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Application of mineralogical, petrological and geochemical tools for evaluating the palaeohdrogeological evolution of the PADAMOT study sites

By A.E. Milodowski, E.L. Tullborg, B. Buil, P. Gomez, M.-J. Turrero, S. Haszeldine, G. England, M.R. Gillespie, T. Torres, J. Ortiz, J. Zacharias, J. Silar, M. Chvatal, L. Strnad, O. Sebek, J.E. Bouch, S.R.N. Chenery, C.A. Chenery and T.J. Shepherd

Abstract

The role of Work Package (WP) 2 of the PADAMOT project – ‘Palaeohydrogeological Data\ud Measurements’ - has been to study late-stage fracture mineral and water samples from\ud groundwater systems in Spain, Sweden, United Kingdom and the Czech Republic, with the aim\ud of understanding the recent palaeohydrogeological evolution of these groundwater systems. In\ud particular, the project sought to develop and evaluate methods for obtaining information about\ud past groundwater evolution during the Quaternary (about the last 2 million years) by examining\ud how the late-stage mineralization might record mineralogical, petrographical and geochemical\ud evidence of how the groundwater system may have responded to past geological and\ud climatological changes.\ud Fracture-flow groundwater systems at six European sites were studied:\ud • Melechov Hill, in the Bohemian Massif of the Czech Republic: a shallow (0-100 m)\ud dilute groundwater flow system within the near-surface weathering zone in fractured\ud granitic rocks;\ud • Cloud Hill, in the English Midlands: a (~100 m) shallow dilute groundwater flow system\ud in fractured and dolomitized Carboniferous limestone;\ud • Los Ratones, in southwest Spain: an intermediate depth (0-500 m) dilute groundwater\ud flow system in fractured granitic rocks;\ud • Laxemar, in southeast Sweden: a deep (0-1000 m) groundwater flow system in fractured\ud granitic rocks. This is a complex groundwater system with potential recharge and\ud flushing by glacial, marine, lacustrine and freshwater during the Quaternary;\ud • Sellafield, northwest England: a deep (0-2000 m) groundwater flow system in fractured\ud Ordovician low-grade metamorphosed volcaniclastic rocks and discontinuous\ud Carboniferous Limestone, overlain by a Permo-Triassic sedimentary sequence with\ud fracture and matrix porosity. This is a complex coastal groundwater system with deep\ud hypersaline sedimentary basinal brines, and deep saline groundwaters in crystalline\ud basement rocks, overlain by a shallow freshwater aquifer system. The site was glaciated\ud several times during the Quaternary and may have been affected by recharge from glacial\ud meltwater;\ud • Dounreay, northeast Scotland: a deep (0-1400 m) groundwater flow system in fractured\ud Precambrian crystalline basement overlain by fractured Devonian sedimentary rocks.\ud This is within the coastal discharge area of a complex groundwater system, comprising\ud deep saline groundwater hosted in crystalline basement, overlain by a fracture-controlled\ud freshwater sedimentary aquifer system. Like Sellafield, this area experienced glaciation\ud and may potentially record the impact of glacial meltwater recharge.\ud In addition, a study has been made of two Quaternary sedimentary sequences in Andalusia in\ud southeastern Spain to provide a basis of estimating the palaeoclimatic history of the region that\ud could be used in any reconstruction of the palaeoclimatic history at the Los Ratones site:\ud • The Cúllar-Baza lacustrine sequence records information about precipitation and\ud palaeotemperature regimes, derived largely from the analysis of the stable isotope (δ18O\ud and δ13C) signatures from biogenic calcite (ostracod shells).\ud • The Padul Peat Bog sequence provided information on past vegetation cover and\ud palaeogroundwater inputs based on the study of fossil pollen and biomarkers as proxies\ud for past climate change.\ud Following on from the earlier EC 4th Framework EQUIP project, the focus of the PADAMOT\ud studies has been on calcite mineralization. Calcite has been identified as a late stage mineral, closely associated with hydraulically-conductive fractures in the present-day groundwater\ud systems at the Äspö-Laxemar, Sellafield, Dounreay and Cloud Hill sites. At Los Ratones and\ud Melechov sites late-stage mineralization is either absent or extremely scarce, and both the\ud quantity and fine crystal size of any late-stage fracture mineralization relevant to Quaternary\ud palaeohydrogeological investigations is difficult to work with. The results from the material\ud investigated during the PADAMOT studies indicate that the fracture fillings at these sites are\ud related to hydrothermal activity, and so do not have direct relevance as Quaternary indicators.\ud Neoformed calcite has not been found at these two sites at the present depth of the investigations.\ud Furthermore, the HCO3\ud - concentration in all the Los Ratones groundwaters is mainly controlled\ud by complex carbonate dissolution. The carbonate mineral saturation indices do not indicate\ud precipitation conditions, and this is consistent with the fact that neoformed calcite, ankerite or\ud dolomite have not been observed petrographically

Topics: Earth Sciences
Publisher: UK Nirex Ltd
Year: 2005
OAI identifier: oai:nora.nerc.ac.uk:11494

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