West Central Phoenix - North Plume | Site Hydrogeology

The site is located within the West Salt River Valley sub-basin of the Phoenix Active Management Area. The Salt River Valley is an alluvial filled basin located in the basin physiographic province.

The site is underlain by the upper alluvial unit (UAU) which consists of silty sands and sandy silts with interbedded clay layers and gravelly sand zones from ground surface to approximately 80 feet below ground surface (bgs). Beneath the UAU, a 35-foot thick fine grained unit consisting of primarily silt, clay with silty sand and gravel interbeds exists, referred to as the middle alluvial unit (MAU).  Beneath the MAU, the lower alluvial unit (LAU) is encountered, which overlies bedrock.

Depth to groundwater has declined in the past several years, attributed principally to the ongoing drought. In 1999, the depth to groundwater in the UAU was approximately 122 feet bgs.  By 2002, the depth to groundwater was approximately 132 feet bgs. Depth to groundwater as of March 2004 was 120 feet to 145 feet bgs.  Currently, depths to groundwater range from approximately 130 feet bgs to 163 feet bgs.  The site sits along a groundwater divide in the UAU with groundwater flowing to the northwest across most of the site and to the southwest in the far southern part of the Site. The coarse grained interval in the MAU acts as a groundwater sink, with water level elevations approximately 30 feet lower than the UAU and approximately 15 feet lower than the LAU. Groundwater flow direction in the coarse grained interval in the MAU is to the west.