|
Discussion |
Department of Geology, The University of Alberta, , Edmonton, Alberta T6G 2E3, Canada.
Dr Clark has presented an excellent review of the theory of the step drawdown test and its applications. The purpose of this communication is to comment on the use of the recovery portion of a step drawdown test to calculate aquifer transmissivity. The Theis method may be applied if the variable discharge rate is replaced by the average of the pumping rates. This average may be expressed as:
where 
= mean pumping rate, V = total volume pumped, t = total pumping time.
Equation (1) is equivalent to Clark's formulation. Although it is a useful working approximation, more accurate results would be obtained if were calculated by weighting each discharge rate by the time elapsed since that rate began. Sternberg (1967) has described such a method which can be modified slightly to determine transmissivity from the recovery portion of a step drawdown test. The transmissivity T, of an infinite, confined aquifer is shown to be:
where te = 2{t-(
10Q(
) d /V)}, = time from start of pumping to time t, 
= time since pumping stopped, Qe = V/t,e, Q() = variable pumping rate, sW' = residual drawdown in the pumped well (replacing the residual drawdown at any point radially distant r from the well in Sternberg's derivation).
For short step drawdown tests with steps of equal length, tc may be approximated by: where n = number of steps, 
t = length of each step, te = time intervals since pumping started, Qti+ti+1 =
...
This 250-word extract was created in the absence of an abstract.
Related articles in Quarterly Journal of Engineering Geology and Hydrogeology:
