Regional trends in matrix porosity and dry density of the chalk of England

Laboratory measurements of porosity and dry density are presented for 2045 core samples from the Chalk of England. The data are subdivided on the basis of gross stratigraphy, i.e. Lower, Middle and Upper Chalk, and into four geographical areas: Northern England, East Anglia, Thames & Chilterns and Southern England. Statistical analysis of the data shows (i) that the porosity distributions for the Upper Chalk of the Southern and Thames & Chilterns regions are indistinguishable, (ii0 that the porosity distributions for the middle and Lower Chalk of the East Anglian region are indistinguishable, and (iii) that the porosity distributions for each of the gross stratigraphical units from all other regions are statistically discrete. Porosities range from 3.3% to 55.5%, with a mean porosity of 34.0%. Dry densities range from 1210 kg/m3 to 2510 kg/m3, with a mean dry density of 1790 kg/m3. In a given region there is a trend of increasing porosity from Lower to Middle to Upper Chalk. There are systematic variations in porosity between the regions. There is a trend of increasing porosity from the Northern England region to the Southern England region, to the Thames & Chilterns region, to East Anglia. No significant systematic variations in porosity-depth gradients were observed. Chalk porosity-depth gradients are typically high, of the order of -0.07 to -0.1 porosity per cent per metre

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

The physical properties of minor aquifers in England and Wales

This report is the result of a three-year collaborative project between the British Geological Survey and the Environment Agency. The aim of the project has been to collect, collate and present information concerning the physical hydraulic properties of the minor aquifers in England and Wales. These properties include hydraulic conductivity, porosity, transmissivity and storage coefficient. In addition, specific capacity (yield per unit drawdown) values are included for many of the formations described, together with yields for those formations where aquifer properties data are sparse. Although the parameters studied were limited in number, the study has proven to be complex for several reasons. Firstly the aquifers themselves are hydraulically complicated. They are bodies of rock, sometimes with indeterminate boundaries, which are heterogeneous either because of sedimentological factors in the case of the Cainozoic aquifers, or because of the effects of fracturing in older formations. This heterogeneity presents several problems. Firstly, hydraulic tests on such materials often violate the classical 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 which are particularly significant for these smaller aquifers. The quality of the pumping tests is variable and many results are from short duration pumping tests which are designed more to assess the yields of boreholes than to examine the properties of the aquifer. Also, data can be very irregularly distributed, being a product mainly of the evolving requirements of groundwater users and not of well-planned resource assessments. This irregular spacing can be both vertical as well as lateral, as in the case of thick structurally complex sequences with only scattered productive horizons. Awareness of these inherent hydrogeological factors dictated the project’s approach, which was to collect both data and knowledge about the aquifers. This permits the report to describe not only the magnitudes and variability of the aquifer parameters at a given tested locality, but also to provide some insight into factors controlling the properties, so that the results can be more confidently extrapolated. Project resources were therefore initially employed in data collection. This involved a detailed search through Agency records, with additional information from BGS, published and unpublished literature. Most of the data obtained were from analysed pumping tests, the results of which were entered in a database. The latter originally housed data on the major aquifers, collected under a preceding project, but the database needed to be significantly altered and expanded so as to manage efficiently the much larger number of aquifers involved. It was also linked with the BGS Core Analysis Database. The result comprises the National Aquifer Properties Database which is now a major UK geoscience resource, with data from more than 8000 pumping test analyses at over 8250 sites. The second main strand of the project was the collection and summarising of knowledge about the aquifers. In addition to the collection of reports of hydrogeological studies and a literature survey, expert opinion was canvassed. The latter is a vital source of information that 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 a group of minor aquifers, subdivided as appropriate on stratigraphic or geographical grounds. The chapters are arranged in order of increasing age. 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 aquifer systems involved and the controls on the data. To that end the review includes geological, geographical and physical hydrogeological aspects of the aquifers. Useful summaries of data from the database are included on the accompanying CD-ROM. The intention of the report is therefore to acquaint the reader with the aquifer properties data values that characterise the aquifers in the context of what is known about the complexities of their hydraulic structure and the physical controls on the data. The reader is specifically dissuaded from taking raw values out of context. A further purpose of the report is 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 minor aquifers in England and Wales, it is apparent that there are many areas in which knowledge is inadequate. For example, a critical comparison of the equivalent aquifer systems in the London and Hampshire basins was not possible in other than the most general terms. Similarly, the lateral variability in aquifer properties in the Lower Cretaceous aquifers of the Weald is suspected to arise partly from fault-controlled compartmentalisation, but the role of the faults is not well enough understood for predictive purposes. For all the effort expended on geological characterisation over almost two centuries of detailed study of English Jurassic rocks, the flow systems of the numerous arenaceous and carbonate minor aquifers of that system are in general poorly characterised. Very localised borehole development and the effects of tapping complex multi-aquifer sequences mean that the fracture-dominant, structurally-affected systems of older rocks of Palaeozoic age are in many cases barely conceptualised. Such gaps in our knowledge are inevitable considering the paucity of data. Nevertheless, the project has provided the first opportunity to review comprehensively the aquifer properties of this second rank of British aquifers whose role is so important in providing local sources of water supply for both private and public use