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Characterising permeability distributions in Chalk catchments

Title

Characterising permeability, groundwater flow and contaminant attenuation in Chalk catchments using tracer techniques.

Overview of the project

First principles suggest that Chalk rivers, springs and wells are protected from land surface contamination by solutes diffusing from fractures into the porous matrix. Using artificial and environmental (CFC) tracers to mimic contaminants, the study investigated the amount of flow that has short residence times and little potential attenuation by diffusion while flowing through voids greater than about l mm wide. It also investgated and what proportion of flow passes slowly through smaller voids, with maximum attenuation, therefore protecting water resources and river ecology.

Aims

The aim was to develop a conceptual model of water and potential contaminant fluxes from land surface to rivers via the Chalk aquifer, to underpin future basin management and risk analysis.

Main findings

Results of single borehole dilution tests at 25 sites show that the Chalk contains preferred flow horizons with vertical gradients of hydraulic head between them. The separations between preferred flow horizons are log-normally distributed with a modal value of about 4m, and 95% between 0.3 and 52m. Tracer tests at about 10 to 100 m scales suggest that flow is predominantly along horizontal fractures. The new data imply that flowing horizons are randomly distributed with respect to depth, and are probably linked by less permeable fractures at a high angle to bedding, forming a sparsely connected network of preferred flow paths. Borehole imaging data showed that flow horizons occurred where small conduits (< 1 to about 20cm diameter), or solutionally enlarged fractures are present.

Tracer tests from stream sinks reveal karstification of the Chalk. Flows from sinks to springs had velocities of thousands of metres per day, which are comparable to those observed in highly karstic aquifers. These results suggest that networks of conduits and solutionally widened fractures connecting between stream sinks and springs do exist. Groundwater flow along these pathways is turbulent and attenuation of contaminants will be low. However, tracer recovery was either almost zero or about 25%. A probable explanation for rapid flow combined with very high attenuation is that flow occurs along multiple pathways and that very few solutionally widened fractures and conduits connect directly to springs. Large proportions (>75% to almost 100%) of the tracers they carried were distributed into narrow fractures and fissures where they were very effectively attenuated by diffusion into the matrix.

Karst features in the Pang and Lambourn area show zoning that corresponds to the structure of permeability in the sub-surface. Zone 1, near the edge of overlying Palaeogene strata, contains stream sinks and has the greatest risk of rapid, low-attenuation transport of contaminants. In Zone 2, further from the Palaeogene, sinking streams are absent but there are frequent dolines, whereas in Zone 3 the sole karst landforms are the dry valleys that typify the Chalk landscapes of southern England. In Zones 2 and 3, groundwater flows moderately rapidly along preferred paths through the fracture network, but attenuation by diffusion is extremely effective. Contaminants from the land surface are likely to be highly diluted before reaching rivers, springs or water supplies. This conceptual model is potentially applicable across the whole of the UK Chalk.

Description of activities

A field survey of 128 stream sinks and 26 springs in the Pang and Lambourn catchments was undertaken to investigate the nature of surface karst development and groundwater inputs to rivers.

Qualitative natural gradient tracer tests using bacteriophages and dye tracers were undertaken at five different sites to establish connections between streams sinks and groundwater outlets.

Quantitative tracer testing using fluorescent dyes was undertaken on three occasions at one site, including a multi tracer test involving four dyes and one bacteriophage tracer. These experiments aimed to investigate contaminant attenuation mechanisms over this groundwater flowpath, specifically determining whether attenuation is dominated by advection-dispersion or double porosity diffusion.

Fifty single borehole dilution tests (SBDTs) were carried out in twenty-five boreholes in the catchments. At all sites at least one "uniform injection" involving tracer injection throughout the saturated length of the borehole was carried out. Repeated tests at selected sites were used to consider the effect of variations in water level on SBDTs at an individual site. Tracer injections at specific points in boreholes were used to identify vertical flows and, together with borehole geophysics and imaging carried out at selected sites, were used to develop a methodology to identify the location of flowing fractures in boreholes. The dataset of flowing fractures was used to consider the controls on the location of flow at the catchment scale. Where borehole imaging data was available, it was used to determine the nature of the flowing horizons identified from the SBDTs.

Areas of application

  • Source protection zone policy
  • Catchment management planning on the Chalk or other aquifers prone to karstification

Related and future work / research needs

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Researchers' details

Principal Investigator:

Co-investigators:

Researcher:

Contributors:

Publications

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

PhD thesis

Dr LD Maurice. Investigations of rapid groundwater flow and karst in the Chalk.

Refereed Journal Papers

Van den Daele, G F A, Barker J A, Connell L D, Atkinson T C , Darling W G & Cooper J D, 2007. Unsaturated flow and solute transport through the Chalk : tracer test and dual permeability modeling. Journal of Hydrology, 342, 157-172.

