Constraining coastal aquifer models by hydrogeophysical imaging of seawater intrusion dynamics: a case study from the Lower Andarax delta, SE Spain
Creators: name: Ogilvy, Richard
name: Kuras, Oliver
name: Nguyen, Frederic
name: Kemna, Andreas
name: Meldrum, Philip
name: Gisbert, Juan
name: Jorreto, Sara
name: Sanchez Martos, Francisco
name: Pulido Bosch, Antonio
name: Engesgaard, Peter
name: Antonsson, Arni
name: Jensen, Karsten
name: Kuras, Oliver
Publisher: name: XVI International Conference on Computational Methods in Water Resources
Year: 2006

description: 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.