Properties of the Chalk aquifer in the Thames Basin

Map showing the distribution of Chalk outcrop in the Thames Basin

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.

The Chalk at outcrop

Chalk outcrop overlain by palaeogene deposits

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 confined Chalk

White Horse, Westbury. Looking N. Close-up. Ledge due to Melbourn Rock (base on Middle Chalk) on left of photo.

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.

Chalk groundwater levels

Groundwater level contours for the Chalk aquifer of the Thames Basin

Hydraulic properties of the Chalk aquifer

Transmissivity data (m2⁄ day)

Location No. of localities No. of tests Median 25th centile 75th centile
Kennet Valley 74 117 830 380 1500
Chilterns 44 62 860 276 2100
Thames 81 88 230 44 990
North Downs 41 57 670 350 1600

Storage coefficient data

Location No. of tests Median 25th centile 75th centile
Kennet Valley 107 0.0075 0.004 0.017
Chilterns 44 0.0029 0.0008 0.028
Thames 41 0.0024 0.0004 0.0047
North Downs 35 0.0036 0.001 0.015

Porosity data (%)

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

Hydraulic conductivity data (m/day)

Location No. of localities Mean Min Max
Thames Basin 18 45.4 2.0 192.7

Papers about the Chalk aquifer in the Thames Basin

Solution-enhanced fracture

BGS projects about the Chalk aquifer

Contact

Contact Stephanie Bricker for further information