Maurice L D, Atkinson T C, Barker J A, Bloomfield J P, Farrant A R & Williams, A.T., 2006. Karstic behaviour of groundwater in the English Chalk. Journal of Hydrology, 330, 63-70.

Johnston P B, Atkinson T C, Odling N E & Barker J A, 2005. Models of tracer breakthrough and permeability in simple fractured porous media. In Shaw, R.P. (ed.) Understanding the Micro to Macro Behaviour of Rock-Fluid Systems. Geological Society, London, Special Publications, 249, 91-102.

Betson M, Barker J, Barnes P & Atkinson T, 2005. Use of synchrotron tomographic techniques in the assessment of diffusion parameters for solute transport in groundwater flow. Transport in Porous Media, 60, 217-223.

Betson M, Barker J, Barnes P, Atkinson T & Jupe A, 2004. Porosity imaging in porous media using synchrotron tomographic techniques. Transport in Porous Media, 57, 203-214.

Taylor R, Cronin A, Pedley S, Barker J & Atkinson T, 2004. The implications of groundwater velocity variations on microbial transport and wellhead protection - review of field evidence. FEMS Microbiology Ecology, 49, 17-26.

Refereed Web-based paper

Mathias S A, Butler A P, Atkinson T C, Kachi S & Ward R S, 2006. A parameter identifiability study of two Chalk tracer tests . Hydrology & Earth System Sciences Discussion, 3, 2437-2471.

Refereed Conference and Discussion papers

Maurice L, Atkinson T, Barker J, Bloomfield J, Farrant A & Williams A, 2005. Importance of karst for groundwater protection in the English Chalk. In Stavonovic, Z & Milanovic, P (eds), Water Resources and Environmental Problems in Karst. Proc. Int. Conf. & Field Seminars, Belgrade & Kotor, 13-19 Sept 2005, 49-54.

Atkinson T C, Lacey R A & Cole J A, 2003. Discussion of 'Estimating water pollution risks arising from road and railway accidents' by R.F.Lacey & J.A.Cole Quarterly Journal of Engineering Geology and Hydrogeology, Vol.36, 185-192. ibid., 36, 367-368.

Conference abstracts

Moreno A, Atkinson T C & Simons R R, 2003. Fracture networks: a case study from the Chalk aquifer in East Anglia. p.83-84 in Krasny, J., Hrkal, Z. & Bruthans, J. (eds), Groundwater in Fractured Rocks. Proceedings of the Prague Conference, International Association of Hydrogeologists, September 2003. IHP-VI, Series on Groundwater No 7, Prague, ISBN 92-9220-002-X.

Van Den Daele G F A, Connell L D, Atkinson T C & Barker J A, 2003. A tracer investigation into the importance of diffusive exchange in the unsaturated zone of the British Chalk. p.301-302 in Krasny, J., Hrkal, Z. & Bruthans, J. (eds), Groundwater in Fractured Rocks. Proceedings of the Prague Conference, International Association of Hydrogeologists, September 2003. IHP-VI, Series on Groundwater No 7, Prague, ISBN 92-9220-002-X

Johnston P, Atkinson T, Odling N & Barker J, 2003. Tracer breakthrough, hydraulic properties and fracture networks at sub-continuum scales in aquifers. p.411-412 in Krasny, J., Hrkal, Z. & Bruthans, J. (eds), Groundwater in Fractured Rocks. Proceedings of the Prague Conference, International Association of Hydrogeologists, September 2003. IHP-VI, Series on Groundwater No 7, Prague, ISBN 92-9220-002-X

Recommended reading

Banks, D., Davies, C., & Davies,W., 1995. The Chalk as a karstic aquifer: evidence from a tracer test at Stanford Dingley, Berkshire, UK. Quarterly Journal of Engineering Geology. 28, S31-S38

Downing, R.A., Price, M., & Jones, G.P. (eds.) (2005). The hydrogeology of the Chalk of North-West Europe. pp. 308. Pub. Oxford University Press. ISBN10: 0198526482

Drew, D & Hotzl, H 1999. Karst Hydrogeology and Human Activities: Impacts, Consequences and Implications. pp. 286. Pub. A.A. Balkema. ISBN-10: 9054104643

Ford, D.C., & Williams, P., 2007. Karst hydrogeology and geomorphology. Pub. Wiley. pp. 576. ISBN-10: 0470849967

Macdonald A.M., Brewerton L. & Allen D.J., 1998. Evidence for rapid groundwater flow and karst type behaviour in the Chalk of Southern England. In: Groundwater pollution, aquifer recharge and vulnerability, Robins NS (Ed), Geological Society of London special publication No. 130, 95-106

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