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The Upper Cretaceous Chalk of southern England is the UK’s most important aquifer, providing more than 75% of the public supply for southeast England, including London. The aquifer also sustains rivers and wetlands, and their associated groundwater dependent ecosystems. However, the aquifer is facing a multitude of threats including over-abstraction, nitrate pollution, and climate change.
The Upper Cretaceous Chalk of southern England is the UK’s most important aquifer, providing more than 75% of the public supply for southeast England, including London. The aquifer also sustains rivers and wetlands, and their associated groundwater dependent ecosystems. However, the aquifer is facing a multitude of threats including over-abstraction, nitrate pollution, and climate change.


The Chalk is a complex aquifer in which groundwater flow is through the matrix, fractures and karstic dissolutional voids. The Chalk matrix has a porosity of around 35% (Bloomfield et al., 1995). The matrix is thought to provide an important contribution to storage, although the size of the pore throats is very small, and therefore the permeability is very low (Price et al., 1993). The average permeability of 977 core samples was only 6.3 x 10-4 m/day (Allen et al., 1997). The matrix is particularly important in solute transport, because solutes move between the matrix and the more permeable parts of the aquifer via diffusion (Foster 1975). The unmodified fracture network provides an important contribution to storage and flow, and has a hydraulic conductivity of about 0.1 m/d, and a transmissivity of about 20 m2/day (Price, 1987). However, it is the dissolutionally enlarged fissures and conduits that make the Chalk such a good aquifer. The median transmissivity from 2100 pumping tests is 540 m2/day, and the 25th and 75th percentiles are 190 and 1500 m2/day respectively (MacDonald and Allen, 2001). Borehole packer testing, logging and imaging have shown that most of this transmissivity comes from a small number of dissolutional voids (e.g. Tate et al., 1970; Schurch and Buckley, 2002). Laterally extensive lithostratigraphical horizons including marl seams, bedding planes, sheet and tabular flint bands, and hard-grounds have an important influence on these groundwater flows. They are all horizons where downward percolation of water may be impeded. Dissolution often occurs where flow is concentrated along these horizons, creating conduits or fissures, especially where they are intersected by joint sets.
The Chalk is a complex aquifer in which groundwater flow is through the matrix, fractures and karstic dissolutional voids. The Chalk matrix has a porosity of around 35% (Bloomfield et al., 1995<ref name="Bloomfield">BLOOMFIELD, J P, BREWERTON, L J and ALLEN, D J. 1995. Regional trends in matrix porosity and dry density of the Chalk of England. ''Quarterly Journal of Engineering Geology ''28, S131–S142 </ref>). The matrix is thought to provide an important contribution to storage, although the size of the pore throats is very small, and therefore the permeability is very low (Price et al., 1993<ref name="Price 1993">PRICE, M, DOWNING, R A and EDMONDS, W M. 1993. The Chalk as an aquifer. In The hydrogeology of the Chalk of North- West Europe. Edited by Downing, R A, Price, M and Jones, G P. 35–59 </ref>). The average permeability of 977 core samples was only 6.3 x 10-4 m/day (Allen et al., 1997<ref name="Allen">ALLEN, D J, BREWERTON, L J, COLEBY, L M, GIBBS, B R, LEWIS, M A, MACDONALD, A M, WAGSTAFF, S J and WILLIAMS, A T. 1997. The physical properties of the major aquifers in England and Wales. BGS Technical Report WD/97/34, Environment Agency R&D Publication 8. 312 pp Atkinson and Smith, 1974. </ref>). The matrix is particularly important in solute transport, because solutes move between the matrix and the more permeable parts of the aquifer via diffusion (Foster 1975<ref name="Foster">FOSTER, S S D. 1975. The Chalk groundwater tritium anomaly — a possible explanation. ''Journal of Hydrology'' 25, 159–165.</ref>). The unmodified fracture network provides an important contribution to storage and flow, and has a hydraulic conductivity of about 0.1 m/d, and a transmissivity of about 20 m2/day (Price, 1987<ref name="Price 1987">PRICE, M. 1987. Fluid flow in the Chalk of England. In Goff, J C & Williams, B P J. (eds), Fluid Flow in Sedimentary Basins and Aquifers. ''Geological Society Special Publication'' 34, 141–156. </ref>). However, it is the dissolutionally enlarged fissures and conduits that make the Chalk such a good aquifer. The median transmissivity from 2100 pumping tests is 540 m<sup>2</sup>/day, and the 25th and 75th percentiles are 190 and 1500 m<sup>2</sup>/day respectively (MacDonald and Allen, 2001<ref name="MacDonald">MACDONALD, A M and ALLEN, D J. 2001. Aquifer properties of the Chalk of England. ''Quarterly Journal of Engineering Geology ''34, 371–384 </ref>). Borehole packer testing, logging and imaging have shown that most of this transmissivity comes from a small number of dissolutional voids (e.g. Tate et al., 1970<ref name="Tate">TATE, T K, ROBERTSON, A S and GRAY, D A. 1970. The hydrogeological investigation of fissure flow by borehole logging techniques. ''Quarterly Journal of Engineering Geology ''2, 195–215 </ref>; Schurch and Buckley, 2002<ref name="Schurch">SCHÜRCH, M and BUCKLEY, D. 2002. Integrating geophysical and hydrochemical borehole log measurements to characterise the Chalk aquifer, Berkshire, United Kingdom. ''Hydrogeology Journal ''10 (6), 610–627 </ref>). Laterally extensive lithostratigraphical horizons including marl seams, bedding planes, sheet and tabular flint bands, and hard-grounds have an important influence on these groundwater flows. They are all horizons where downward percolation of water may be impeded. Dissolution often occurs where flow is concentrated along these horizons, creating conduits or fissures, especially where they are intersected by joint sets.


