First International Conference on Saltwater Intrusion and Coastal Aquifers— Monitoring, Modeling, and Management. Essaouira, Morocco, April 23–25, 2001 INFLUENCE OF A HIGH RECHARGE EPISODE FOLLOWING A DROUGHT PERIOD IN AN AQUIFER AFFECTED BY SEASONAL SEAWATER INTRUSION (GRANADA COAST, S. SPAIN)

One interesting hydrogeological feature of the small (less than 4 km) Almuñécar alluvial aquifer -andalusian mediterranean coast, Spainis related to its seasonal intrusion-extrusion cycles. This is thought to be a consequence of the coarse and clean nature of the sediments and the correspondent high values of hydraulic conductivity (around 500 m/day). The recharge (14 hm/y as an average) comes mainly from the infiltration of surface water from a catchment of slightly more than 100 km. In normal hydrologic conditions, the underground discharge to the sea (of some 7 hm/y) is restricted to the wet season (around winter). The maximal pumping/recharge values are found between June and September and induce negative piezometric levels throughout most of the aquifer as well as a saltwater intrusion of some 3-4 hm. Then, the decrease of such ratio in autumn is followed by the recovery of both piezometry and groundwater quality. A severe drought has happened between 1991-1995, followed by an abnormally rainy period (1995-1997). A general hydrological model of the overall basin as well as the mathematical simulation of the groundwater flow indicate that the recharge to the aquifer has doubled the normal values. Freshwater discharge from the aquifer to the sea (around 19 hm/y) has taken place even during the summer of 1996. Electrical conductivity logs in a number of monitoring wells have been useful in characterising the very active extrusion process (resulting in a piston-flow flushing of the aquifer) between December 1995 (end of the drought, when most of the EC values were higher than 10-20 mS/cm, showing different preferential pathways of seawater penetration) and February 1996, as a consequence of rainfall values of near 800 mm registered in such a short interval of time. In addition, a number of borehole dilution tests have allowed to evaluate the Darcy (5-10 m/m·d ) and flow (50-100 m/d) velocities in the aquifer associated to this important recharge period. INTRODUCTION In spite of its small sizeless than 4 kmthe alluvial aquifer of Almuñécar (Granada province, Andalusia, South Spain) has a great economic interest. This is mainly due to the special climatic conditions (mild temperatures) which come out from the barrier effect against cold north winds induced by a mountain range located few kilometers from the sea shore (Figure 1). This enables a special type of irrigated crops which extend over the totality of the alluvial plain as well as over a part of the valley slopes: specially custard apples and avocados, all of them fairly rare as intensive plantations on the European continent, and because of it the term “Tropical Coast” has been chosen to name touristically this Mediterranean coastal strip. Figure 1.General location and sketch of the Verde River basin. 1: alluvial aquifer, 2: carbonate aquifer, 3: metapelitic materials, 4: springs, 5: gallery. 0 1 2 km M E D I T E R R A N E A N S E A 1362 GRANADA