In Africa, groundwater is the major source of drinking water and its use for irrigation is forecast to increase substantially to combat growing food insecurity. Despite this, there is little quantitative information on groundwater resources in Africa, and groundwater storage is consequently omitted from assessments of freshwater availability. Here we present the first quantitative continent-wide maps of aquifer storage and potential borehole yields in Africa based on an extensive review of available maps, publications and data. We estimate total groundwater storage in Africa to be 0.66 million km 3 (0.36‐1.75 million km 3 ). Not all of this groundwater storage is available for abstraction, but the estimated volume is more than 100 times estimates of annual renewable freshwater resources on Africa. Groundwater resources are unevenly distributed: the largest groundwater volumes are found in the large sedimentary aquifers in the North African countries Libya, Algeria, Egypt and Sudan. Nevertheless, for many African countries appropriately sited and constructed boreholes can support handpump

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Quantitative maps of groundwater resources in Africa

Temporal and spatial variability of groundwater–surface water fluxes: Development and application of an analytical method using temperature time series

Vadose zone flow model parameterisation using cross-borehole radar and resistivity imaging

Cross-borehole geoelectrical imaging, in particular electrical resistivity tomography (ERT) and transmission radar tomography, can provide high-resolution images of hydrogeological structures and, in some cases, detailed assessment of dynamic processes in the subsurface environment. Through appropriate petrophysical relationships, these tools offer data suitable for parameterising and constraining models of groundwater flow. This is demonstrated using cross-borehole radar and resistivity measurements collected during a controlled vadose zone tracer test, performed at a field site in the UK Sherwood Sandstone. Both methods show clearly the vertical migration of the tracer over a 200 h monitoring period. By comparing first and second spatial moments of changes in moisture content predicted from a numerical simulation of vadose zone flow with equivalent statistics from two- and three-dimensional ERT and cross-borehole radar profiles the effective hydraulic conductivity is estimated to be approximately 0.4 m/d. Such a value is comparable to field estimates from borehole hydraulic tests carried out in the saturated zone at the field site and provides valuable information that may be utilised to parameterise pollutant transport models of the site.

Vadose Zone Flow Model Parameterisation Using Cross-Borehole Radar and Resistivity Imaging

Appropriate regularizations of geophysical inverse problems and joint inversion of different data types improve geophysical models and increase their usefulness in hydrogeological studies. We have developed an efficient method to calculate stochastic regularization operators for given geostatistical models. The method, which combines circulant embedding and the diagonalization theorem of circulant matrices, is applicable for stationary geostatistical models when the grid discretization, in each spatial direction, is uniform in the volume of interest. We also used a structural approach to jointly invert cross-hole electrical resistance and ground-penetrating radar traveltime data in three dimensions. The two models are coupled by assuming, at all points, that the cross product of the gradients of the two models is zero. No petrophysical relationship between electrical conductivity and relative permittivity is assumed but is instead obtained as a by-product of the inversion. The approach has been applied to data collected in a U.K. sandstone aquifer in order to improve characterization of the vadose zone hydrostratigraphy. By analyzing scatterplots of electrical conductivity versus relative permittivity together with petrophysical models a zonation could be obtained with corresponding estimates of the electrical formation factor, the water content, and the effective grain radius of the sediments. The approach provides greater insight into the hydrogeological characteristics of the subsurface than by using conventional geophysical inversion methods.

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Improved hydrogeophysical characterization using joint inversion of cross‐hole electrical resistance and ground‐penetrating radar traveltime data

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.

C.S. Cheney

Papers

The physical properties of major aquifers in England and Wales