Ground-water development in East St. Louis area, Illinois. Urbana, IL ...

indicate that stabilization of the piezometric surface during
1956 was mostly due to the effects of the Mississippi
River. During much of the latter part of the drought
there were long periods when little water was in the
small streams and lakes in the interior portion of the
East St. Louis area, and these hydrologic features had
for practical purposes negligible influence on water levels.
Computations made with equation 4, taking into consideration
the Mississippi River (recharge boundary)
and accumulated periods of little or no recharge directly
from precipitation, indicate that the piezometric surface
for 1956 can be duplicated by using a time period of
5 years in estimating water-level declines.
Production wells were grouped into centers of pumping,
and the average discharges during the period 1952-
1956 for each pumping center were determined. The analog
model was coupled to the excitation-response apparatus
and the pulse generator was connected to junctions
at locations of pumping centers. The output of the
pulse generator was adjusted in accordance with discharge
data and a maximum time period of 5 years. The oscilloscope
was connected to terminals representing observation
wells and water-level declines were computed. Thus,
water-level declines everywhere in the aquifer were described.
The total water-level decline, based on the superposition
theorem, at each terminal was obtained by summation
of individual effects of each pumping center.
Only the effects of pumping centers were taken into account
and the average stage of the Mississippi River was
assumed to be the same in 1956 as it was in 1900. However,
records show that the average stage of the Mississippi
River was about 11 feet lower in 1956 than in 1900.
The effect of the change in the average stage of the
river on water levels was estimated by coupling the,
pulse generator to junctions in the analog model along
the river and measuring water-level changes due to the
given change of the stage of the river with the oscilloscope
connected to junctions in the interior portions of
the analog model.
The above water-level declines due to the decline in
river stage were superposed upon water-level changes
due to pumpage, and a water-level change map covering
the period 1900 to December 1956 was prepared. A piezometric
surface map (figure 61B) was constructed by
superposing the water-level change map on the piezometric
surface map for 1900.
Features of the piezometric surface map prepared
with data from the analog computer and the piezometric
surface map prepared from actual water-level data are
generally the same, as shown in figure 61. A comparison
of water-level elevations for selected pumping centers,
based on the analog computer and actual piezometric
surface maps, are given in table 31. The average slope of
Table 31. Comparison of Analog Computer and Actual
Piezometric Surface Maps for December 1956
Water-level elevation
(ft above msl)
Pumping
center Analog computer Actual
Alton area 375 375
Wood River area 375 375
Granite City area 345 350
National City area 365 365
Monsanto area 360 355
Caseyville area 400 400
the piezometric surface in areas remote from pumping
centers from both maps was 5 feet per mile. A comparison
of gradients from analog computer and actual piezometric
surface maps in the vicinity of pumping centers
is given in table 32.
Table 32. Comparison of Analog Computer and Actual
Hydraulic Gradients of Piezometric Surface Maps
for December 1956
Average gradient (ft/mi)
Pumping
center Analog computer Actual
Alton area 15 15
Wood River area 15 15
Granite City area 20 30
National City area 10 10
Monsanto area 20 25
Differences in analog computer and actual piezometric
surface maps are not significant when considered in relation
to the accuracy and adequacy of geohydrologic
data. The close agreement between analog computer and
actual piezometric maps indicates that the analog computer
may be used to predict with reasonable accuracy
the effects of future ground-water development and the
practical sustained yield of existing pumping centers.
PRACTICAL SUSTAINED YIELDS OF EXISTING PUMPING CENTERS
In 1962 water levels were not at critical stages in
any pumping center and there were areas of the aquifer
unaffected by pumping. Thus, the practical sustained
yield of existing pumping centers exceeds total withdrawals
in 1962. The practical sustained yield is here de-
fined as the rate at which ground water can be continuously
withdrawn from wells in existing pumping centers
without lowering water levels to critical stages or
exceeding recharge. Ground water withdrawn from wells
less than 1 mile from the river was not considered.
56