Geophysical investigations of the vadose zone in the Valley of Hermosillo aquifer, Sonora, Mexico

Abstract

Seven Schlumberger soundings and three azimuthal soundings were conducted in the Valley of Hermosillo, Sonora, northwestern Mexico, in order to investigate the vadose zone above the Valley of Hermosillo aquifer. This coastal aquifer system, which is considered unconfined in its upper part, consists of alluvial material. The resistivity soundings show that lateral changes of the resistivity can be interpreted with respect to a 'background' value of 100 Ωm. Resistivity values less than 50 Ωm were found near the coast and near the population centers Hermosillo and Miguel Alemán. These relatively low values may reveal the presence of low quality pore water due to irrigation with ground water mixed with brackish ground water from sea water intrusions and with untreated sewage, or as a result of pollution by domestic and/or industrial sewage. The porosity of these low resistivity areas was estimated to be on the order of 25 %, based on Archie's law. Azimuthal resistivity surveys show that there is only superficial resistivity anisotropy which may be related to a rearrangement of grains by infiltrated irrigation water.