Constraining coastal aquifer models by ...

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XVI International Conference on Computational Methods in Water Resources (CMWR-XVI) Ingeniørhuset

Constraining coastal aquifer models by hydrogeophysical imaging of seawater intrusion dynamics: a case study from the Lower Andarax delta, SE Spain
Author:Richard Ogilvy <rdo@bgs.ac.uk> (British Geological Survey)
Oliver Kuras <oku@bgs.ac.uk> (British Geological Survey)
Frederic Nguyen <f.nguyen@fz-juelich.de> (Institute of Chemistry and Dynamics of the Geosphere)
Andreas Kemna <a.kemna@fz-juelich.de> (Institute of Chemistry and Dynamics of the Geosphere)
Philip Meldrum <pime@bgs.ac.uk> (British Geological Survey)
Juan Gisbert <jgisbert@ual.es> (University of Almeria)
Sara Jorreto <sjorreto@ual.es> (University of Almeria)
Francisco Sanchez Martos <fmartos@ual.es> (University of Almeria)
Antonio Pulido Bosch <apulido@ual.es> (University of Almeria)
Peter Engesgaard <pe@geol.ku.dk> (University of Copenhagen)
Arni Antonsson <arni@geol.ku.dk> (University of Copenhagen)
Karsten Jensen <khjgeol.ku.dk> (University of Copenhagen)
Presenter:Oliver Kuras <oku@bgs.ac.uk> (British Geological Survey)
Date: 2006-06-18     Track: Special Sessions     Session: Hydrogeophysical data fusion
DOI:10.4122/1.1000000489

Appropriate characterisation, modelling and management of coastal aquifers requires knowledge of the dynamic balance between freshwater and saltwater present in the aquifer as a result of natural marine intrusion or other salinisation processes of geological or anthropogenic origin. Monitoring this balance is essential for the timely detection and prediction of groundwater deterioration and the sustainable management of abstraction schemes. Recent advances in hydrogeophysical methods allow more accurate non-invasive assessments of intrusion dynamics and evolution, thus providing vital input to the generation and refinement of conceptual and numerical hydraulic models of coastal aquifers. The purpose of this study of the Lower Andarax delta in the Andalucía region of southeast Spain is to combine time-lapse electrical resistivity tomography (ERT) with conventional intrusive methods (borehole sampling, wireline and core logging) and hydraulic tests in order to (1) image the saline/freshwater interface, (2) obtain an improved understanding of the intrusion process and its seasonal variation and (3) generate detailed and realistic models of a coastal aquifer. The Andarax river basin is an alluvial valley situated in one of the most arid regions in Europe. Its infrequent and irregular precipitation determines the regime of land use and groundwater abstraction. The Lower Andarax catchment serves as a test area for the project “Sustainable Management of Water Resources by Automated Real-time Monitoring (ALERT)” under the European Union Sixth Framework Programme, which aims to develop strategies for monitoring and managing vulnerable coastal zones. A number of manual ERT surveys conducted in the Andarax riverbed over a period of several months have revealed the shape of the mixing zone at high resolution. Present-day seawater intrusion appears to be confined to the shallow Quaternary part of the aquifer, where a sloping front has been detected down to depths of 70- 80 m over a distance of approximately 1300 m from the shore. Early indications show that this front is subject to seasonal change. Deeper formations of Pliocene age appear to store significant amounts of saline water, however the results suggest that these formations may exhibit much lower permeabilities and that therefore the deeper regions of the aquifer may remain static over the timescale of the observations. The manual surveys will be followed by the permanent deployment of an automated monitoring system during 2006. This system will provide volumetric images of the subsurface at regular intervals or “on demand”, thereby obviating the need for expensive repeat surveys and manual intervention. Using calibrated geophysical- hydrological relationships, the geophysical monitoring data will be integrated with hydraulic data obtained from newly drilled exploratory boreholes as well as a range of existing freshwater abstraction wells.