Aquifer properties of the Chalk of England

Abstract

Aquifer properties data from 2100 pumping tests carried out in the Chalk aquifer have been collated as part of a joint British Geological Survey / Environment Agency project. The dataset is highly biased: most pumping tests have been undertaken in valley areas where the yield of the Chalk is highest.Transmissivity values from measured sites give the appearance of log-normality, but are not truly log- The data indicate several trends and relationships in Chalk aquifer properties. Transmissivity is highest in the harder Chalk of Yorkshire and Lincolnshire (median 1800 m²/d). Throughout much of the Chalk aquifer a direct relation is observed between transmissivity and storage coefficient, reflecting the importance of fractures in governing both storage and transmissivity. Pumping tests undertaken in unconfined conditions give consistently higher measurements of transmissivity than in confined areas, probably as a result of increased dissolution enhancement of fractures in unconfined areas. At a catchment scale the data illustrate a relation between transmissivity and winter flowing streams. Word Count 7500QJEGH 34:371-384Alan MacDonaldPage 2 of 26 submitted QJEGHThe Chalk is the most important aquifer within the UK. It accounts for 60% of the groundwater used inEngland and Wales and 20% of the total water used in England and Wales (UK Groundwater Forum 1998). To protect and manage this unique resource it is essential to know and understand the variations of aquifer properties throughout the Chalk.Against this background a three-year collaborative project between the British Geological Survey (BGS) and the Environment Agency (EA) was devised. The aim of the study was to collect, collate and present information concerning the physical properties of the major aquifers in England and Wales (Allen et al. 1997). This included creating a database of available pumping tests for the major aquifers. This paper presents an analysis and interpretation of all available Chalk data in the Aquifer Properties Database. OVERVIEW OF THE DEVELOPMENT OF CHALK AQUIFER PROPERTIESThe presence and development of fractures gives the Chalk the properties of an aquifer. Without fractures, the permeability and specific yield of the Chalk would be negligible. Furthermore, without solution enhancement of the fractures, the high transmissivity of the Chalk would be impossible, and without further concentration of groundwater flow and dissolution of chalk, conduits and karstic features would not be observed. The Chalk is often referred to as possessing dual porosity (Price 1987;Barker 1991;Price et al. 1993). In a classic dual porosity aquifer, the matrix pores provide the storage, and the fractures provide the permeable pathways to permit groundwater flow. Groundwater movement within the Chalk is more complex: the high porosity (produced by the coccoliths) is not readily drained, due to the very small pore throats (Price et al. 1976), therefore effective groundwater storage is primarily within the fracture network and the larger pores....

Spatio-temporal modelling of the status of groundwater droughts

“…The Chalk aquifer is a dual porosity, dual permeability limestone aquifer, where groundwater storage and flow are primarily dependent on the nature of the fracture system which may locally be sub-karstic (Bloomfield, 1996;Allen et al, 1997;MacDonald and Allen, 2001;Downing et al, 2005;Maurice et al, 2006). Groundwater levels in the Chalk respond to local hydro-climatology, they are sensitive to episodes of drought and depend on a combination of catchment characteristics, such as unsaturated zone thickness, and intrinsic hydraulic properties of the aquifer, such as hydraulic diffusivity (Bloomfield and Marchant, 2013).…”

“…9b) should reflect any spatial heterogeneity in transmissivity (T) and storativity (S) of the Chalk at a regional scale. MacDonald and Allen (2001) analysed the aquifer properties of the Chalk based on 2100 pumping tests in four regions: southern; the Thames Basin (including the North Downs); East Anglia; and Yorkshire and Lincolnshire, but the results were inconclusive. They found that the distribution of T and S was broadly similar for all four regions, and that within a given region transmissivity and storage co-efficients could vary by over five orders of magnitude.…”

Accounting for groundwater in future city visions

“…It is located in the estuary of the River Thames catchment with the Chalk aquifer, the most important aquifer within the UK (MacDonald and Allen, 2001), lying at depth beneath the city. The aquifer is covered and confined by clays with interbedded sand and gravel deposits.…”

Determining fracture properties by tracer and thermal testing to assess thermal breakthrough risks for ground source heating and cooling in the Chalk

“…The Chalk aquifer is typically hydrogeologically linked to the Thanet Sand Formation which is a fine grained sandstone. The intact chalk is generally considered to be impermeable (Bloomfield 1995, Macdonald 2001) but fissures and fractures can make the overall rock mass highly permeable. Water flow can be rapid where significant fracturing occurs.…”