Site hydrogeology includes unconsolidated and intermingled clay, silt, sand and gravel layers associated with stream channel and alluvial fan deposition from the surface to about 260 feet below the ground surface (ft bgs). This is locally designated as the Upper Unit.

From about 260 ft bgs to nearly 1,000 ft bgs are weakly consolidated silt, silty sand and gravel layers with locally occurring moderately to well-cemented siltstone associated with former playa, alluvial fan and fluvial deposits. This is locally designated as the Middle Unit.

A leaky aquitard (layer of low permeability) separates the Upper Unit and Middle Unit near the former Williams Air Force Base (WAFB).

Historically, groundwater levels in the vicinity of the WAFB have fluctuated as groundwater pumping volumes increased and decreased. Static groundwater, circa 1900, was reported to be approximately 80 ft bgs near the WAFB. By the 1970s, substantial groundwater drawdown was attributed to agriculture and municipal groundwater withdrawal. Since the late 1970s, however, groundwater levels have recovered (risen) 0.5 to 3 ft per year. In recent years, groundwater has been encountered between 123-160 ft bgs across the site. 

Groundwater flow direction is somewhat variable, ranging west-to-east and west-to-northeasterly near ST012. Please note that groundwater flow direction has shifted when compared to historic data recorded during high yield extraction well pumping years.