Personnel: Jared West, Simon Bottrell, Noelle Odling, Samuel Allshorn
Project period: 3 years from October 2004
Groundwater represents a high proportion of the public, industrial and agricultural water supply in many parts of England and is a vital resource in many other countries, but its quality is increasingly under threat from rising levels of pollution. Protection of groundwater resources requires better understanding of unsaturated zone processes, which can significantly influence recharge water quality. The upper, unsaturated zone of groundwater aquifers can provide a protective buffer, in which pollutants may be delayed, degraded or attenuated before reaching the water table. This protection is much diminished if a proportion of recharge 'short circuits' to the saturated zone as rapid flow via high permeability pathways such as fractures. Understanding flow behaviour in their unsaturated zones is vital to predicting their vulnerability to pollution.
The
occurrence of occasional rapid unsaturated flow in the Chalk in Yorkshire
is confirmed by the presence of short half-life contaminants such as pesticides
and pathogenic organisms in water abstracted from supply wells. However,
no attempt has been made to estimate the groundwater travel time spectrum
for the unsaturated zone of the Chalk. Other important factors, such as
the seasonal variability of unsaturated zone travel times, and the relation
between the spectrum of travel times and the fracture network characteristics
have also never been addressed. Resolving these issues will allow future
trends in groundwater quality in the Chalk aquifer, and the potential impact
of changes in climate and land use.
This project will be centred around characterisation of the groundwater flow, storage and pollutant transport behaviour of the unsaturated zone of fractured rock aquifers by sampling groundwaters deep within the unsaturated zone. Samples will be obtained from railway tunnels. Samples will be analysed for artificial tracers such as the optical brightener Photine-CU and fluorescein, introduced at the ground surface, and also for the stable isotopes 180 and/or 2H, as well as major ions and nitrate. Artificial tracers will be used firstly to define the catchment areas of the underground sampling points, and secondly, in conjunction with environmental tracers, to determine unsaturated zone residence times and storage characteristics. The amounts of 180 and 2H in rainfall vary seasonally, and comparison their variation in rainfall and at sample collection points within the unsaturated zone will allow determination of groundwater residence times and mixing (dispersive) behaviour. The natural isotopic tracers will provide complementary data to that from the artificial tracers because they are introduced across the ground surface rather than at discrete points. Finally, the results of the tracer experiments will be combined with observations of the fracture network in the Chalk in discrete fracture/permeable matrix flow modelling.
References
Barker, A P; Newton, R J; Bottrell, S H; Tellam, J H 1998. Processes affecting
groundwater chemistry in a zone of saline intrusion into an urban sandstone
aquifer. Applied Geochemistry, 13, 735-749,
Bottrell, S.H. and Atkinson, T.C. 1992. Tracer study of flow and storage
in the unsaturated zone of a karstic limestone aquifer. In 'Tracer Hydrology'
Hotzl and Werner (Eds), Balkema, 207- 211.
Bottrell, S.H. Leosson, M.A. and Newton, R.J. 1996. The origin of brine
inflows at Boulby potash mine, UK. Transactions of the Institution of Mining
and Metallurgy (sect. B) 105: B159-B164.
Odling, N.E. and Webman, I. 1991. A 'Conductance' Mesh approach to the permeability
of natural and simulated fracture patterns. Water Resources Research, 27,
2633-2643.
Odling N.E. and Roden J. 1997. Contaminant transport in fractured porous
rocks using natural fracture geometries. Journal of Contaminant Hydrology,
27, 263-283.
Slater, L., Zaidman, M.D., Binley, A.M. and West, L.J., 1997. Electrical
imaging of saline tracer migration for the investigation of unsaturated
zone transport mechanisms. Hydrology and Earth System Sciences, 1. 291-302.
Zaidman, M.D. Middleton R.T., West L.J., Binley A.M. 1999. Geophysical investigation
of unsaturated zone transport in the Chalk in Yorkshire. Quarterly Journal
of Engineering Geology, 29,185-198