Discussion. Operation of a Recharge Borehole.

Abstract

The Paper itself is of limited, rather specialized interest, but much interest is being shown in the subject of artificial recharge generally. The Paper is therefore presented as a vehicle for discussion on current prospects for the use of artificial recharge particularly in Britain, and the practical and economic problems envisaged.93. In connexion with the work described at the Birmingham Racecourse some interesting facts have been determined since the Paper was written.94. Contrary to expectation, the rest level in the borehole has risen steadily some 40 ft over the last six years. This appears to be partly the result of progressively reduced annual abstraction in this area of the Birmingham aquifer and partly the result of increased winter rainfall in the latter half of the period.95. A continuous recording of rest water level in the borehole over a period of four weeks has recently been obtained. This showed that intermittent pumping operations in neighbouring boreholes causes regular changes of rest water level amounting to several feet at the racecourse borehole. Such changes would explain irregularities in the recharge performance curves, for example between points C and G and the sharp dip at point D in Fig. 5. The recording also showed that there is a definite correlation between changes of rest water level and barometric pressure. Barometric efficiency, as defined by J a~o b ,~ works out to be very close to unity, which implies that the storage coefficient of the aquifer derives mostly from elasticity of the water and very little from elasticity of the sandstone. On this basis the storage coefficient of the aquifer should be of the order of 1 X 10-4, which supports the kind of explanation offered in Q 44.96. Some investigation of the porosity and permeability of the sandstone has also provided useful information. Analysis of the pumping test referred to in 5 4 together with the results obtained in $0 31 and 42, show that the permeability increases with depth in the borehole from 7 X 10-' to 2 X 10-5 ft/s. Laboratory tests of small samples of sandstone from the borehole gave results in this range, which tends to confirm that water flows through pores in the aquifer rather than along fissures. On this basis the effective diameter of the pore passages can be estimated to range from about 5 microns to about 25 microns. These dimensions are of critical importance in the mechanism of plugging of the borehole by solids in the recharge water.97. Laboratory tests of the porosity of small samples of sandstone from the borehole gave a minimum value of 0.142. From this value it can be estimated that the water recharged during operation of the scheme never travelled further than about 100 ft from the borehole.In light of this, 5 62 needs qualification. While 100 ft is hardly 'aconsiderable distance from the borehole' in relation to the nearest neighbouring borehole, it is still very considerable in relation to the diameter of the pore passages of the aquifer. M r A. Hunter Blair, Water Research AssociationThe stan...