1 THE SALINE INTRUSION IN THE COSTA DE HERMOSILLO AQUIFER IN SONORA, MÉXICO; A CHALLENGE TO RESTORE

The Costa de Hermosillo aquifer is located in the northwestern Pacific sector of Sonora, México, southwest of the City of Hermosillo. The area constitutes a sedimentary basin originated by normal faulting oriented NW-SE and NE-SW, probably developed during the basin-and-range tectonic events and during the opening of the Gulf of California. The geology of the study area includes four units: 1) Quaternary alluvium, 2) Miocene marine sediments, 3) Miocene gravels and sands, 4) crystalline basement (granite and volcanic rocks). The Quaternary sediments host the main aquifer, in which an excessive pumping rate, has exceeded the natural recharge for the last 50 years. In consequence, a saline intrusion has been penetrating into the continent, at a rate of about 650 m per year, in average. Three saline intrusion fronts have been identified, located between 28 and 32 km inland. This over-extraction originated the loss of hydraulic pressure in the aquifer, causing subsequent lowering of the piezometric water level. Our studies show the existence of a single aquifer, not two as previously postulated since 1968. Therefore, the extraction policy was based on a mistaken hydrogeological model, causing the water level decrease. The overpumping caused the formation of different aquifer zones, changing the original hydrogeological conditions. At present, every aquifer functions with individual hydrodynamic characteristics. This point is extremely important in order t