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