CHAPTER 6
FIELD APPLICATION OF THE ANALYSES
Field data was acquired to test the theoretical analysis presented in the previous
chapters. Time-drawdown data was obtained from a well test at a site in northern South
Carolina. The field site is located in the vicinity of Gaffney, Spartanburg County, South
Carolina, which is in the Blue Ridge Physiographic province (Fig 6.1). The topography
of the region is characterized by hills with gentle slopes and valleys that are cut by
streams.
Geology/Hydrology of Area
The area contains three weathered zones: surficial residual soil; saprolite; and a
transitional zone overlying fractured rock. Saprolite is weathered rock that contains the
structure of the bedrock and the transitional zone is less weathered, retaining chunks of
unweathered bedrock. Beneath the transition zone is fracture bedrock containing two
major metamorphic rock types. The northwestern part of the site is composed of felsic
metasediments and metavolcanics, whereas the southeastern part is composed of mafic
metasediments and metavolcanics (Fig. 6.2). The rock layering strikes northeast and dips
moderate to steeply southeast (RMT, Inc. 1995).
The two bedrock units are separated by a fault that strikes N50E, and dips 70
degrees to the southeast. The fault appears to have moved primarily with a
N
Stream
Ridge
Fault
Stream
500 feet
Fig. 6.1
Schematic of field site. The ridge is the recharge area with groundwater
flow suspected mainly in the SE direction.
(http://geography.about.com/science/geography/gi/dynamic/offsite.htm?sit
e=http://fermi.jhuapl.edu/states/sc%5F0.html Sterner, John Hopkins
University)
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N
70
Stream
Ridge
70
Stream
500 feet
Fig. 6.2
Surface geology of field site. Light gray is felsic metavolcanic and
metasediment rocks. Dark gray is mafic metavolcanic and metasediment
rocks. Fault occurs at the heavy line. Striking N50E and dipping 70
southeast with right lateral movement. Southeast (mafic) side is up
thrown and northwest (felsic) side is down thrown.
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component of normal motion, down to the north, and with a possible component of right
lateral slip. This fault plane is parallel to the foliation in the metamorphic rocks (RMT,
Inc., 1995). Groundwater in the area occurs in the saprolite, transition zone, and in the
moderately fractured bedrock below the transition zone. These three hydrostratigraphic
units grade into one another and thus comprise a single aquifer beneath the site. (RMT,
Inc., 1995).
The Well Test
A well test was performed at the site using well B-4 on March 9, 1993, by RMT,
Inc.. The test started at 5:00 pm and continued for 48 hours. The average pumping rate
was 9.75 gallons per minute (gpm). The pumping well, B-4, is screened from the water
table to 20 feet into bedrock with a total screen length of 100 feet. Two piezometers,
BW-2 and BW-109, were used in the analysis, as they are the piezometers available from
the test that are screened within the transition zone. Piezometer BW-2 is located
northeast of the pumping well at a distance of 232 feet and piezometer BW-109 is located
southwest of the pumping well at a distance of 148 feet. The pumping well is located
west of the fault at a distance of approximately 280 feet measured on a line perpendicular
from the fault (Fig 6.3).
Analysis of the Drawdown Curves
Semi-log time-drawdown plots were prepared for the pumping well, B-4 (Fig
6.4), as well as for piezometers, BW-109 and BW-2 (Fig 6.5). There is negligible
response from piezometer BW-2; therefore, it was not used in the analysis. With the
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N
B-4
BW2
BW-109
L
N
500 feet
Fig. 6.3
Location of piezometers BW-2 and BW-109 in relation to the pumping
well B-4 and the fault.
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50
40
s
30
20
10
0
10
100
1000
10000
100000
2
ds/dln(t2)
1.5
1
0.5
0
100
1000
10000
100000
t/rt2
Fig. 6.4
Semi-log time-drawdown plot of pumping well B-4. Derivative plot of
semi-log time-drawdown curve showing minimum slope of m = 0.5.
