Geochemical indicators of deep groundwater movements at Sellafield, UK A. Bath a, * , H. Richards b , R. Metcalfe c , R. McCartney d , P. Degnan e , A. Littleboy e,1 a Intellisci, 104 Main Street, Willoughby-on-the-Wolds, Loughborough LE12 6SZ, UK b 9 New Road, Dursley, Gloucestershire GL11 6PN, UK c Quintessa K.K., 303 1090-3 Terakawado-cho, Mizunami-shi, Gifu 509-6121, Japan d GeoScience, Falmouth Business Park, Bickland Water Road, Falmouth TR11 4SZ, UK e UK Nirex, Curie Avenue, Harwell, Didcot OX11 0RH, UK Received 1 December 2003; accepted 13 September 2005 Available online 30 January 2006 Abstract Boreholes were drilled in an area of 52 km 2 at Sellafield, on the coast of northwest England, to investigate groundwater conditions in Permo-Triassic sedimentary rocks and underlying Ordovician metavolcanic basement. The basement rocks are fractured with low permeability and were being investigated as a potential host for a nuclear waste repository until the project was stopped in 1997. Water samples were collected by pumping from discrete intervals in 19 deep boreholes to a maximum depth of 1950 m. Chemical analyses of the water samples showed a range of salinities from fresh/brackish to brine and, in addition, various isotopic analyses were carried out. Data for Na, K, Ca, Mg, Sr, Cl, SO 4 , HCO 3 , Br, 18 O/ 16 O, 2 H/ 1 H, 3 H, 14 C, 13 C/ 12 C, 36 Cl/Cl, 87 Sr/ 86 Sr, 34 S/ 32 S, 4 He and other inert gas contents are reported here for 16 of the boreholes. The reliabilities of most of these data as representative of in situ conditions are considered to be high, although pH, alkalinity and tritium are less reliable because of contamination with drilling fluid, and carbon isotopic data are definitely erroneous. The groundwater system in the sedimentary cover rocks and basement formation comprises water masses with different origins, ages and salinities. Fresh water in the sedimentary rocks, typically to 350 m depth in the centre of the area, was mostly recharged during the Holocene. Fresh and brackish waters, which occur locally at greater depth (about 700 m), have stable isotopic compositions that indicate cold climate recharge, probably in the late Pleistocene. Saline water in the basement formation contains meteoric water that recharged during the Pleistocene, i.e. between 10 ka and 2 Ma ago. Brine in offshore basinal sedimentary rocks, west of the investigated area, is more than 2 Ma old and probably originated as meteoric recharge during the Tertiary period, becoming saline by dissolution of halite. The most likely origin of salinity in groundwaters in basement rocks here is that it is primarily evolved from basinal brine that migrated from overlying sedimentary rocks before Tertiary erosion. Saline groundwater in the basement is now chemically distinct from the basinal brine, for example having a higher Br/Cl ratio. Mixtures between the groundwater types occur in salinity transition zones between the major water masses. The transition zone between fresh/brackish and saline water in the centre of the area is a few tens of metres thick with a steep salinity gradient and is located at the boundary between sedimentary cover rocks and basement. The transition between saline basement water and basinal brine near to the coast is 500 m wide. 36 Cl data suggest that groundwater movements and mixing of solutes at this transition have occurred within the last 1.5 Ma. 0375-6742/$ - see front matter D 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.gexplo.2005.09.003 * Corresponding author. Tel./fax: +44 1509 889229. E-mail addresses: abath@intellisci.co.uk (A. Bath), hrichards@anhaga.freeserve.co.uk (H. Richards), metcalfe@ns.quintessa.co.jp (R. Metcalfe), mccartney@geoscience.co.uk (R. McCartney), paul.degnan@nirex.co.uk (P. Degnan), anna.littleboy@csiro.au (A. Littleboy). 1 Present address: CSIRO Exploration and Mining Division, Brisbane 4069, Australia. Journal of Geochemical Exploration 90 (2006) 24 – 44 www.elsevier.com/locate/jgeoexp