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A strategic programme for NERC Lowland catchment research
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Contaminant attenuation - a radiochemical approach

Title

Contaminant attenuation in Chalk groundwater - A new approach using radiochemistry.

Overview of project

The aim of this NERC studentship was to investigate a potential radiochemical method for estimating the Chalk aquifer's capacity to attenuate contaminants. Contaminant solutes are advected by groundwater flow through fractures, but are slowed and attenuated by molecular diffusion into immobile water in the pores of the Chalk. Fracture apertures are the key factor controlling both advection and diffusion effects.

In principle apertures may be estimated by comparing dissolved radon gas in fracture water with uranium-series isotope activities in the rock matrix, as radon release and contaminant attenuation are both governed by similar mechanisms of molecular diffusion. The project tested the robustness of a radon-derived transport model developed by Atkinson and Barker through a series of lab experiments and field observations. The results were compared against other more traditional tracer experiments.

Aims

  • To test the assumption that the Chalk aquifer possesses a homogeneous distribution of radon precursors, by determining uranium and radium in core material from the Pang/Lambourn LOCAR boreholes.
  • To test the assumption that double-porosity behaviour dominates for solute transport and can be characterised by an effective diffusion time, despite the heterogeneity of the fracture flow.
  • To interpret measurements of radon concentrations made across the LOCAR catchments in terms of the contaminant attenuation capacity of the Chalk.

Main findings

  • The uranium content of the chalk is not uniform but shows litho-stratigraphic variations over vertical intervals of a few meters. In addition radium, the immediate precursor of radon, is sometimes out of equilibrium with its precursor uranium-238
  • The radon content of groundwater evolves in a complex fashion that is a function of the duration and rate of pumping and is not in general representative of a specific layer of chalk. To establish the relationship between radon production in the chalk matrix and radon activity in groundwater it is necessary to take water samples from packered intervals for which radium assays of chalk are also available.
  • There is significant variation in the calculated diffusion times calculated by our U-Ra model, due primarily to the variation in uranium concentration, the 226Ra/238U activity ratio within each core section and the high degree of uncertainty in the rate of radon emanation.
  • The calculated values of characteristic diffusion times are much larger than those determined from other tracer-derived tests that have been undertaken in similar Chalk aquifers.
  • These findings suggest that it may be difficult to deduce reliable transport parameters by routine application of our current model, because each site would require detailed characterisation.

Description of activities

Dissolved radon concentrations have been measured at approximately monthly intervals at ten spring locations and six borehole locations during 2005-2007. It is evident there are both differences in magnitude between individual locations, and that there is a consistent temporal trend throughout the year, with activity peaking in the summer months.

In addition a more detailed vertical profiling of a LOCAR borehole using a packer system was undertaken in summer 2006. Radon samples were also obtained during a much larger pump test from an adjacent production borehole.

The uranium concentration of a 100 m section of Seaford Chalk has been assayed at a 1m resolution by using a luminescence spectrometer. Further analysis of a much smaller section of core was undertaken at a much finer (cm) scale. These data were compared with radium activity measurements assayed by measuring the radon ingrowth of solid Chalk samples (by liquid scintillation techniques). It is clear that both radionuclides are subject to litho-stratigraphic control and that both the concentration and isotope activity ratio are subject to significant variation.

From our work on subsets of solid and disaggregated core material it is also clear that the radon emanation rate from chalk particles to the matrix pores cannot be assumed to be 100%. In fact for silt particles (~63 µm) average rates of about 20% are more common.

Areas of application

Groundwater protection is an integral part of the EU Water Framework Directive and Groundwater Daughter Directive.

Related work

Continuing work at University College, London investigating the dual porosity behaviour of the Chalk aquifer with regards to modelling and prediction of contaminant attenuation of other dissolved species.

Researchers' details

Main contact:

Publications

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

PhD thesis

Quinn, S. Contaminant attentuation in chalk groundwater: a new approach using radiochemistry. PhD thesis, University College London.

Recommended reading

Andrews, J. N. and D. F. Wood, Mechanism of radon release in rock matrices and entry into groundwaters, Transactions of the Institution of Mining and Metallurgy, B81, 198-209, 1972.

Atkinson, T.C., Barker, J.A., Ward, R.S. & Low, R.G., 2001. Radon : An indicator of solute transport in double porosity aquifers. In Seiler, K.-P. & Wohnlich, S. (eds.), New Approaches Characterising Groundwater Flow. Proc. XXXI Congr. Int. Ass. Hydrogeol., Munich, September 2001, 1, 441-445.

Barker, J. A. and S. S. D. Foster, A Diffusion Exchange Model for Solute Movement in Fissured Porous Rock, Quarterly Journal of Engineering Geology, 14(1), 17-24, 1981.

Cuttell, J. C., J. W. Lloyd and M. Ivanovich, A Study of Uranium and Thorium Series Isotopes in Chalk Groundwaters of Lincolnshire Uk, Journal of Hydrology, 86(3-4), 343-365, 1986.

Younger, P. L. and T. Elliot, Chalk Fracture System Characteristics - Implications for Flow and Solute Transport (Vol 28, Pg S39, 1995), Quarterly Journal of Engineering Geology, 28, 199, 1995, including subsequent Discussion, M. Price, QJEG, 29, 93-94, 1996.

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