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Use of electromagnetic waves to detect the intrusion of sea water in the aquifer of the Maneadero Valley in Ensenada, Baja California

Violeta LUJAN FLORES student laureate

°1980 Mexico
Master’s in Applied Geophysics, Center for Scientific Research and Higher Education of Ensenada, Mexico, 2006

Utilización de ondas electromagnéticas para detectar la invasión de agua marina en el acuífero del Valle de Maneadero en Ensenada, B.C.

Throughout the world sea water intrusion poses a major threat to fresh groundwater reserves in coastal areas, where the water requirements of the usually dense populations lead to the heavy utilization of groundwater. Violeta Lujan Flores’s thesis was written in the context of this problematic situation and aims to contribute to a solution.
The area studied is the Maneadero Valley in the north of Baja California (Mexico), where the phreatic aquifer is in hydraulic contact with the Pacific. The groundwater in this area is of vital importance to agrarian activity (intense horticulture, which supplies the whole country) as well as providing the drinking water of the nearby town of Ensenada: 40% of Ensenada’s drinking water comes from the groundwater of this valley. The flourishing agrarian sector attracts many farmers from the south of the country.
Sea water intrusion in the aquifer already affects an area of 15,000 hectares of agricultural land. If irrevocable damage to the development of this area is to be avoided, it is essential to halt this intrusion, by the judicious control of groundwater use. Geophysical techniques, such as electromagnetic prospection, can be useful here for the accurate mapping of the present situation as well as for following up the situation as it evolves (the effects of measures taken, for example).
In her thesis Violeta Lujan Flores has measured four electromagnetic (audiomagnetotelluric) profiles and has developed a method of interpretation, making use of the four elements of the impedance tensor, from which series and parallel impedances were derived. A 2D inversion was then used to produce a 2D substrate resistivity model. The geological structure corresponds to the known structural information. Salinization is clearly revealed in the low resistivity values. Using Archie’s Law the salinity of the groundwater was assessed; measurements on groundwater from various wells provided confirmation of the salinity obtained by geophysical techniques.

Report: Prof. K. Walraevens, Applied Geology and Hydrogeology Laboratory, Department of Geology and Soil Science, Universiteit Gent, Belgium