U.S. Geological
Survey
Open-File Report 01-60
Stratigraphic
and Structural Characterization of the OU-1 area at
the former George Air Force Base, Adelanto, Southern California
By
R. D. Catchings, G. Gandhok, and M.R. Goldman
INTRODUCTION The former
George Air Force Base (GAFB), now known as the Southern California Logistics
Airport (SCLA), is located in the town of Adelanto, approximately 100
km northeast of Los Angeles, California (Fig. 1). In this report, we present
acquisition parameters, data, and interpretations of seismic images that
were acquired in the OU-1 area of GAFB during July 1999 (Fig. 2). GAFB
is scheduled for conversion to civilian use, however, during its years
as an Air Force base, trichlorethylene (TCE) was apparently introduced
into the subsurface as a result of spills during normal aircraft maintenance
operations. To comply with congressional directives, TCE contaminant removal
has been ongoing since the early-tomid 1990s. However, only a small percentage
of the TCE believed to have been introduced into the subsurface has been
recovered, due largely to difficulty in locating the TCE within the subsurface. Because TCE
migrates within the subsurface by ground water movement, attempts to locate
the TCE contaminants in the subsurface have employed an array of ground-water
monitoring and extraction wells. These wells primarily sample within a
shallow-depth (~40 m) aquifer system. Cores obtained from the monitoring
and extraction wells indicate that the aquifer, which is composed of sand
and gravel channels, is bounded by aquitards composed largely of clay
and other fine-grained sediments. Based on well logs, the aquifer is about
3 to 5 m thick along the seismic profiles. A more thorough understanding
of the lateral variations in the depth and thickness of the aquifer To better
characterize the shallow (upper 100 m) stratigraphy beneath GAFB, the
US Environmental Protection Agency (USEPA) contracted the US Geological
Survey (USGS) to acquire three seismic reflection/refraction profiles
within an area known as Operational Unit #1 (OU-1). The principal objective
of the seismic survey was to laterally characterize the subsurface with
respect to structure and stratigraphy. In particular, we desired to (1)
laterally map stratigraphic units (particularly aquifer layers)
that were previously identified in monitoring wells within the OU-1 area
and (2) identify structures, such as faults and folds, that affect the
movement of ground water. Knowledge of lateral variations in stratigraphic
units and structures that may affect those units is useful in constructing
ground-water flow models, which aid in identifying possible TCE migration
paths within the subsurface. Stratigraphic and structural |
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Created: 8-16-01
Last modified: 8-17-01 (cad)
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