomalies in electrical properties of the
subsurface, which causes some energy to be reflected back to the receiving
antenna and some to be transmitted downward to deeper material.
The amplitude or strength of the EM energy reflected from subsurface
materials depends on contrasts in the electrical properties (conductivity and
dielectric constants) of those materials.
The reflected signal is amplified, transformed to the audio-frequency
ranges, recorded, processed, and displayed.
Recorded data displays the two-way travel time for a signal to pass
through the subsurface, reflect, and return to the surface.
The observed time for the reflected signal to return to the antenna from a
subsurface feature is an indication of the depth to the reflector.
The two-way reflection time can be converted to depth if the EM wave
velocity of the subsurface soils is known. In
the absence of such information, an approximate time to depth conversion can
be estimated by using published values of material velocity for different soil
types.
GPR surveys were conducted with a GSSI SIR-2 instrument to verify the
information obtained by the EM survey.
Two GPR profiles were created in the northeast-southwest direction
and three were created in the southeast-northwest direction (Figure
B9-4). A 300 mega-hertz (MHz)
antenna with a range setting of 90 nanoseconds (ns) was used for all profiles.
The survey was conducted in accordance with procedures outlined in Volume
1-4, Sampling and Analysis Plan, DO5068 Addendum, Section 2.4.
The individual GPR survey profiles were conducted over anomalies that
were detected during the EM-31 survey. Additional
surveys were also conducted at the site to provide background information.
If no anomalies were identified during the EM‑31 survey, the GPR
was used to gather additional background information for the site.
The GPR profiles were sequentially numbered with a GPR profile number as they
were created throughout the day. Therefore,
each profile is identified with a sequential number that is not related to the
number of profiles created at the site.
In
accordance with the approved work plan, a soil gas survey was not performed in
association with the current investigation conducted for SWMU B-9.
In 1996, samples were collected from two borings in an area thought at the
time to be SWMU B-9. It was later
determined that these samples were located within the nearby AOC-44, which is
approximately 300 feet north. These
samples were analyzed by ITS Laboratory in Richardson, Texas, and were later
found to be unusable by the Environmental Protection Agency (EPA) for site
closure purposes. Therefore, in
March 2000 rework samples were collected, but they were collected within the
SWMU B-9 boundary.
In March 2000, three soil borings were advanced and nine soil samples were
collected at SWMU B-9. These
surface and subsurface soil samples were the first collected from SWMU B-9. The boring locations are shown on Figure B9-5. Samples
collected between zero and one-foot bgs are considered surface soil samples.
Two of the samples were collected as part of the laboratory rework
field effort, though no surface soil samples were previously analyzed from
SWMU B-9. The third sample was collected as part of the RL53 DO field
effort. Since the geophysical
survey revealed no anomalies, the samples were collected from areas that would
provide a general characterization of the site.
The samples were submitted to APPL Laboratories in Fresno, California,
for VOC (SW-8260B) and SVOC (SW-8270C) analysis to DataChem Laboratories in
Salt Lake City, Utah, for explosives (SW-8330) analysis, and to O’Brien and
Gere Laboratories in East Syracuse, New York for metals analysis.
Metals analyses included SW-6010B for barium, chromium, copper, nickel,
and zinc; SW-7060A for arsenic; SW-7131A for cadmium; SW-7471A for mercury;
and SW-7421 for lead. A total of nine environmental samples, one field duplicate,
one matrix spike, one spike duplicate, and one trip blank were submitted for
analysis.
Samples collected above the bedrock were obtained using a decontaminated
hollow-stem auger and split-spoon sampler.
Rock samples were obtained by air rotary methodology using a
decontaminated core barrel. Since
no geophysical anomalies were detected, borings were advanced in locations
that would accurately characterize the subsurface conditions at SWMU B-9.
Boring depths were determined by the work plan, which called for
borehole terminus after drilling through five feet of competent bedrock (Volume
1-1, Work Plans, RL17 Addendum). All
decontamination, sample preparation and handling followed those protocols
established in the Field Sampling and Analysis Plan (Volume
1-4, Field Sampling Plan, Quality Assurance Project Plan). Environmental sampling also included the collection and
submittal of QA and QC samples at those frequencies outlined in the AFCEE QAPP
(Volume 1-4, Quality Assurance Project
Plan).
A review of the boring logs generated during the subsurface investigation (Appendix B) identifies a thin veneer of soil present at this site. The SWMU B-9 surface soil samples were loose, dry, silty clays with cobbles. Materials representative of the Upper Glen Rose Formation were typically encountered at depths of less than one foot bgs. The Upper Glen Rose was identified to consist of materials varying from limestone to clayey marl. At the time of sampling, no evidence of contamination was identified either through visual observation or with a photoionization detector (PID) instrument. Soil boring logs from the borings advanced in 2000 are presented in Appendix C.
In accordance with the approved work plan, groundwater samples were not
collected in association with the investigation conducted for SWMU B-9.
Groundwater was not encountered in any of the soil borings.
On April 9 and 10, 1997, a UXO sweep was performed by UXO specialists from UXB
International, Ashburn, Virginia. The
UXO specialists traversed the site in a systematic manner to visually identify
UXO on the surface. In addition,
they used Schonstedt magnetometers to assist in identification of metal at or
near the surface.
The geophysical surveys revealed no evidence of subsurface anomalies related
to past waste disposal activities. There
was little variation in the data that were recorded during the EM survey,
which can be interpreted as homogeneous and consistent soil and bedrock
profiles throughout SWMU B-9 (Figure
B9-6 and
Figure B9-7). In-phase
readings during the EM survey ranged from a minimum of –1.537 ppt, to a
maximum of 0.836 ppt. The contour
interval in Figure B9-6 is only 0.3 ppt, and the slight variations near SB01,
SB02, and SB03 are not considered great enough to be considered anomalies.
Quadrature-phase readings ranged from a low of 12.60 mS/m, to a high of
16.33 mS/m.
The GPR surveys were conducted to further investigate the information obtained
by the EM survey. Like the EM
survey, the GPR also revealed no evidence of subsurface anomalies. The GPR profile included in this report
(Figure B9-8) represents the typical 300 MHz antenna survey profile
that was produced at SWMU B-9. Resolution
of the profile was poor due to the homogeneous nature of the clay-rich soil
and underlying bedrock; however, the shallow dipping bedrock can be seen in
the GPR profile. The vertical
scale on the profile, time in ns, can be converted into feet using the
following formula: