A new method of combined techniques for characterization and monitoring of seawater interface in an alluvial aquifer
Abstract
Understand the dynamics of the fresh-salt water interface in aquifers is a key issue to comprehend mixing process
and to quantity the discharge of nutrients in to coastal areas. In order to go beyond the current knowledge in
this issue an experimental site has been set up at the alluvial aquifer Riera Argentona (Barcelona – Spain). The
site comprises 16 shallow piezometers installed between 30 and 90 m from the seashore, with depths ranging
between 15 and 25 meters. The seawater interface is being monitored using several techniques, the combination
of which will help us to understand the spatial and temporal behaviour of the mixing zone and the geochemical
processes occurring there. Specially the deepest piezometers are equipped with electrodes in order to perform
cross-hole electrical resistivity tomography (CHERT). In addition, all piezometers are also equipped with Fiber
Optic cable to perform distributed temperature measurements. Two single steel armoured fibre optic cable lines of
around 600m length were installed in all boreholes. The objective is to use the cable both as passive and active
temperature sensor. The first is being done for the continuous monitoring of temperature whereas; the second
provides a higher temperature resolution used to monitor field experiments.
Periodic CHERT measurements are carried out between the piezometer equipped with electrodes, resulting
in parallel and perpendicular vertical cross sections of the site resistivity. The position of the fresh-salt water
interface can be identified due to the resistivity contrast between the saline and fresh water. Preliminary results
of periodic distributed temperature measurements will be also be used to monitor the position of the mixing zone
thanks to the contrast and seasonal temperature changes. Periodic down-hole EC profiles will be used to validate
the method.
Acknowledgements
This work was funded by the projects CGL2013-48869-C2-1 y CGL2013-48869-C2-2-R of the Spanish Government.
We would like to thank SIMMAR (Serveis Integrals de Manteniment del Maresme) and the Consell
Comarcal del Maresme in the construction of the research site