Across much of the UK, the Chalk has not traditionally been considered or managed as a karst aquifer, perhaps because caves are rare. However, it has been recognised for some time that karst processes and conduit flow may be common in many areas of the Chalk aquifer. Although not renowned for its caves, the Chalk does contain some significant cave systems. The largest chalk cave in Britain is Beachy Head Cave, near Eastbourne which is approximately 400 m long. Caves are more common in northern France where some are several kilometres long. As in more classical karst regions, surface karst features such as sinking streams, springs, dolines and dry valleys can be very common in the Chalk (Cooper et al., 2011). There are also many fossil sediment filled karst features associated with the Chalk-Palaeogene unconformity when the climate was warmer and wetter (Newell, 2014).
Across much of the UK, the Chalk has not traditionally been considered or managed as a karst aquifer, perhaps because caves are rare. However, it has been recognised for some time that karst processes and conduit flow may be common in many areas of the Chalk aquifer. Although not renowned for its caves, the Chalk does contain some significant cave systems. The largest chalk cave in Britain is Beachy Head Cave, near Eastbourne which is approximately 400 m long. Caves are more common in northern France where some are several kilometres long. As in more classical karst regions, surface karst features such as sinking streams, springs, dolines and dry valleys can be very common in the Chalk (Cooper et al., 2011<ref name="Cooper">COOPER, A H, FARRANT, A R, and PRICE, S J. 2011. The use of karst geomorphology for planning, hazard avoidance and development in Great Britain. ''Geomorphology ''134, 118–131. </ref>). There are also many fossil sediment filled karst features associated with the Chalk-Palaeogene unconformity when the climate was warmer and wetter (Newell, 2014<ref name="Newell">NEWELL, A J, 2014. Palaeogene rivers of southern Britain: climate extremes, marine influence and compressional techtonics on the southern margin of the North Sea Basin. ''Proceedings of the Geologists' Association'', 125 (5–6). 578–590. </ref>).


