The physical properties of major aquifers in England and Wales

This report is the result of a three-year collaborative project between the British Geological Survey and the National Rivers Authority (now the Environment Agency). The aim of the project has been to collect, collate and present information concerning the physical hydraulic properties of the major aquifers in England and Wales. The properties addressed are those which are substantially invariant with time; permeability and porosity, transmissivity and storage coefficient. These properties have been investigated for the six main aquifers; the Chalk, the Lower Greensand, the Jurassic limestones, the Permo-Triassic sandstones, the Magnesian Limestone and the Carboniferous Limestone. Although the parameters studied were limited in number, the study has proven to be both broad and complex for several reasons. Firstly the aquifers themselves are hydraulically complicated. They are in the main heterogeneous, fractured bodies of rock, sometimes with indeterminate boundaries. This presents a double problem; hydraulic tests on such materials often violate the classic assumptions used in the test analysis, and the complexity of the aquifers makes interpolation between data points difficult. Secondly the physical properties of the aquifers are often scale dependent, so that the value of a parameter at one scale may not be appropriate for use at a larger or smaller scale. Thirdly there are problems of data quality and quantity. The quality of the pumping tests is variable and many results are from pumping tests of short duration which are designed more to assess the yields of boreholes than to examine the properties of the aquifer. Also, data obtained from boreholes tend to be clustered in high yielding areas, making an assessment of the true variation of hydraulic properties across an aquifer difficult. As a result of these difficulties the approach to the project has been to collect both data and knowledge about the aquifers, in order that the report can address not only the magnitudes and variability of the aquifer parameters, but also to provide some insight into factors controlling the properties. To this end project resources were used in two distinct ways. Initially the main effort of the project was put into data collection. This involved a detailed search principally through the records of the former NRA, with additional information from BGS, industry and published and unpublished literature. Most of the data obtained were from pumping tests, and these were digitised and stored in a database designed for the project. The database was linked with the BGS Core Analysis Database to form a large set of basic data for the aquifers under consideration. The second main strand of the project was the collection of knowledge about the aquifers. This took the form both of collecting reports of hydrogeological studies carried out on the aquifers and of canvassing expert opinion (a vital source of information which is not often published). The results of these two approaches are synthesised in this report. After the introductory sections each chapter takes the form of a detailed review of the physical properties of one aquifer (subdivided as necessary). The purpose of the review is to present the magnitudes and variability of the data (mainly from the database, but with other examples) in the context of current understanding of the controls on the data. To that end the review encompasses appropriate aspects of the geological, geographical and physical hydrogeological nature of the aquifers. Summaries of data from the database are also presented in the form of appendices on an accompanying CD-ROM. The intention of the report is therefore not only to acquaint the reader with the aquifer properties data values which characterise the aquifers, but also to show the perceived complexity of their hydraulic structure and the physical controls on the data — there is therefore an overt intention to dissuade the reader from taking raw values out of context. A further purpose of the report is to provide a comprehensive set of references by which the reader can obtain more detailed information about particular areas of interest in an aquifer. As a result of the collection and review of information about the physical properties of the aquifers it is apparent that there are many areas in which knowledge is inadequate. For example the scale dependence of aquifer properties in the Permo-Triassic sandstones, and in particular the effects of fractures, are perceived to be important but are poorly understood. In the Chalk the extent to which the aquifer may be considered to be karstic, in the sense of allowing rapid flow to occur in discrete zones of high permeability, is an often debated issue on which there has been little research. Many other areas of uncertainty are apparent in the information presented in this report; however this is an important function of the study, for by summarising the extent of available knowledge its inadequacies will be more readily seen.