The Chalk Group forms the most important aquifer unit within the Thames Basin, supplying water for drinking water public consumption and supporting river flows within chalk bournes characteristic of the Berkshire and Marlborough Downs, the Chilterns and North Downs.
The Chalk is best described as a dual porosity medium with groundwater flow occurring within both the matrix and through fractures. Most of the aquifer's storage is derived from secondary porosity within these fractures.
The mixing of fracture groundwater with pore water in the aquifer matrix exerts an important control on Chalk hydrochemistry and the fate of pollutants such as nitrate.
Most fractures within the Chalk occur parallel to the bedding plane within the top 60 metres with a corresponding decrease in permeability with depth. A strong topographical control on transmissivity is also evident with high transmissivity values occurring within valleys decreasing towards the interfluves.
Where unconfined the Chalk is drained by intermittent streams within incised peri-glacial valleys.
Groundwater baseflow contributions are high (mean Baseflow Index or BFI 0.95) and spring lines are well developed at the intersection of hardgrounds such as the Tottenhoe Stone and at the base of the Chalk sequence.
The high fracture permeability and low matrix porosity give rise to comparatively large seasonal water table variations and associated stream-head migration up dry valleys.
The Chalk aquifer is folded through the London Basin syncline and becomes confined by Palaeogene deposits.
The Palaeogene deposits promote acidic runoff at Chalk margins causing development of solution-enhanced karstic features with groundwater velocities up to 5 km per day recorded.
The Chalk provides a significant groundwater resource in the London Basin and considerable effort has been made in recent years to ensure the groundwater levels within the basin remain stable.
|Location||No. of localities||No. of tests||Median||25th centile||75th centile|
|Location||No. of tests||Median||25th centile||75th centile|
|Location||No. of tests||Mean||Min||Max||Median|
|Thames and Chilterns (Middle Chalk)||356||31.4||9.5||52.6||31.8|
|Thames and Chilterns (Lower Chalk)||158||26.6||11.6||39.5||27|
|Location||No. of localities||Mean||Min||Max|
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