Locally the Chalk may host a greater density of surface karst features than other more classic karst aquifers such as the Carboniferous limestones of Derbyshire, yet it is rarely treated as a karstic aquifer. There is a rather simplistic tendency to equate karstic groundwater flow with the occurrence of extensive cave systems. Yet subsurface dissolution features are frequently encountered in the Chalk during construction projects (Edmonds, 2008), and tracer testing has demonstrated rapid groundwater flow over distances of up to 20 km (e.g. Harold, 1937; Atkinson and Smith, 1974). Even where there is little evidence of surface karst, the Chalk often has high transmissivity due to the solutional enlargement of fractures to form fissures and conduits. Given the evidence for karstic features, rapid groundwater flow, and considerable spatial heterogeneity, the Chalk is perhaps better described as a weakly cavernous karst aquifer rather than weakly- or non-karstic.
Locally the Chalk may host a greater density of surface karst features than other more classic karst aquifers such as the Carboniferous limestones of Derbyshire, yet it is rarely treated as a karstic aquifer. There is a rather simplistic tendency to equate karstic groundwater flow with the occurrence of extensive cave systems. Yet subsurface dissolution features are frequently encountered in the Chalk during construction projects (Edmonds, 2008<ref name="Edmonds">EDMONDS, C N. 2008. Karst and mining geohazards with particular reference to the Chalk outcrop, England. ''Quarterly Journal of Engineering Geology and Hydrogeology'', 41, 261–274. </ref>), and tracer testing has demonstrated rapid groundwater flow over distances of up to 20 km (e.g. Harold, 1937<ref name="Harold">HAROLD C. 1937. The flow and bacteriology of underground water in the Lee Valley. ''Metropolitan Water Board 32nd Annual Report'', 89–99. </ref>; Atkinson and Smith, 1974<ref>ATKINSON, T C and SMITH D I. 1974. Rapid groundwater flow in fissures in the Chalk: An example from South Hampshire. ''Quarterly Journal of Engineering Geology ''7, 197–205 </ref>). Even where there is little evidence of surface karst, the Chalk often has high transmissivity due to the solutional enlargement of fractures to form fissures and conduits. Given the evidence for karstic features, rapid groundwater flow, and considerable spatial heterogeneity, the Chalk is perhaps better described as a weakly cavernous karst aquifer rather than weakly- or non-karstic.


The Pang and Lambourn catchments in Berkshire are good examples of chalk catchments with a high density of surface karst features (Figure 1). These catchments provide an excellent  location in which to discuss the extent to which Chalk can be viewed as a karstic aquifer, and the importance of such features to water resource management.
The Pang and Lambourn catchments in Berkshire are good examples of chalk catchments with a high density of surface karst features (Figure 1). These catchments provide an excellent  location in which to discuss the extent to which Chalk can be viewed as a karstic aquifer, and the importance of such features to water resource management.
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During this field trip we will visit some of the stream sinks and one of the major springs in the Pang catchment (Figure 2). We will also look at the contact between the  Chalk  and the overlying Palaeogene deposits in a quarry, and observe some sub-surface dissolutional fissures and conduits via a borehole CCTV camera system.
During this field trip we will visit some of the stream sinks and one of the major springs in the Pang catchment (Figure 2). We will also look at the contact between the  Chalk  and the overlying Palaeogene deposits in a quarry, and observe some sub-surface dissolutional fissures and conduits via a borehole CCTV camera system.


[[Image:OR15042_fig1.jpg|thumb|center|500px|'''Figure 1''' Surface karst in the Pang and Lambourn catchments, UK. Geology based on 1:50,000 scale DigMapGB data. Contains Ordnance Survey data © Crown copyright and database rights 2015. Hill-shaded topography is NEXTMap Britain elevation data from Intermap Technologies.]]
[[Image:OR15042_fig1.jpg|thumb|center|500px|'''Figure 1''' Surface karst in the Pang and Lambourn catchments, UK. Geology based on 1:50 000 scale DigMapGB data. Contains Ordnance Survey data © Crown copyright and database rights 2015. Hill-shaded topography is NEXTMap Britain elevation data from Intermap Technologies.]]
 
[[Image:OR15042_fig2.jpg|thumb|center|500px|'''Figure 2''' Field trip sites in the Pang catchment. Contains Ordnance Survey data © Crown copyright and database rights 2015. Geology based on 1:50 000 scale DigMapGB data.]]
 
