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Discussion |
Thames Conservâncy Division, Thames Water Authority, , Nugent House, Reading.
May I congratulate Lewis Clark on his paper covering the analysis and planning of step drawdown tests. I think the subject of well performance is due for a ‘shake-up’ and this paper will hopefully initiate some useful scientific discussion in the right direction.
I have two main criticisms to make. The first one is that the author considers only continuous step drawdown tests; he makes no mention of conducting discontinuous or intermittent step drawdown tests in which each drawdown step is followed by a recovery period of at least equal duration. I have used both types of test and have found the latter provides the more reliable results for the following reasons:
(i) the problem of errors caused by fluctuations in pumping rate within a given step affecting the reliability of data from subsequent steps does not arise;
(ii) the problem of errors caused by extrapolating the time-drawdown trend in order to calculate incremental drawdown does not arise;
(iii) the analysis of transmissivity from the time-drawdown data is straightforward; each drawdown step is treated simply as a short constant rate test, and since errors caused by (i) and (ii) will not exist, the values thus obtained will be more reliable;
(iv) there will be as many recovery steps to analyse as there are drawdown steps, and hence more ‘stabs’ at transmissivity. The recovery ‘Q’ values are the same as the preceding drawdown step value; the problem of estimating an averaged or weighted effective ‘Q’ for the recovery test does not
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