Impact of micro-karstic flow on Chalk aquifer function and water quality

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Highlights and Novelty

  • Opportunity for investigating the impacts of contaminants on water quality and security.
  • Investigation of one of the world’s most important aquifers, the Cretaceous Chalk.
  • Determination of micro-karst extent in chalk, to better understand its development, and identify the impact of rapid micro-karstic flow processes on water quality [chalk has previously not been considered a karst aquifer in most areas, as surface karst features absent; recent work has highlighted the widespread nature of micro-karst].
  • Working with a very experienced team including British Geological Survey staff to conduct fieldwork using e.g. novel tracer technology to identify micro-karst pathways.
  • Identification of preventive actions to be taken by Water Company partner to facilitate improved future groundwater resource management and resource sustainability.

Background

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, water quality is consistently 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 solution 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 micro-karst development in the Chalk enabling rapid groundwater flow over long distances through relatively small karst features, but its nature 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 solution 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 micro-karst development in aquifers like the Cretaceous Chalk. Using these approaches we aim to identify key factors controlling development, the importance of different void types (widened fractures, conduits etc.) in the Cretaceous Chalk, and the impact of these on contaminant transport and therefore water quality.

Objectives

  1. To identify key controls on development of micro-karstic flow paths in soluble rocks such as the Chalk.
  2. To identify extent of Chalk micro-karst development in areas where surface macro-karst features (swallow holes etc.) are absent, as well as where these surface macro-karst features are evident .
  3. To simulate evolution of micro-karstic features in the Chalk aquifer and their impact, and/or predict the pollution vulnerability of groundwater in chalk and karstic aquifers more widely.
  4. To investigate how degree of micro-karst development influences nitrate and pesticide concentration trends in Chalk groundwater abstractions, and their response to extreme weather events.

Figure 1. Chalk micro-karst features in borehole image (right) and time-series borehole dilution test data (left)

 

Figure 2: Borehole tracer injection (left) and chalk stream sinkhole (right), southern England.

The ultimate goal is to provide a coherent understanding of micro-karst development in chalk aquifers, within the framework of previously-developed models for karst development in soluble rocks, and the impact of micro-karst development on contaminant transport, groundwater pollution vulnerability and therefore on the quality of abstracted groundwater and that of chalk-fed springs and streams.

Methods

There is scope for the project methods to be tailored to the specific skills background and interests of the student in order to develop ownership. For example, the data collection phase of the project will include some (but not necessarily all) of the following elements:

  • identification of swallow holes and boreholes for tracer testing to borehole/spring abstractions using existing remote sensing data and geophysical borehole logs held by the CASE partner organisation,
  • geomorphological field mapping in order to identify surface expressions of karst features
  • single borehole dilution testing (SBDT) and/or borehole geophysical logging to determine whether solutional fissures/conduits are present at specific horizons;
  • injecting tracers (dyes or bacteriophage) into suitable boreholes/swallow holes/soakaways and sampling of pumped abstractions and springs to identify links to abstraction boreholes, followed by further tracer tests on identified connections to obtain more quantitative data (tracer breakthrough curves);
  • Analysis of existing abstracted water quality data i.e. nitrate and short residence indicators (turbidity, coliforms, short residence time pesticides) held by the CASE partner organisation.

The interpretation phase of the project will include hydrogeological conceptual model development for the catchments investigated, incorporating the extent of micro-karst network development and its impact on nitrate and pesticide concentrations seen in abstraction wells. Depending on the specific background of the student, this phase of the project can then be directed towards either numerical simulations i.e. of micro-karst development in chalk and its impact on contaminant transport e.g. modelling tracer breakthrough curves or development of semi-quantitative groundwater vulnerability mapping approaches for chalk terrains, incorporating the knowledge gained.

Impact and expected outcomes

Investigation of the global environmental challenge presented by rising contaminant concentrations in abstracted groundwater is of international importance. It is anticipated that this project will have a tangible impact on risk management strategies for groundwater contamination, as the outputs will be used for decision making by stakeholders in the UK such as the CASE partner company and Environment Agency but also more widely. By conducting fieldwork (e.g. geomorphological mapping of surface karst features, tracer tests between sinkholes and abstractions) and developing modelling or vulnerability mapping approaches based on the results, you will provide a coherent explanation of micro-karst development in chalk aquifers, and its impact on contaminant transport and therefore on the quality of abstracted groundwater. The outputs of this investigation will be transferrable to other karstic aquifer systems, and constitute the basis of a new generation of groundwater vulnerability mapping approaches, leading to high impact publication outputs.