==References==


[[Image:OR15042_fig2.jpg|thumb|center|500px|'''Figure 2''' Field trip sites in the Pang catchment. Contains Ordnance Survey data © Crown copyright and database rights 2015. Geology based on 1:50,000 scale DigMapGB data.]]
[[category:OR/15/042 Groundwater in Cretaceous carbonates: KG@B field trip 21st June 2015 | 01]]
[[category:OR/15/042 Groundwater in Cretaceous carbonates: KG@B field trip 21st June 2015 | 01]]

Latest revision as of 15:12, 28 July 2015

MAURICE, L, FARRANT, A R, BUTCHER, A AND ATKINSON, T C. 2015. Groundwater in Cretaceous carbonates: KG@B field trip 21st June 2015. British Geological Survey Internal Report, OR/15/042.

The Upper Cretaceous Chalk of southern England is the UK’s most important aquifer, providing more than 75% of the public supply for southeast England, including London. The aquifer also sustains rivers and wetlands, and their associated groundwater dependent ecosystems. However, the aquifer is facing a multitude of threats including over-abstraction, nitrate pollution, and climate change.

The Chalk is a complex aquifer in which groundwater flow is through the matrix, fractures and karstic dissolutional voids. The Chalk matrix has a porosity of around 35% (Bloomfield et al., 1995[1]). The matrix is thought to provide an important contribution to storage, although the size of the pore throats is very small, and therefore the permeability is very low (Price et al., 1993[2]). The average permeability of 977 core samples was only 6.3 x 10-4 m/day (Allen et al., 1997[3]). The matrix is particularly important in solute transport, because solutes move between the matrix and the more permeable parts of the aquifer via diffusion (Foster 1975[4]). The unmodified fracture network provides an important contribution to storage and flow, and has a hydraulic conductivity of about 0.1 m/d, and a transmissivity of about 20 m2/day (Price, 1987[5]). However, it is the dissolutionally enlarged fissures and conduits that make the Chalk such a good aquifer. The median transmissivity from 2100 pumping tests is 540 m2/day, and the 25th and 75th percentiles are 190 and 1500 m2/day respectively (MacDonald and Allen, 2001[6]). Borehole packer testing, logging and imaging have shown that most of this transmissivity comes from a small number of dissolutional voids (e.g. Tate et al., 1970[7]; Schurch and Buckley, 2002[8]). Laterally extensive lithostratigraphical horizons including marl seams, bedding planes, sheet and tabular flint bands, and hard-grounds have an important influence on these groundwater flows. They are all horizons where downward percolation of water may be impeded. Dissolution often occurs where flow is concentrated along these horizons, creating conduits or fissures, especially where they are intersected by joint sets.

Across much of the UK, the Chalk has not traditionally been considered or managed as a karst aquifer, perhaps because caves are rare. However, it has been recognised for some time that karst processes and conduit flow may be common in many areas of the Chalk aquifer. Although not renowned for its caves, the Chalk does contain some significant cave systems. The largest chalk cave in Britain is Beachy Head Cave, near Eastbourne which is approximately 400 m long. Caves are more common in northern France where some are several kilometres long. As in more classical karst regions, surface karst features such as sinking streams, springs, dolines and dry valleys can be very common in the Chalk (Cooper et al., 2011[9]). There are also many fossil sediment filled karst features associated with the Chalk-Palaeogene unconformity when the climate was warmer and wetter (Newell, 2014[10]).

Locally the Chalk may host a greater density of surface karst features than other more classic karst aquifers such as the Carboniferous limestones of Derbyshire, yet it is rarely treated as a karstic aquifer. There is a rather simplistic tendency to equate karstic groundwater flow with the occurrence of extensive cave systems. Yet subsurface dissolution features are frequently encountered in the Chalk during construction projects (Edmonds, 2008[11]), and tracer testing has demonstrated rapid groundwater flow over distances of up to 20 km (e.g. Harold, 1937[12]; Atkinson and Smith, 1974[13]). Even where there is little evidence of surface karst, the Chalk often has high transmissivity due to the solutional enlargement of fractures to form fissures and conduits. Given the evidence for karstic features, rapid groundwater flow, and considerable spatial heterogeneity, the Chalk is perhaps better described as a weakly cavernous karst aquifer rather than weakly- or non-karstic.

