EP 1110-1-27
27 Jan 00
(2) The utility of data derived from the step-drawdown test is in the ability to:
Determine characteristics about both the well and the aquifer simultaneously (aquifer and
well loss).
Extrapolate or interpolate the performance of the well at various discharge rates, using
measured data points as a reference.
Determine the operating characteristics of the well pump used.
(3) If performed immediately after a well is constructed, the step-drawdown test provides an
estimate of the efficiency of the well and effectiveness of the well development phase of the well
construction, and the baseline well and pump performance for comparison in the future. First checks of a
well design's criteria or assumptions can also be made and adjusted as needed. It is highly recommended
that all of these wells be pre-developed immediately after the well screen and filter pack are installed.
This procedure gives a far greater chance of removing both drilling fluid solids and natural fines, and of
replacing any of the filter pack that subsides due to consolidation, etc.
(4) Note that well loss does not increase linearly to the discharge rate; therefore, well efficiency
and specific capacity are not constant and decline with increasing discharge rate. This relationship makes
comparing well performance data through time and various discharge rates difficult without normalizing
the data to the same discharge rate. The equation describing well and aquifer loss to interpolate or
extrapolate pumping water levels should be used as needed. The equation can also be used to estimate
specific capacity and efficiency at the intended discharge rate. Changes in well performance will then be
apparent.
(5) For step-test data to be useful in calculating well, pump, and aquifer performance parameters:
Data must be accurately gathered, with data collected at standard intervals of decreasing
frequency as recommended (Helweg, Scott, and Noble (1983), and Driscoll, 1986).
Each step must be of a sufficient length of time for either the water level decline to stabilize
or the decline trend to be established on a semi-log plot of drawdown versus time (but does
not have to be long).
The effects of interference (such as other wells turning on and off) must be factored into the
analysis.
(6) HTRW sites may impose restrictions on optimal step testing methodology. For example, a
five-step test with pressure measurement is recommended to determine pump wear. However, pumping
contaminated ground water requires collection of the fluid. Perfecting the gathering of pump wear data
from a three-step test, and learning to extrapolate from short steps may be a necessary compromise in
methodology.
d. Constant rate and slug tests. Constant rate pumping tests and slug tests (in which an
instantaneous charge of water or a solid object is introduced into a well) are employed predominantly to
determine aquifer characteristics, that is, transmissivity, hydraulic conductivity, and storage coefficient.
Their utility in well maintenance and rehabilitation is less direct than with step-drawdown tests, but data
derived from these tests can be used in preliminary calculations of expected well hydraulic parameters.
(1) Constant-rate pumping tests.
(a) With knowledge of aquifer characteristics, the theoretical drawdown in the aquifer at the well
screen for a given discharge rate can be calculated and compared with the observed drawdown at the
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