Alluvial coastal aquifers in the Gulf Extensional Province under the influence of extreme weather events: the case of San Felipe-Punta Estrella aquifer, BC Mexico

Abstract Groundwater in semiarid regions is frequently the only water source. In addition, coastal cities worldwide, such as the city of San Felipe, have significant hydric pressure growth due to tourism and migration. In this work, a gravity survey allowed to delineate the geometry of the San Felipe-Punta Estrella aquifer basement identifying an uplift of the metamorphic and granitic basement according to the regional extensional tectonic model at approximately 400 m below the surface. Furthermore, groundwater geochemistry and isotopic composition interpreted in conjunction with the aquifer geometry allowed for pointing out at least three hydrological implications. First, the granitic uplift reduces the overall volume of the groundwater reservoir, with no apparent reduction in water availability. However, the uplift acts as a deep flow barrier that induces vertical upflows, modifying regional flows. Second, the high fluoride concentration in groundwater can be related to the dissolution of fluoride-bearing minerals, fluoride mobilization at high Na/Ca ratios and high pH, and active regional thermal systems. The third finding was determining the source of groundwater recharge from groundwater stable isotope signature, which is the moisture surges from hurricanes in the eastern Pacific Ocean. These structural and hydrogeological issues have crucial implications for water quality and availability in semiarid regions with high hydric pressure to promote a sustainable water extraction policy.