Revised On: Feb. 18, 2024 - 9:00 p.m.

The Yuma, Arizona area is underlain by thick sequences of non-marine and marine sedimentary rocks. However, only the upper several hundred feet of these sediments are hydrologically important. This is because the upper layers are extremely transmissive and yield sufficient quantities of water to wells.

From lowest to uppermost, the upper layers are described as the wedge zone, the coarse gravel zone, and the upper fine-grained zone. The wedge zone overlies the marine sedimentary Bouse formation and consists of interbedded sands, gravels and cobbles. The wedge zone is approximately 2,500 feet thick in the area and pinches out against the basin bounding ranges.

The coarse gravel zone overlies the wedge zone, varying from zero to 100 feet in thickness. The coarse gravel zone consists of fluvial deposits of coarse gravels, including cobbles and boulder size material. The coarse gravel zone is the principal aquifer for the Yuma, Arizona area. The coarse gravel zone is generally found at a depth of 100 feet in the low lying valley areas near the site, and at a depth of about 180 feet below the Yuma Mesa where the site is located. However, the coarse gravel zone is not present beneath the site.

The upper fine-grained zone is the uppermost saturated unit which overlies the coarse gravel zone. The upper fine-grained zone is up to 200 feet thick and is characterized as sands and silts and may have an extensive clay layer which can locally affect groundwater movement. The Yuma Mesa is a remnant of the upper fine-grained zone which is mostly missing in the nearby low lying valley areas.

Depth to groundwater at the site is approximately 75 feet below ground surface (bgs). Groundwater flow direction at the site is generally to the northwest.