The Lower Greensand beneath Slough has provided a constant and reliable supply of high quality water to local industry since 1909. The source of this water has always been uncertain and it is thought that it is either derived from storage or recharged from the outcrops. An alternative explanation is presented that assumes downward leakage from the Chalk through the Upper Greensand and Gault, together with recharge occurring under the Chilterns to the north of Slough. The clay minerals in the Gault have removed the calcium ions, thus softening the water. The mineral glauconite in the Lower Greensand and other factors may also affect the hydrochemistry. The structure of the strata beneath Slough is such that there is a corridor of more transmissive Lower Greensand abutting the London Platform. This has enabled flow to occur from north to south since the Pleistocene so that the saline connate groundwater has been largely flushed out and replaced by water of high quality.