Identifying groundwater contributions to baseflow forms an essential part of surface water body characterisation. The Gortinlieve catchment (5 km2) comprises a headwater stream network of the Carrigans River, itself a tributary of the River Foyle, NW Ireland. The bedrock comprises poorly productive metasediments that are characterised by fracture porosity. We present the findings of a multi-disciplinary study that integrates new hydrochemical and mineralogical investigations with existing hydraulic, geophysical and structural data to identify the scales of groundwater flow and the nature of groundwater/bedrock interaction (chemical denudation). At the catchment scale, the development of deep weathering profiles is controlled by NE-SW regional scale fracture zones associated with mountain building during the Grampian orogeny. In-situ chemical denudation of mineral phases is controlled by micro- to meso-scale fractures related to Alpine compression during Palaeocene to Oligocene times. The alteration of primary muscovite, chlorite (clinochlore) and albite along the surfaces of these small-scale fractures has resulted in the precipitation of illite, montmorillonite and illite-montmorillonite clay admixtures. The interconnected but discontinuous nature of these small-scale structures highlights the role of larger scale faults and fissures in the supply and transportation of weathering solutions to/from the sites of mineral weathering. The dissolution of primarily mineral phases releases the major ions Mg, Ca and HCO3 that are shown to subsequently form the chemical makeup of groundwaters. Borehole groundwater and stream baseflow hydrochemical data are used to constrain the depths of groundwater flow pathways influencing the chemistry of surface waters throughout the stream profile. The results show that it is predominantly the lower part of the catchment, which receives inputs from catchment/regional scale groundwater flow, that is found to contribute to the maintenance of annual baseflow levels. This study identifies the importance of deep groundwater in maintaining annual baseflow levels in poorly productive bedrock systems.
Bibliographical noteMark Russell is thanked for analytical assistance with powder XRD analysis and the discussion of results. Robbie Goodhue kindly provided access to the TCD collection of Clay Mineral Society Source Clays. John Meneely and Pat McBride from the School of Geography, Archaeology and Palaeoecology (QUB) are thanked for assistance with the separation of the < 2 μm clay fraction and the preparation of polished thin sections respectively. John Finn and Zuansi Cai provided help with sampling during the 2013 field season. Marie Archbold and Alison Orr provided useful information on sampling and sample details for 2010–2012 baseflow monitoring. Discussion of Gortinlieve bedrock geology with Janka Nitsche and Katarina Pilatova helped to identify suitable field outcrops. J.C. gratefully acknowledges the support of the Queen's University Researcher Training and Development Fund for the FTIR analyses. The comments of three anonymous reviewers helped to improve the clarity of the manuscript. Damia Barcelo is thanked for his editorial assistance. This study was undertaken as part of a research grant funded by the Irish Department of Communications, Energy and Natural Resources under the National Geoscience Programme 2007–2013. The views expressed are the authors' own and do not necessarily reflect the views and opinions of the Minister for Communications, Energy and Natural Resources.
- poorly productive bedrock aquifers
- fracture porosity