1.1.1
CSSA
General information
regarding the history and environmental setting of CSSA is provided in the CSSA
Environmental Encyclopedia
(Volume 1-1,
Background Information Report). In
that report, data regarding the geology, hydrology, and physiography are also
available for reference.
1.1.2
SWMU B-6
1.1.2.1 Site Description
SWMU B-6
was initially identified during a records review in 1993. According to the
records, the site was used for miscellaneous solid waste disposal for an unknown
amount of time and during an unknown time period (Parsons, 1993b).
Aerial photographs from 1934, 1957, 1962, 1966, 1973, 1978, 1985, 1991,
1996, and 1999 were reviewed, but no disturbed ground in the vicinity of SWMU
B-6 was apparent. The site was
mapped during a field investigation conducted on March 6, 1996. SWMU B-6 is covered in native grasses and shrubs and is
surrounded by a cover of native Live Oak trees. The site occupies approximately 0.17 acres of land.
Background
information regarding the location, size, and known historical use of SWMU B-6
is included in the Environmental Encyclopedia (Volume 1-2, SWMU B-6).
Volume 1-2 also includes a
Chronology
of Actions and a
Site-Specific Work Plan Addendum for SWMU B-6.
1.1.2.2
Potential Sources of Contamination
Potential COCs at
SWMU B-6 include VOCs, SVOCs, and metals. Metals
(arsenic, barium, cadmium, chromium, copper, lead, mercury, nickel, and zinc)
may occur due to the potential metallic nature of any waste that may have been
disposed at the site. VOCs have
occurred at other waste disposal sites at CSSA and are a contaminant typically
associated with disposal areas. SVOCs
may be present if burning took place at the SWMU.
1.1.2.3
Site Location
SWMU B-6 is located
in the center of the western half of the North Pasture (Figure
B6-1), and is approximately 2,300 feet from the western facility boundary.
Figure
B6-1 is based on the 1998 aerial photograph.
The site is roughly elliptical in shape and covers about 0.17 acres in
area. A dirt road runs along the
western side of the site.
Livestock water
supply well CS-G is approximately 450 feet north of the site, and Salado Creek
is approximately 700 feet southwest of the site.
The closest SWMU is SWMU B-22, about 830 feet southwest of SWMU B-6.
1.2.1 - Site Soils and Topography
SWMU B-6 occurs in
the gently undulating Tarrant soil association
(Figure
B6-2).
These soils occur on
nearly level to gently sloping areas, generally with 1 to 5 percent slopes. The soils are dark-colored, very shallow, calcareous, and
clayey, and are best suited for native grasses and
range use. They develop over hard
limestone and have scattered stones and gravel on the surface and within the
surface layer. Detailed
descriptions of CSSA soil types are provided in the CSSA Environmental
Encyclopedia
(Volume 1-1, Background
Information Report, Soils and Geology).
SWMU B-6 lies on
the lower slope of a hill east of the Salado Creek floodplain.
The ground surface elevation of the site area is approximately 1,305 feet
above sea level, sloping moderately to the southwest.
Drainage from the site flows southwest across a narrow dirt road and
vegetated land towards Salado Creek. The
site vegetation is predominantly native grasses, with large oaks near the
perimeter.
1.2.2 - Geology
The Upper Glen Rose
Formation is the uppermost geologic stratum in the area of SWMU B-6 (Figure
B6-3). The Upper Glen Rose
consists of beds of blue shale, limestone, and marly limestone, with occasional
gypsum beds. Generally, it outcrops in stream valleys and at the ground surface
where soils are poorly developed or eroded. Where present at CSSA, the thickness
of the Upper Glen Rose is estimated to be up to 150 feet thick.
It is underlain by the Lower Glen Rose Limestone, which is estimated to
be 300 feet thick beneath CSSA. The
Lower Glen Rose is a massive, fossiliferous, vuggy limestone that grades upwards
into thin beds of limestone, marl, and shale.
The Lower Glen Rose is underlain by the Bexar Shale facies of the Hensell
Sand, which is estimated to be up to 150 feet thick under the CSSA area.
The Bexar Shale consists of silty dolomite, marl, calcareous shale, and
shaley limestone. The geologic
strata dip approximately 10 to 12 degrees to the south-southeast at CSSA.
Based on published
information, there are two known major fault (shatter) zones at CSSA: the North
Fault Zone and the South Fault Zone. SWMU
B-6 is located approximately 3,660 feet north of the northern edge of the North
Fault Zone that trends to the northeast along the north end of the inner
cantonment (Figure B6-3). Additional
information on structural geology at CSSA can be found in the Environmental
Encyclopedia
(Volume 1-1,
Background
Information Report, Soils and Geology).
1.2.3
- Hydrology
In general, the
uppermost hydrogeologic layer at CSSA is the unconfined Upper Trinity aquifer,
which consists of the Upper Glen Rose Limestone.