The Pang and Lambourn catchments in Berkshire are good examples of chalk catchments with a high density of surface karst features (Figure 1). These catchments provide an excellent location in which to discuss the extent to which Chalk can be viewed as a karstic aquifer, and the importance of such features to water resource management.

During this field trip we will visit some of the stream sinks and one of the major springs in the Pang catchment (Figure 2). We will also look at the contact between the Chalk and the overlying Palaeogene deposits in a quarry, and observe some sub-surface dissolutional fissures and conduits via a borehole CCTV camera system.

Figure 1 Surface karst in the Pang and Lambourn catchments, UK. Geology based on 1:50 000 scale DigMapGB data. Contains Ordnance Survey data © Crown copyright and database rights 2015. Hill-shaded topography is NEXTMap Britain elevation data from Intermap Technologies.
Figure 2 Field trip sites in the Pang catchment. Contains Ordnance Survey data © Crown copyright and database rights 2015. Geology based on 1:50 000 scale DigMapGB data.

References

  1. ↑ BLOOMFIELD, J P, BREWERTON, L J and ALLEN, D J. 1995. Regional trends in matrix porosity and dry density of the Chalk of England. Quarterly Journal of Engineering Geology 28, S131–S142
  2. ↑ PRICE, M, DOWNING, R A and EDMONDS, W M. 1993. The Chalk as an aquifer. In The hydrogeology of the Chalk of North- West Europe. Edited by Downing, R A, Price, M and Jones, G P. 35–59
  3. ↑ ALLEN, D J, BREWERTON, L J, COLEBY, L M, GIBBS, B R, LEWIS, M A, MACDONALD, A M, WAGSTAFF, S J and WILLIAMS, A T. 1997. The physical properties of the major aquifers in England and Wales. BGS Technical Report WD/97/34, Environment Agency R&D Publication 8. 312 pp Atkinson and Smith, 1974.
  4. ↑ FOSTER, S S D. 1975. The Chalk groundwater tritium anomaly — a possible explanation. Journal of Hydrology 25, 159–165.
  5. ↑ PRICE, M. 1987. Fluid flow in the Chalk of England. In Goff, J C & Williams, B P J. (eds), Fluid Flow in Sedimentary Basins and Aquifers. Geological Society Special Publication 34, 141–156.
  6. ↑ MACDONALD, A M and ALLEN, D J. 2001. Aquifer properties of the Chalk of England. Quarterly Journal of Engineering Geology 34, 371–384
  7. ↑ TATE, T K, ROBERTSON, A S and GRAY, D A. 1970. The hydrogeological investigation of fissure flow by borehole logging techniques. Quarterly Journal of Engineering Geology 2, 195–215
  8. ↑ SCHÜRCH, M and BUCKLEY, D. 2002. Integrating geophysical and hydrochemical borehole log measurements to characterise the Chalk aquifer, Berkshire, United Kingdom. Hydrogeology Journal 10 (6), 610–627
  9. ↑ COOPER, A H, FARRANT, A R, and PRICE, S J. 2011. The use of karst geomorphology for planning, hazard avoidance and development in Great Britain. Geomorphology 134, 118–131.
  10. ↑ NEWELL, A J, 2014. Palaeogene rivers of southern Britain: climate extremes, marine influence and compressional techtonics on the southern margin of the North Sea Basin. Proceedings of the Geologists' Association, 125 (5–6). 578–590.
  11. ↑ EDMONDS, C N. 2008. Karst and mining geohazards with particular reference to the Chalk outcrop, England. Quarterly Journal of Engineering Geology and Hydrogeology, 41, 261–274.
  12. ↑ HAROLD C. 1937. The flow and bacteriology of underground water in the Lee Valley. Metropolitan Water Board 32nd Annual Report, 89–99.
  13. ↑ ATKINSON, T C and SMITH D I. 1974. Rapid groundwater flow in fissures in the Chalk: An example from South Hampshire. Quarterly Journal of Engineering Geology 7, 197–205
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