The “Costa de Hermosillo” Acuifer is located in west central Sonora, México, southwest of the City of Hermosillo. The geology of the area is comprised of rocks of various ages, from paleozoic to quaternary rocks and are made of sedimentary and intrusive and volcanic igneous rocks. Altough the information is very scarce to know with precision the sub-surface stratigraphy, a study of the sediments descriptions in several wells permits to recognize four units: 1) quaternary alluvium, 2) miocenic (?) marine sediments, 3) miocenic gravels and sands, 4) crystalline basement (granite and volcanic rocks). The area constitutes a sedimentary basin produced by the generation of dropped blocks originated by normal faulting oriented NW-SE and NE-SW. The “Costa de Hermosillo” aquifer experienced an extreme groundwater extraction during the first 20 years of pumping (1947-1967) and reached a peak in extraction in 1965 of 1,200 Hm3. This extreme pumping produced an extensive withdrawal of groundwater lowering the regional water table 65 meters below sea level at the center of the aquifer, and has induced seawater intrusion of about 32 km into the aquifer, thus increasing the salinity considerably and causing an acceleration of water salinization. A new hidrogeologic model is presented here since the old “two aquifers model” (a confined lower and an unconfined upper aquifers). We present new data and information which are the result of a variety of techniques used to define a new hydrogeological model, to define the actual position and evolution of the mixing zone and the migration of the saline intrusion into the aquifer. Also we calculate the recharge of freshwater in 151.6 Hm3/year and the saline intrusión as a recharge coming from the sea with a volume of 98 Hm3/year, the pumping rate of fresh water used in the balancing equationis 527 Hm3/year.