Transmissivity values are not available for the Upper Glen Rose.
Regionally, groundwater flow is thought to be enhanced along the bedding
contacts between marl and limestone; however, the hydraulic conductivity between
beds is thought to be poor. This
interpretation is based on the observation that static well levels are
discordant in adjacent wells completed in different beds.
Principle development of solution channels is limited to evaporite layers
in the Upper Glen Rose Limestone. Groundwater
discharge occurs predominantly via natural springs, seeps, and by groundwater
pumping. Regional groundwater flow
in the Upper Trinity aquifer appears to be to the southeast.
The Middle Trinity
aquifer is unconfined and functions as the primary source of groundwater at CSSA.
It consists of the Lower Glen Rose Limestone, the Bexar Shale, the Cow
Creek Limestone, and the Hammett Shale. The
Lower Glen Rose Limestone outcrops north of CSSA along Cibolo Creek and within
the central and southwest portions of CSSA.
As such, principle recharge into the Middle Trinity aquifer is via
precipitation infiltration at outcrops. At
CSSA, the Bexar Shale is interpreted as a confining layer, except where it is
fractured and faulted, therefore allowing vertical flow from the up-dip Cow
Creek Limestone into the overlying, but down-dip Lower Glen Rose.
Fractures and faults within the Bexar Shale may allow hydraulic
communication between the Lower Glen Rose and Cow Creek Limestones.
In general, groundwater within the Middle Trinity aquifer at CSSA flows
in a north to south direction, with some seasonal fluctuations.
However, the local flow gradient may vary depending on rainfall,
recharge, and possibly well pumping.
The nearest surface
water body is the intermittently flowing Salado Creek, which is approximately
700 feet southwest of SWMU B-6 (Figure
B6-1).
Salado Creek flows
generally south through CSSA and exits the facility just inside the southwest
corner of the East Pasture.
No site-specific
information regarding groundwater is available.
Well CS-G, which is an inactive livestock water supply well, is located
approximately 450 feet north of SWMU B-6. Between
1994 and 2000, static water levels for Well CS-G have ranged between 246
(October 1997) and 316 (January 1997) feet BTOC.
Well completion information is not available for this well.
1.2.4 - Cultural Resources
Cultural
resources are prehistoric and historic sites, structures, districts, artifacts,
or any other physical evidence of human activity considered important to a
culture, subculture, or community for scientific, traditional, or religious
purposes. No potential sites of cultural importance are close to SWMU
B-6. The nearest cultural resource
site is a Paleoindian and late Archaic site consisting of a burned rock midden
which was recommended as ineligible for listing in the National Register of
Historic Places. This site is
located approximately 200 feet southeast of SWMU B-6.
1.2.5 - Potential Receptors
A land use survey
evaluating local and possible future uses of groundwater and surface water, a
water well survey, and sensitive environmental areas at CSSA was completed
during December 15 and 16, 1999. The
results of this survey, along with results from a more in-depth survey to
identify potential receptors, points of human exposure, and possible constituent
pathways is presented in Section 3
of the Technical
Approach Document for Risk Evaluation
(Volume 1-6).
Land within
0.25 mile of SWMU B-6 is classified a “Evergreen Forest Land” in Figure
1.1 of the Technical Approach Document for Risk Evaluation.
There are no buildings within 0.25 mile of the site, but a pond known as
the “Drop Zone Tank” is located approximately 1,500 feet northwest of the
site area.
As described in
Section 1.2.3, Well CS-G, is located approximately 450 feet north of SWMU B-6.
Well CS-G was formerly used for livestock watering purposes; however,
sucker rods in the well are no longer operational. Dichloromethane was detected in this well at a level of 6.0
µg/l in 1994, but levels of this
chemical did not exceed the MCL (5.0 μg/L) in subsequent sampling
events. No other COCs have been
detected at levels above the MCL.
A small herd of
cattle is maintained on CSSA by the USDA ARC.
The cattle roam freely throughout the Inner Cantonment and in selected
areas of the North Pasture. CSSA
also manages wild game species for the purpose of hunting.
White-tailed deer, axis deer, and wild turkey all roam freely throughout
CSSA. A map of deer hunting stands
that overlook mechanical feeders and planted food plots is located in Figure
5.2 of the Technical Approach
Document for Risk Evaluation (Volume
1-6). SWMU B-6 is located approximately 900 feet
north east of hunting stand number 24. Four
water reservoirs are maintained at CSSA for the purpose of sport fishing.
Two of the reservoirs are located in the northwestern and northeastern
portions of the North Pasture while the other two reservoirs are located near
the western boundary of the Inner Cantonment.
The nearest
potential habitat for local endangered species is a potential Black-capped Vireo
habitat located approximately 1,500 feet southeast of the site.
A potential Golden-cheeked Warbler habitat is located approximately 2,000
feet east of the site area.