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

city of Wood River is given in figure 30. The specific capacity
of the collector well declined from a peak of 270
gpm/ft in August 1954 to about 50 gpm/ft in March 1963.
A part of the decline in specific capacity can be attributed
to the partial clogging of the laterals by incrustation
and with sand and silt. Mechanical cleaning of one
lateral in June 1962 increased the specific capacity from
about 50 gpm/ft to 55 gpm/ft.
Figure 29.
Relation between well-loss constant and
drawdown due to well loss
and sometimes impossible to restore the original capacity
if the well-loss constant is greater than 40 sec 2 /
ft 5 .
Periodic well treatment by acidizing or other methods
has been used successfully to rehabilitate old wells.
However, in many cases wells are abandoned as their
yields decrease and new wells are drilled nearby.
Based on data for production wells which have been
in service a number of years, the average specific capacity
of wells in the East St. Louis area is about 30 gpm/ft.
An average well yield of 450 gpm can be obtained with
a long service life if sufficient screen is provided.
A graph showing the decrease of specific capacity of
a collector well owned by the Shell Oil Refinery near the
Table 19. Theoretical Decreases in Specific Capacity
Due to Increases in Well-Loss Constant
Coefficient
of
transmissibility
(gpd/it)
Pumping
rate
(gpm)
Well-loss
coefficient
of 1 sec 2 /ft 5
Speci-
Draw- fic capadown*
city*
(ft) (gpm/ft)
Well-loss
coefficient
of 5 sec 2 /ft 5
Well-loss
coefficient
of 10 sec 2 /ft 5
Specific
capa-
city*
(gpm/ft)
Drawdown*
(ft)
Specific
Draw- capadown*
city*
(ft) (gpm/ft)
300,000 900 9.3 96.9 25.3 35.6 45.3 19.9
250,000 900 10.3 87.4 26.3 34.2 46.3 19.4
200,000 900 11.9 75.6 27.9 32.2 47.9 18.8
150,000 900 14.4 62.5 30.4 28.6 50.4 17.9
100,000 900 19.7 45.7 35.7 25.2 55.7 16.1
300,000 450 3.7 122.2 7.7 58.4 12.7 35.4
250,000 450 4.2 110.7 8.2 54.9 13.2 34.1
200,000 450 4.9 91.9 8.9 50.6 13.9 32.4
150,000 450 6.1 73.8 10.1 44.5 15.1 29.8
100,000 450 8.4 53.6 12.4 36.3 17.4 25.9
*Theoretical
Figure 30.
Well Design Criteria
Specific-capacity data for collector well,
1954 to March 1963
Walton (1962) gave criteria for well design in unconsolidated
formations in Illinois. Screen design criteria
are applicable to industrial, municipal, and irrigation
wells. The objective is to design an efficient and economical
well with a service life of at least 10 years.
According to Ahrens (1957) artificial pack wells are
usually justified when the aquifer is homogeneous, has a
uniformity coefficient less than 3.0, and/or has an effective
grain size less than 0.01 inch. The uniformity coefficient,
C u , is the ratio of the sieve size that will retain
40 percent of the aquifer materials to the effective size.
The sieve size that retains 90 percent of the aquifer materials
is the effective size. In addition, an artificial pack
is sometimes needed to stabilize well-graded aquifers
having a large percentage of fines in order to avoid excessive
settlement of materials above the screen or to
permit the use of larger screen slots. The uniformity coefficients
based on mechanical analyses of samples in
figures 26 through 28 are less than 3 and/or the effective
grain size is less than 0.01 inch, indicating that an
artificial pack well should be constructed at each site.
Selection of the artificial pack is based on the mechanical
analysis of the aquifer. A criterion that has been
successfully used in Illinois is that the ratio of the 50
percent sizes of the pack and the aquifer (the P-A ratio)
be 5 (Smith, 1954). Artificial packs should range
in thickness from 6 to 9 inches (Walton, 1962).
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