Evolution of the Marine Evolution of the Marine Intrusion Using Intrusion Using

Geophysical Methods after Geophysical Methods after 25 years in the Motril-25 years in the Motril-

Salobreña Aquifer Salobreña Aquifer (Southern Spain)(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

1981

2006

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.