CEMP-E
TI 814-01
3 August 1998
where:
r1
=
distance to nearest observation well in feet
r2
=
distance to farthest observation well
h1
=
saturated thickness in feet at nearest observation well
h2
=
saturated thickness at farthest observation well
For artesian conditions, again, as shown in figure 5-4, the formula becomes:
528 Q log (r2/r1)
K
=
_______________
b (h22 - h12)
(eq. 5-4)
where:
h1
=
head in feet at nearest observation well
h2
=
head at farthest observation well
d. Aquifer Testing. Where existing wells or other data are insufficient to determine aquifer
characteristics, a pumping test may be necessary to establish values used for design. Testing
consists of pumping from one well and noting the change in water table at other wells as
indicated in figures 5-3 and 5-4. Observation wells are generally set at 15 to 150 m (50-500 ft)
from a pumped well, although for artesian aquifers they may be placed at distances up to 300
m (1000 ft). A greater number of wells allows the slope of the drawdown curve to be more
accurately determined. The most common methods of aquifer testing are:
- Step Drawdown Method. Involves pumping one well and observing what happens in
observation wells. The well is pumped at slow constant rate until the water level stabilizes. It
is then pumped at a higher rate until the water level again stabilizes. At least three steps are
normally performed.
- Recovery Method. Involves shutting down the pumping well and noting the recovery
water levels in the pumping well and its observation wells.
- Slug Test. Involves the introduction or removal of a "slug" or volume of water into the
well then measuring the rise or fall in water level. The test can also be performed by inserting
and removing a solid cylinder into the water.
- Bailer Test. Water is removed from the well using a bailer of known volume, as rapidly
as possibly until the well is empty or the water level stabilizes. The volume and unit of time are
noted.
e. Testing Objectives. A simplified example is given in appendix B. When conducting
aquifer tests by methods such as the drawdown method, it is important to note accurately the
yield and corresponding drawdown. A good testing program, conducted by an experienced
geologists, will account for, or help to define, the following aquifer characteristics:
5-9
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