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

wells contain slotted pipe screens. Irrigation wells range
in diameter from 8 to 16 inches and usually have pack
thicknesses of 6 to 8 inches. Lengths of slotted pipe
screens range from 10 to 40 feet.
Service Life of Wells and Collector Wells
One of the problems in the East St. Louis area associated
with the development of ground-water resources
is the short life expectancy of wells. According to a
study by Bruin and Smith (1953), the median service
life of municipal wells terminating in sand and gravel
formations in the East St. Louis area is about half that
for similar municipal wells in other parts of the state.
Nearly all of the wells retired in the area were taken
out of service either because the screens had become partially
clogged or the wells had filled with sand.
The results of mechanical analyses presented by
Bergstrom and Walker (1956) are shown in figures 26
through 28. According to Bergstrom and Walker the
analyses must be accepted with caution because the conditions
of collecting most of the samples are not known,
and because of the highly variable nature of the valleyfill
deposits in the area. A careful examination of the
mechanical analysis curves suggests that the valley-fill
deposits contain a rather high percentage of fine materials
which could, under heavy pumping conditions,
migrate toward a screen and partially clog the well wall
and screen openings. As indicated by data in the files of
industries and municipalities, specific capacities of existing
production wells decrease markedly after a few years
and in some cases after a few months of operation.
Specific capacities are generally determined by the driller
after completion of the well by pumping the well at
different rates for short periods of time, generally less
than 24 hours, and by frequently measuring drawdowns
in the pumped well. This method of measuring specific
capacity is continued by industrial and municipal personnel
periodically.
It is a general practice of industries and municipalities
to place a well in operation and pump it at high rates,
often about 1000 gpm. As the result of heavy pumping,
fine materials migrate towards the well and partially
clog screen openings and the voids of the formation surrounding
the well. The well-loss constant increases rapidly
and, because well loss varies as the square of the discharge
rate, drawdown increases rapidly. The relation
between well-loss constant and drawdown due to well
loss is shown in figure 29. As drawdown increases the specific
capacity and, therefore, the yield of the well decreases.
Typical decreases in specific capacity due to
increases in the well-loss constant are given in table 19.
Theoretical specific capacities of wells with a nominal
radius of 15 inches and with 40 feet of screen given in
table 19 were determined for values of the coefficient of
transmissibility ranging from 100,000 to 300,000 gpd/ft,
Figure 26.
Mechanical analyses of samples from wells
27