In the southern half of the Tucson Airport Remediation Project (TARP) project area, the regional aquifer is composed of two hydrostratigraphic units: the upper zone of the regional aquifer and the lower zone of the regional aquifer. The regional aquifer in the northern portion of the project area is composed of only one hydrostratigraphic unit called the undivided 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 about 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; it extends from about 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. The undivided regional aquifer in the northern part of the TARP project area is composed mainly of coarse-grained materials.

Depth to groundwater varies from 80 to 240 feet bgs and generally gets deeper in a northward direction. The general groundwater direction flows toward the north-northwest. More detailed descriptions of the hydrogeology of the TARP project area can be found in reports and studies available at the Tucson International Airport Area Information Repository.