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A strategic programme for NERC Lowland catchment research
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Investigation of flow heterogeneity in the Chalk: field studies and stochastic models

Title

Investigation of groundwater flow heterogeneity in Chalk aquifers using detailed borehole arrays and stochastic modelling.

Aims

This project sought to provide an improved understanding of saturated flow through Chalk, the UK's primary aquifer. This is important for the further development of groundwater flow and transport modelling and for the interpretation of pumping and tracer test data, which are generally used to calibrate and/or condition such models.

Main findings

Research achievements:

  • Characterisation of flow heterogeneity at the local scale: important progress has been made on how various types of data can be integrated in order to quantify borehole/aquifer interaction under ambient and pumped conditions.
  • Prediction of flow heterogeneity using fracture flow modelling: The work has made a major contribution in demonstrating that the observation of a fracture (either visually or from caliper logs) does not necessarily indicate permeability or, more importantly, flow. It has also shown that flow horizons may change markedly under pumped and ambient conditions. It appears, therefore, that constructing discrete fracture flow models for such systems may not be feasible (due to insufficient data to constrain such models). However, the equivalent porous medium approach can also be misleading, particularly when using pumping test data to represent ambient conditions. Furthermore, great care is required when applying transport properties derived from the radially convergent tracer tests to groundwater contamination problems. A method for recovering tracer injection inputs in fractured rocks involving a limited number of flow horizons has also been developed.
  • Up-scaling of hydraulic parameters: the research has also pointed to the challenges faced in applying local data (where an artificial perturbation has been imposed on the system) to regional models. The research has indicated that a correct representation of the impact of the borehole needs to be included in any data analysis. It also indicates that the traditional representation of the effect of vertical heterogeneity in groundwater models, using a linear variation in hydraulic conductivity, whilst supported to a certain extent by packer test data, does not necessarily reflect actual flow horizons.

New modelling tools:

  • Improved pumping test solution for unconfined aquifers.
  • Correction to the Hvorslev shape factors.
  • Semi-analytical solution for determining hydraulic conductivities from double packer tests in open boreholes.
  • A new solution for representing the behaviour of finite diameter wells in anisotropic aquifers.

A method for recovering tracer injection inputs in fractured rocks involving limited flow horizon has also been developed.

These modelling tools have been used to provide new insights into anisotropy and heterogeneity present in the Chalk and the effect of water table elevation on pumping test parameters. In addition, they has provided detailed knowledge of the vertical structure of Seaford/Lewis Nodular Chalk in terms of fractures and hydraulic conductivity. This has included the development of a purpose-built software tool for fracture analysis of borehole images and a critical review of the methods used to interpret packer test results. The project has led to the development of an integrated modelling methodology for the analysis and interpretation of open boreholes in Chalk using a range of geophysical and tracer data. Researchers used it to demonstrate the existence of a limited number of active flow horizons during pumped conditions. The work has provided new and important insights into the effects of borehole/aquifer interaction. It has shown that flows can be substantially modified during pumping tests and hence do not necessarily reflect ambient conditions. This has important implications for the calibration of regional and local groundwater models. It has also highlighted the importance of including borehole dilution tests prior to and during pumping tests to elucidate these effects. Finally, the presence of localised flow pathways during pumping tests is likely to result in the occurrence of turbulent flow over a significant distance (eg 10m) from the well.

Description of activities

Fieldwork was undertaken at the Trumpletts Farm LOCAR site in the Pang Catchment where three 100 m deep boreholes are located less than 150 m apart.

A range of field investigation techniques were employed to investigate local scale flow heterogeneity:

  • standard suite of geophysical methods (caliper, temperature, conductivity, pulsed heat flow, etc.)
  • video fracture logging
  • pumping tests under different water table conditions
  • double packer permeability tests
  • single borehole dilution tests
  • radially convergent tracer tests.

The data collected were analysed using existing tools and those developed by the project, e.g. a purpose built software tool for fracture analysis of borehole images.

New modelling techniques were developed to improve interpretation of single borehole dilution tests, radial flow tracer tests and pumping tests.

Areas of application

This work has important implications for the calibration of regional and local groundwater models. These are used in water resources planning which is directly linked to the Habitats Directive and the Water Framework Directive.

Related and future work

Work on a new pumping test solution incorporating the effects of turbulent flow is ongoing.

Researchers' details

Principal Investigator:

Co-investigators:

Other researchers:

Publications

All publications from this and other LOCAR projects are listed in the publications database.

Refereed journal papers

Mathias, S.A., Butler, A.P. 2007. Flow to a finite diameter well in a horizontally anisotropic aquifer with wellbore storage. Water Resources Research, 43, W07501, doi:10.1029/2006WR005839.

Mathias, S.A., Butler, A.P. 2007. Shape factors for constant-head double-packer permeameters. Water Resources Research, 43, W06430, doi:10.1029/2006WR005279.

Mathias, S.A., Butler, A.P. and Williams, A.T. 2007. Recovering tracer test input functions from fluid electrical conductivity logging in fractured porous rocks. Water Resources Research, 43(7), W07443, doi:10.1029/2006WR005455.

Mathias, S. A., and Butler, A. P. 2006 Linearized Richards' equation approach to pumping test analysis in compressible aquifers, Water Resources Research, 42, W06408, doi:10.1029/2005WR004680.

Mathias, S.A., Butler, A.P. 2006. An improvement on Hvorslev's shape factors. Geotechnique, 56(10), 705-706.

Mathias, S.A., Butler, A.P., Atkinson, T.C., Kachi, S. and Ward, R.S. 2006. A parameter identifiability study of two chalk tracer tests, Hydrology and Earth System Sciences Discussions, 3, 1/35.

Williams, A.T., Bloomfield, J., Griffiths, K. and Butler, A.P. 2006. Characterising the vertical variations in hydraulic conductivity within the Chalk aquifer, Journal of Hydrology, 330, 53-62.

Conference proceedings

Bloomfield J.P., Butler A.P., Cobbing J.E., Griffiths K.J., Moreau M., Williams A.T., Peach D.W. and Binley A. 2003. Flow heterogeneity in the fractured Chalk aquifer of southern England. In 'Groundwater in Fractured Rocks' (Editors Krasny J, Hrkal Z and Bruthans J), IHP-VI Series on Groundwater No 7, p. 29-30. ISBN 92-9220-002-X.

Recommended reading

Allen., D.J., Brewerton, L.J., Coleby, L.M., Gibbs, B.R., Lewis, M.A., MacDonald, A.M., Wagstaff, S.J., and Williams, A.T. 1997. The physical properties of the major aquifers in England and Wales. BGS Technical Report WD/97/34, Environment Agency R&D Publication 8. 312pp

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