Analysis of the migration of Cl, 131I and 3H in a shallow sand aquifer

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Nuclear energy and safety

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• Publié par: EUROPEAN-COMMISSION-DIRECTORATE-GENERAL-FOR-THE-INFORMATION-SOCIETY-AND-MEDIA-DI
• Nombre de lectures: 15
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Commission of the European Communities
nuclear science
and technology
Analysis of the migration of CI, 131l and 3H
in a shallow sand aquifer
Report
EUR 13635 EN S1
Commission of the European Communities
nuclear science
and technology
Analysis of the migration of CI, 131l and 3H
in a shallow sand aquifer
M. A. Sen, G. M. Williams, G. P. Wealthall
British Geological Survey
Fluid Processes Research Group
Keyworth, Nottingham NG12 5GG
United Kingdom
Topical report
Work carried out under cost-sharing contract
No FI1W/0064 UK with the European Atomic Energy Community,
in the framework of its third R&D programme on 'Management
and storage of radioactive waste', Part A, Task 4
'Geological disposal studies'
Directorate-General
Science, Research and Development
PAU. rUîîCP Eiblioth.
1991 NC EUR 13635 EN Published by the
COMMISSION OF THE EUROPEAN COMMUNITIES
Directorate-General
Telecommunications, Information Industries and Innovation
L-2920 Luxembourg
LEGAL NOTICE
Neither the Commission of the European Communities nor any person acting
on behalf of then is responsible for the use which might be made of
the following information
Cataloguing data can be found at the end of this publication
Luxembourg: Office for Official Publications of the European Communities, 1991
ISBN 92-826-2992-9 Catalogue number: CD-NA-13635-EN-C
© ECSC-EEC-EAEC, Brussels · Luxembourg, 1991
1 Printed in France PREFACE
This study lies within the MIRAGE Π research programme (Migration of RAdionuclides in
the GEosphere) set up by the Commission of the European Communities and supported by
the UK Department of the Environment The work forms part of the project entitled "In situ
determination of the effects of organics on the mobility of radionuclides in controlled
conditions of groundwater flow" which is being carried out by the British Geological Survey
at Drigg in Cumbria, on land owned by British Nuclear Fuels Pic. The study involves the
detailed geochemical and hydrogeologicai characterisation of a confined aquifer of glacial
sand, the laboratory scale investigation of radionuclide sorption processes and how these are
affected by the presence of natural and anthropogenic organic compounds. Ultimately the
results of field hydraulic testing and laboratory studies of radionuclide sorption will be used
to predict the outcome of a field tracer experiment using conservative and reactive
radionuclide species.
This report deals with the validation of the conservative tracers chloride (as NaCl), iodine -
131 (as KI), and tritium (as HTO), and includes the analysis of the hydraulic properties of
the aquifer in which the borehole array has been located.
Undertaking tracer tests is impossible without the help of a team of colleagues. Thanks are
given to the following who provided essential support in the field:-G. P. Wealthall,
M. Brightman, A. J. Cook, D. Haigh, S. P. Hitchman and P. D. Roberts.
The authors are also grateful to British Nuclear Fuels pic for permission to use the Drigg site
and the UK Department of the Environment and the Commission of the European
Communities for funding the work.
Other reports within the series include:-
Williams, G. M., Alexander, L. S., Hitchman, S. P., Hooker, P. J., Noy, D. J., Ross,
C. A. M., Stuart, A. and West, J. M., 1985. In situ radionuclide migration studies in a
shallow sand aquifer (Part 1). Report of the British Geological Survey, Fluid Processes
Research Group, FLPU 85 - 7.
Williams, G. M., Alexander, L. S., Hitchman, S. P., Hooker, P. J., Noy, D. J., Ross,
C. A. M., Stuart, A. and West, J. M., 1985. In situ radionuclide migration studies in a
shallow sand aquifer (Part 2, appendices). Report of the British Geological Survey, Report
No. FLPU 85 - 10.
m Williams, G. M., Hooker, P. J., Ross, C. Α. Μ., Noy, D. J., Cook, Α., Hitchman, S. P.,
Wealthall, G. P., 1986. In situ radionuclide migration studies at Drigg April 1985-March
1986. Report of the British Geological Survey, Fluid Processes Research Group FLPU
86-4.
Warwick, P., Shaw, P., Williams, G. M. and Hooker, P. J., 1988. Preliminary studies of
cobalt complexation in groundwater. British Geological Survey Fluid Processes Research
Group Report No.WE/88/12, also in Radiochemica Acta, 44/45,59 - 63.
