Evolution of the Marine Intrusion Using Geophysical Methods after 25 years in the Motril-Salobreña Aquifer (Southern Spain) C. Duque, M.L. Calvache, A. Pulido- Bosch, A. González Ramón, J. C. Rubio, J.A. Navarro, M. López- Chicano and W. Martín-Rosales University of Granada. Spain

Location Very high desertification risk Mediterranean Sea

Human activity affecting groundwater in the last 25 years Recent contruction of a dam Agriculture activities as crops with a lot of irrigation demand, irrigation channels and greenhouses Changes in land uses (contruction of golf courses and associated urbanization) The influence of the river and the irrigation return has been estimated over 60 % of the yearly inputs to the aquifer!

Problems for detecting the location of the saline wedge No boreholes with enough depth near the coastline The quatenary sediments of the coastal aquifer make drilling very expensive The coastal aquifer is more than 200 m thick near the coast (using gravimetry) Geophysics is a good option

Geophysical surveys In 1981 the first geophysical study for detecting the saline wedge was carried out using Vertical Electric Soundings (VES) In 2006 a new one was developed with Time Domain Electromagnetic Soundings (TDEM) together with a Gravimetry survey

Combination of gravimetry and TDEM in 2006

Geophysical surveys

Comparing results The result was very similar, so the comparison is possible Marine intrusion was not detected in 1981 due to the limitation of VES In 2006 TDEM soundings allowed us to increase the prospecting depth and reach the saline wedge

Comparing results In profile B it was possible to detect a low resistivity zone in both geophysical surveys The shape of this zone is interpreted as saltwater intrusion in 1981, but they did not have the information related with the gravimetry survey (2006). This could be a lower permeability area that avoided flushing by freshwater So it looks like a situation without changes in the last 25 years

Comparing results A detailed comparison of the measurements near the sea shows that the main differences can be related to the improvement of the geophysical techniques

Water table evolution It appears there are no changes in the last 25 years related with salt water intrusion The relation between water table and seawater encroachment can be another way to verify these observations Four points in differents locations of the aquifer were selected for this proposal Is this the start of the dam effect? or only another dry period as 1995?

Conclusions It is important to note the utility of the comparison of geophysical techniques, but it must be a careful task due to the improvement of the measurement techniques. This work shows the stability of the marine intrusion during the last 25 years in the Motril-Salobreña aquifer. The water table presents an important drop from 2005, when the dam commenced operation, so it is important to quantify the impact of the human actions in this area. The monitoring of the groundwater changes will allow us to prevent undesirable effects in one of the highest quality water aquifers in the Southeast of Spain.