The regional aquifer at the Tucson International Airport Area (TIAA) is composed of two hydrostratigraphic units: the upper zone of the regional aquifer and the lower zone of the regional aquifer. The upper zone of the regional aquifer is composed mainly of gravelly sand with some clayey sand and sandy clay, and it extends to a depth of approximately 200 feet below ground surface (bgs). The lower zone of the regional aquifer is composed mainly of relatively finer materials including clayey sand with lenses of gravelly sand and sandy clay, and it extends from approximately 300 feet bgs to an unknown depth. Separating the upper and lower zones of the regional aquifer is a thick clayey sequence termed the middle aquitard. This unit generally prevents contamination in the upper zone from reaching the lower zone.

At the Airport Property is a hydrostratigraphic unit referred to as the Shallow Groundwater Zone (SGZ). The SGZ is composed of fine-grained silts and clays with some sandy and gravelly interbeds referred to as the gravelly sub-unit (GSU). A clay aquitard separates the SGZ and GSU from the upper zone of the regional aquifer. Groundwater levels in the SGZ and GSU have been significantly higher than the upper zone of the regional aquifer. Just west of the Airport Property the aquitard is absent, allowing groundwater to flow from the SGZ and GSU into the regional aquifer. However, recently the water levels in the upper zone of the regional aquifer have risen to the same levels as the SGZ and GSU. More detailed descriptions of the hydrogeology of the Airport Property Project Area can be found in reports and studies available at the TIAA Information Repository.