Training

You will work under the supervision of Dr. Jared West and Prof. Simon Bottrell within the Institute of Applied Geoscience in the School of Earth and Environment at the University of Leeds, with additional support for fieldwork from Drs. Louise Maurice and Andrew Farrant (British Geological Survey). The CASE partner for the projects is Affinity Water. Logistical support for fieldwork and bacteriophage tracer analysis will be provided by BGS and Water Company staff, who have extensive fieldwork experience. This project provides a high level of specialist scientific training in area including: (i) analysis of water quality; (ii) tracer testing approaches; (iii) use of specialist software for numerical simulation of flow and/or transport in fractured aquifer systems (iv) groundwater pollution vulnerability mapping. Co-supervision will involve regular Skype meetings with the BGS and CASE supervisors, plus two field seasons for geological and hydrogeological fieldwork. There is also a requirement to undertake a minimum 3 month duration internship at CASE partner organisation (Affinity Water) premises. Internships are likely to be undertaken early, as they provides opportunities to collate background information held by the partner, for example geophysical borehole logs and water quality data. You will have access to a broad spectrum of training workshops put on by the Faculty at Leeds that include an extensive range of training workshops in technical aspects, through to managing your degree, to preparing for your viva (http://www.emeskillstraining.leeds.ac.uk/).

Student profile

You will have a degree in Geoscience or Environmental Science background (includes Earth Science, Environmental Science, Geology, Geophysics, Hydrology, and Physical Geography), the ability to undertake fieldwork and wet chemical laboratory work and to analyse and interpret numerical data. You will need willingness to EITHER learn to apply numerical modelling codes (using existing modelling software) OR to develop groundwater vulnerability mapping approaches using GIS-based tools. You will need to undertake fieldwork in the field areas relevant to CASE partner (i.e. South East of England), such as geomorphological mapping and tracer testing, with support from the staff at BGS Wallingford and CASE partner staff.

For further information related to the project or any other specific questions concerning what the successful applicant will be expected to do and required educational background, please contact the lead supervisor. We encourage interested applicants to get in touch and arrange an informal Skype meeting to discuss details of the project prior to application.

References

Key Supervisor Publications

Allshorn SL; Bottrell SH; West LJ; Odling NE (2007) Rapid karstic bypass flow in the unsaturated zone of the Yorkshire chalk aquifer and implications for contaminant transport, In: Parise M; Gunn J (Ed) Natural and Anthropogenic Hazards in Karst Areas: Recognition, Analysis and Mitigation, Geological Society Special Publications, Geological Society of London, pp.111-122.

Hartmann S; Odling NE; West LJ (2007) A multi-directional tracer test in the fractured Chalk aquifer of E. Yorkshire, UK, J CONTAM HYDROL, 94, pp.315-331. doi: 10.1016/j.jconhyd.2007.07.009

Maurice, L D, Atkinson, T A, Barker, J A, Bloomfield, J P, Farrant, A R, and Williams, A T. 2006. Karstic behaviour of groundwater in the English Chalk. Journal of Hydrology 330 53–62. 10.1016/j.jhydrol.2006.04.012

Maurice, L D, Atkinson, T C, Williams, A T, Barker, J, and Farrant, A R. 2010. Catchment scale tracer testing from karstic features in a porous limestone. Journal of Hydrology. 389 (1–2) 31–4. 10.1016/j.jhydrol.2010.05.019

Maurice, L D, Atkinson, T C, Barker, J, Williams, A T, and Gallagher, A. 2012. The nature and distribution of flowing features in a weakly karstified porous limestone aquifer. Journal of Hydrology. 438–439, 3–15. 10.1016/j.jhydrol.2011.11.050

Medici G, West LJ, Banwart SA. 2019. Groundwater flow velocities in a fractured carbonate aquifer-type: Implications for contaminant transport. Journal of Contaminant Hydrology. 222, pp. 1-16

Medici G, West LJ, Chapman PJ, Banwart SA. 2019. Prediction of contaminant transport in fractured carbonate aquifer-types; case study of the Permian Magnesian Limestone Group (NE England, UK). Environmental Science and Pollution Research. 26(24), pp. 24863-24884

Parker AH; West LJ; Odling NE (2018) Well flow and dilution measurements for characterisation of vertical hydraulic conductivity structure of a carbonate aquifer, Quarterly Journal of Engineering Geology and Hydrogeology, . doi: 10.1144/qjegh2016-145

West L J; Odling N (2007) Characterization of a Multilayer Aquifer Using Open Well Dilution Tests, Ground Water, 45, pp.74-84. doi: 10.1111/j.1745-6584.2006.00262.x

 

Other key references

Cook, S J, Fitzpatrick, C M, Burgess, W G, Lytton, L, Bishop, P, and Sage, R. 2012. Modelling the influence of solution-enhanced conduits on catchment-scale contaminant transport in the Hertfordshire Chalk Aquifer. In: Groundwater resources modelling a case study from the UK, Geological Society Special Publication 364, p. 205- 225

Edmonds, C. 2008. Improved groundwater vulnerability mapping for the karstic Chalk aquifer of south east England. Engineering Geology 99 (95–108).

El Janyani, S., Dupont, J.P., Massei, N., Slimani, S. and Dörfliger, N., 2014. Hydrological role of karst in the Chalk aquifer of Upper Normandy, France. Hydrogeology Journal, 22(3), pp.663-677.

Foley, A, Cachandt, G, Franklin, J, Willmore, F, and Atkinson, T. 2012. Tracer tests and the structure of permeability in the Corallian limestone aquifer of northern England, UK. Hydrogeology Journal 20, 483–498.

Hartmann A, Goldscheider N, Wagener T, Lange J, Weiler M., 2014. Karst water resources in a changing world: Review of hydrological modeling approaches. Reviews of Geophysics 52(3):218-42.