Abstract

I have received letters from Mr D. M. Milne and Dr P. F. Worthington pointing out an error in the application of the analytical methods in this paper, (published in 1977 in Q. Jl Engng Geol. 12, 125–143). I would therefore like to take this opportunity to clarify the matter.

The source of the error lies in the use of mixed time-units in the evaluation of the storage coefficient S. Time is given in minutes while the discharge rates are given in m3/day throughout the paper. These units, if applied consistently throughout an analysis, will give correct values for the various well-loss equations and the aquifer transmissivity. The value of the storage coefficient, however, will be 1440 times too great.

The storage coefficient is obtained (p. 139) by the evaluation of the equation:

The discharge rates used in obtaining a and T in this equation are expressed in m3/day. However, a is derived through Figs. 4 and 5 using the specific drawdown at unit time, t = 1 minute, in order to remove the expression (b log t) from (a + b log t) (p. 132). The units in this equation above have to be made consis- tent with this derivation and the equation therefore becomes:

where 1440 = number of minutes in one day.

This procedure was carried out in the original paper in order to get the storage coefficient in Table 2 but was not adequately explained in the text.

It is possible to remove the source of

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