Peachey, D. and Williams, G. M., 1987. Characterisation of humic material for inter-
laboratory comparison. British Geological Survey Fluid Processes Research Group Report
No. FLPU 87-5. 19pp.
West, J. Μ. , 1988. The influence of microbial activity on the degradation of acetate and
EDTA. British Geological Survey Fluid Processes Research Group Report No.WE/88/40.
Haigh, D. G., Williams, G. M., Hooker, P. J., Ross, C. A. M., 1988. The influence of
organics on the sorption of cobalt by glacial sand in laboratory batch experiments. British
Geological Survey Fluid Processes Research Group Report No.WE/88/20.
Falck, W. E., 1988. Modelling the interaction between natural organic matter and metal
cations: a review. British Geological Survey Fluid Processes Research Group Report
No. WE/88/49.
Falck, W. E., Quinn, G. W, Duffield, J. R., Williams, D. R., 1988. Chemical speciation
modelling studies on groundwater in a shallow glacial sand aquifer. PART 1: General
principles. British Geological Survey Fluid Processes Research Group Report
No. WE/88/48.
Haigh, D. G., Higgo, J. J. W, Williams, G. M., Ross, C. A. ML, Falck, W. E., Allen,
M. A. and Warwick, P., 1989. The influence of organics on the sorption of Np, Eu, U, I,
Sr, and Cs by glacial sand in laboratory batch experiments. British Geological Survey Fluid
Processes Research Group Report No.WE/89/16.
Longworth, G., Wilkins, M. A. and Ivanovitch, M., 1988. Role of geocolloids in transport
of naturally occurring radionuclides. MRS conference proceedings, Berlin, October 1988.
IV EXECUTIVE SUMMARY
Two tracer tests with conservative species have been carried out in a shallow glacial sand
aquifer at the BGS in-situ research site at Drigg in Cumbria. Groundwater is re-circulated
between two boreholes 3.41 m apart and the tracer(s) are released in a third "on-line"
borehole 0.94 m from the recharge well. The movement of tracers is monitored by removing
groundwater from three multi-level sampler installations, using a vacuum extraction
manifold. Radio-tracer migration is also monitored by means of three gamma probes on
automatic ranging winches controlled by micro-computer, and connected to a multi-channel
analyser.
The migration of inactive chloride (as NaCl) was used to provide initial information on the
hydraulics of the array and to test the safety of the instrumentation and experimental protocol,
prior to using radiotracers. Iodine -131 (as KI) and tritium (as HTO) were subsequently
introduced under the same hydraulic conditions as used for the chloride test. The objectives
of the tests were to determine the transport properties of the aquifer, to compare the use of the
three 'conservative' tracers and to provide a reference for further experiments involving
chemically reactive solutes.
The flow system generated by the pumping/recharge well duplet was initially modelled using
an analytical solution for a homogeneous aquifer. This showed that, in the vicinity of the
tracer release and monitoring wells, parallel flow lines could be expected. Consequently 1-D
and 2-D models can be used to investigate the groundwater flow and solute migration rates.
As data on the hydraulic structure of the aquifer were sparse a 2-D layered model was used
to estimate thec conductivity of each layer from the tracer breakthroughs. A 1-D
analytical solution to the advection-dispersion equation was used to estimate the dispersion
lengths of each layer.
A comparison of the hydraulic conductivity values estimated from the breakthrough times
with those from directc tests shows a similar structure for both; a high conductivity
zone sandwiched between two low conductivity zones. However, the actual values of the
breakthrough estimated conductivities were several times higher than the hydraulic test
results. There are a number of possible explanations for this including borehole disturbance
increasing the conductivity in the line of the array. Head measurements in the line of the
array recorded in future experiments should help to resolve this issue.
Longitudinal dispersion lengths ranged from 0.5cm to 5cm. The chloride values are possibly
higher than the iodide values on average but, considering the fitting method and goodness of the fits, no more detailed conclusions should be drawn than that the dispersion lengths are of
the order of a centimetre and 50-150 times less than the path length.
The chloride breakthrough curves from the first test are similar to the iodide and tritium
profiles from the second test. There are some differences, however, implying that the aquifer
properties are slightly altered by the running of the tests. Thus future tests with reactive
tracers should be carried out with simultaneous injection of a conservative tracer for
comparison. The iodide and tritium breakthrough curves have almost identical shapes so that
all three tracers, chlo