Environmental Engineering Reference
In-Depth Information
Table 6.4 Monthly groundwater recharge in
millimeters for Beaverdam Creek Basin, Maryland,
for April 1950 to March 1952 (Rasmussen and
Andreasen,
1959
).
area of 152 km
2
and has an average thickness
of 18 m. Average depth to the water table is
about 2 m. The aquifer is recharged by rainfall;
discharge is to local surface-water bodies and
to regional pumping centers. Six observation
wells were instrumented with pressure trans-
ducers for monitoring water levels and rain-
fall gauges. Readings were made at 5-minute
intervals over a period of 3 years beginning in
August, 2000.
An automated time-series procedure was
used to estimate recharge from the water-level
and rainfall data. Exact details of the proced-
ure are given in Crosbie
et al
. (
2005
); a gen-
eral description is presented here. The rate of
water-level recession at each well was deter-
mined by analyzing the record for water-level
declines in the absence of rainfall. The reces-
sion rate was determined to be a linear func-
tion of water-table height, with higher rates
for higher water-table heights. The procedure
checks that rainfall precedes a water-table rise;
if a rise occurs when there is no rainfall, the
rise is not included in the recharge calcula-
tion. It also checks for instances of water levels
being influenced by entrapped air by analyzing
the rate of water-level decline after each rise.
If that decline is too rapid, the height of the
water-table rise is adjusted to remove the effect
of entrapped air.
Three methods were used to calculate
specific yield: Equation (
6.4
) with
S
r
set to
residual water content as determined from
laboratory measurements, analysis of aquifer
pumping tests, and a water-budget approach
(Equation (
6.13
)). This third option was deter-
mined to be the most appropriate. For calcu-
lating recharge, this value of
S
y
was adjusted
to account for depth to the water table.
Figure
6.10
shows water levels measured in one obser-
vation well along with calculated values of
S
y
and estimated recharge rates. The procedure
in this study is attractive because it eliminates
much of the subjectivity in applying the water-
table fluctuation method, although estimated
recharge may be sensitive to parameters for
assessing the effects of entrapped air and
the maximum time lag between rainfall and
water-table rise.
1950
1951
1952
January
27
82
February
42
54
March
44
107
April
23
15
May
50
55
June
7
74
July
37
49
August
7
27
September
30
10
October
0
23
November
50
119
December
72
79
was 1.05 m; average annual recharge was
estimated at 0.54 m.
Base flow,
Q
bf
, change in storage in the satu-
rated zone, Δ
S
gw
, and evapotranspiration from
groundwater,
ET
gw
, were quantified by using a
modified form of Equation (
6.1
):
∆= ++
(6.15)
R
S
gw
Q
bf
ET
gw
Δ
S
gw
was calculated as
S
y
Δ
H
n
/ Δ
t
, where Δ
H
n
is the net change in head over each month
(i.e. the difference in head between the end
and the beginning of the month). Base flow
was determined by streamflow hydrograph
separation, and
ET
gw
was calculated as the
residual of Equation (
6.15
). Average annual
base flow was estimated to be 0.27 m; annual
estimates of Δ
S
gw
and
ET
gw
were 0.02 and 0.25,
respectively.
Example: Tomago sand beds
Crosbie
et al
. (
2005
) applied the water-table fluc-
tuation method to estimate recharge to the
Tomago sand beds, an unconfined aquifer con-
sisting of unconsolidated aeolian deposits of
fine sands, near Newcastle, Australia. The aqui-
fer, which supplies about 25% of the potable
water used in the Newcastle region, covers an
