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Impact of karstic flow on Chalk aquifer function and water quality

Dr Jared West (SEE), Prof. Simon Bottrell (SEE), Dr Louise Maurice (BGS), Dr Andrew Farrant (BGS), Dr Alister Leggatt (Affinity Water, CASE supervisor)

Project partner(s): British Geological Survey and Affinity Water (CASE)

Contact email: l.j.west@leeds.ac.uk

Summary

Context

The Cretaceous Chalk aquifer represents the most important groundwater resource in the UK and is also important ecologically for chalk stream ecosystems. Similar aquifers exist in France, Belgium, Netherlands, and Israel. The extent to which chalk aquifers show development of karstic features (widened fractures and conduit development due to dissolution by groundwater flow) is of interest because where karstic features connect sources of contaminants directly to borehole abstractions, water quality may be poor. Where flow is more distributed because karst features less developed and smaller, water quality is generally better. Karst is generally associated with distinctive and often spectacular landforms, including caves, and results in rapid groundwater flow in subsurface streams and rivers, as well as flow through smaller solutional voids, although the latter are less well understood due to their inaccessibility. The Chalk is often not considered a karst aquifer because caves are rare, and surface karst features are small and until recently not well documented. Recent work has highlighted the potential importance of karst in the Chalk enabling rapid groundwater flow over long distances, but the nature of chalk karst and how it impacts groundwater flow and contaminant transport is not well understood. Recent advances in tracer testing techniques offer the opportunity to study the smaller sized solutional voids in karst aquifers, both in classical karst aquifers and in the Chalk. The development of tracers such as bacteriophage (non-harmful virus particles small enough to pass through fractured aquifer systems) and in modelling the development of karst networks enable a new way forward via systematic investigation of the extent of karst development in key areas of the Cretaceous Chalk coupled with numerical simulations of karst network development. Using this approach will test hypotheses about the key factors controlling karst development, the interactions between different void types in karst aquifers, and the impact of these on water quality.

Objectives

1. To identify key controls on development of karstic flow paths in soluble rocks such as the Chalk.

2. To test the hypothesis that chalk karst development also occurs in areas where surface karst features are absent (so rapid subsurface karstic flow may still be occurring).

3. To investigate the extent that karst development is stratigraphically constrained in Chalk (i.e. controlled by geological layering).

4. To investigate how degree of karst development is affecting nitrate and pesticide concentration trends in Chalk groundwater abstractions.

The ultimate goal of this research effort is to provide a coherent explanation of karst development in chalk aquifers, within the framework of previously-developed self-organising network models for karst development in soluble rocks, and the impact of karst development on water quality in chalk.

Methods

Data collection is likely to take place in the following areas within the Southern Province Chalk: i) Chilterns area (Affinity Water Company area) north of London, ii) South Downs area (Portsmouth Water area) south of London.  It may include: identification of swallow holes and boreholes for tracer testing to borehole/spring abstractions using existing data and geomorphological mapping; using borehole geophysical logging/single borehole dilution tests (SBDTs) to determine whether solutional fissures/conduits are present at flow horizons; injecting bacteriophage tracers into suitable boreholes/swallow holes/soakaways to identify links to abstraction boreholes, followed by dye tracers tests to obtain more quantitative data; sampling of pumped abstractions and springs to identify hydraulic connections and tracer migration rates; collation of existing Water Company time series and spatial data for nitrate and short residence indicators in abstracted water (turbidity, coliforms, short residence time pesticides).

Interpretational approaches will include hydrogeological conceptual model development: determination of stratigraphy; locating flowing horizons by their stratigraphic/structural location; mapping karstic flow linkages between boreholes/swallow holes and springs; evaluation of links between karst networks, stratigraphic and structural features with the Chalk, and their relation to nitrate and pesticide concentrations seen in abstraction wells.

Modelling approaches will aim to identify of the key controls on karst development and assist with devising improved strategies for groundwater and land management. Depending in your background and interest, approaches may i) involve permeable reactive transport simulation of karst development ii) numerical simulation of contaminant transport via karst networks, iii) development of groundwater vulnerability mapping approaches for chalk terrains. Knowledge gained will form basis of recommendations for groundwater management in chalk and karstic terrains.

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Related undergraduate subjects:

  • Earth science
  • Earth system science
  • Environmental science
  • Geochemistry
  • Geological science
  • Geology
  • Geophysical science
  • Geoscience
  • Natural sciences
  • Physical geography