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8
s
6
4
2
0
1.5
m = ds/dln(t/r2)
1
0.5
0
-0.5
0.0001
0.001
0.01
0.1
1
t/r^2
t/r2
BW-109
Fig. 6.5
BW-2
Semi-log time-drawdown plot of piezometers BW-109 and BW-2.
Derivative plot of semi-log time-drawdown curve showing minimum
slope of m = 0.5.
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perpendicular distance from the pumping to the fault being 280 feet, the critical region
around the pumping well was determine to be approximately 84 feet, or 0.3L. Neither of
the available piezometers are within the critical region, so only a late-time semi-log
straight line is expected.
The drawdown curves for B-4 and BW-109 have a similar shape, the slope
increases to a transition period where the curve shifts downward and then approaches a
new straight line with approximately the same slope as the first (Figs. 6.4 and 6.5). This
shape is similar to the type curves produced from a change in storativity from low to high
in a 2-Domain model as well as a high transmissivity strip in a 3-Domain model. With an
evaluation of the geology of the area, it was concluded that this field case was acting as a
3-Domain model where the strip or fault zone is more transmissive than the matrix
The properties of the matrix were determined using the semi-log straight-line
method on data from BW-109. The head change over 1 log cycle is, s = 4.5 feet and the
x-intercept is, to=39 (Fig 6.6). Using equations (39) the transmissivity of the matrix was
determined to be 0.053 ft2/min. The storativity of the matrix was determined to be
0.0002 using equation (40); however, this value is highly uncertain because the
piezometer is outside of the critical region.
The derivative of the semi-log time-drawdown plot was taken from data at BW109 and B-4 to estimate the transmissivity of the fault. The data were noisy, so a moving
average filter was used with a window of 44 measurements. Measurements during the
well test were taken every minute during the first hour and 40 minutes and then every 5
minute to the end of the test. Once the filter was applied, the derivative was scaled using
127
9
8
7
6
s
5
4
3
2
1
0
1
10
100
1000
10000
t (min)
Fig. 6.6
Semi-log time-drawdown plot from piezometer BW-109 showing semilog straight-line method. to = 39 min s = 4.5 ft giving a Tm=0.053
ft2/min and Sm=0.0002.
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ds d
ds 4Tm

d ln( t s ) d ln( t f ) Q
(43)
where ds/dtf is the filtered derivative of the field data (ft/min), Tm is the transmissivity in
ft2/min, and Q is the average pumping rate in ft3/min.
The minimum dimensionless slope for both the piezometer and the pumping well
is 0.5 (Fig. 6.4, Fig 6.5). This value was used in equation (38) to determine that Tssd = 34.
These values for Tssd and Km along with the distance L, measured off the map, were
entered into equation (36) to determine the transmissiveness of the fault Tss = 24 ft2/min.
The thickness of the fault is unknown, but I will assume that the fault occurs as a zone of
fractures 10 to 20 feet wide. This gives a value of 2.4 ft/min for the hydraulic
conductivity of the fault zone. The transmissivity of the matrix is Tm =0.05 ft2/min,
which gives the conductivity of the matrix as Km = 0.003 ft/min assuming the thickness of
the aquifer is 21.5 ft (the length of the well screen at of 21.5 ft). Little is known about
how the hydraulic conductivity changes with depth at this site, so the assumption of the
thickness of the transition zone is only approximate.
These values suggest that the hydraulic conductivity of the fault zone is about 500
times that of the matrix. The assumptions involved in this estimate probably require that
the hydraulic conductivity contrast be approximated to within 2 to 3 orders of magnitude.
This seems feasible for a fault zone cutting fractured rock.
It is important to point out that the data from this test are relatively sparse, and
that other interpretations are possible. For example, the data shown here could be
explained by a model that considered a saprolite zone with a storativity that is
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significantly greater than the transition zone. Additional data are required to resolve this
ambiguity.
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