The Chalk and overlying Tertiary sands of the London Basin comprise the aquifer system of a classical artesian basin, confined and underlain by the London Clay and the marls of the Lower Chalk, respectively. The piezometric surface was at or near ground level until extensive over-abstraction dewatered the aquifer system. Experimental artificial recharge has been carried out in the Lee Valley at three different periods since the 1890's, the most recent experiments leading directly to a full-scale operational recharge scheme.
Groundwater quality throughout the area shows considerable natural variation, closely related to the geology. The major dewatering and more recent refilling of the aquifer have led to various regional and local changes in groundwater quality, and these are discussed in relation to the hydrogeology of the system. The experimental and prototype recharge schemes have shown that large-scale, artificial recharge of this multiple aquifer system is a viable proposition; effects on groundwater quality are generally negligible except locally at two sites, where slight problems have been encountered, but these may only be temporary.