Environmental Engineering Reference
In-Depth Information
80
4.5
Figure 6.8
Hydrograph of
water levels in a piezometer
screened in the Mintaro Shale,
and bar chart of daily rainfall
at Clare, South Australia. The
magnitude of the seasonal
water-level fluctuations and
the independently estimated
recharge rate are consistent with
a value of specific yield close to
the total porosity. Short-term
fluctuations in response to daily
rainfall events are generally
absent due to attenuation of
short wavelength variations by
the large storage capacity of
the well. (With kind permission
from Springer Science+Business
Media:
Hydrogeology Journal
,
“Using groundwater levels to
estimate recharge”, v. 19, 2002,
p. 102, R. W. Healy and P. G.
Cook, Figure 11.)
70
5.0
60
50
5.5
40
6.0
30
20
6.5
10
0
7.0
1997
1998
1999
This is not attributed to a lack of recharge, but
rather attenuation of these short-term signals
by the large storage capacity of the piezometer
(50 mm inside diameter) relative to that of the
fractures. The annual cycle in water level is
approximately 2 m, which is consistent with
a recharge rate of approximately 40 mm/year
and a specific yield (0.02) that is closer to the
matrix porosity than to the fracture porosity.
Estimating specific yield in fractured rocks
is problematic. Aquifer tests in fractured-rock
systems are usually unreliable for determin-
ing
S
y
(Bardenhagen,
2000
). The water-budget
method is perhaps the most widely used tech-
nique for estimating specific yield in fractured-
rock systems because it does not require any
assumptions concerning flow processes. Gburek
and Folmar (
1999
) and Heppner
et al
. (
2007
) used
a water-budget approach to determine
S
y
for a
fractured sandstone, siltstone, and shale system
in east-central Pennsylvania. The depth to the
water table was approximately 7 m. Pan lysim-
eters were installed at 1 to 2 m depths beneath
undisturbed soil columns. These lysimeters
were designed to capture all downward-moving
water. The rate of water percolation measured
by the lysimeters was assumed equal to the
recharge rate. Water levels were measured in
wells near the lysimeters, and
S
y
was calculated
from Equation (
6.2
). An average value for
S
y
of
0.009 was obtained by Gburek and Folmar (
1999
)
for eight events between 1993 and 1995. At that
same study site, Gburek
et al
. (
1999
) compared
the recession of well hydrographs with the
base-flow recession curve for a stream drain-
ing the aquifer over a 40-day period. Through
calibration of a groundwater flow model, spe-
cific yield was estimated to be 0.01 in the over-
burden and 0.005 in the highly fractured rocks
within which most wells were screened. Moore
(
1992
) compared stream-flow hydrographs with
slopes of groundwater recession curves for
shale and limestone aquifers in Tennessee and
estimated a specific yield of 0.002. In spite of
the efforts that have been devoted to evaluating
S
y
in fractured-rock systems, it remains unclear
whether estimates of
S
y
generated for these sys-
tems are of sufficient accuracy to permit their
use in estimating recharge.
Example: Beaverdam Creek Basin
Rasmussen and Andreasen (
1959
) studied the
water budget of the Beaverdam Creek water-
shed on the Delmarva Peninsula of Maryland.
