Hydrostratigraphy of the Alto Deep Well: Implications for sustainable groundwater recharge into the distal Victorville fan sediments southwestern Mojave Desert, California

The Alto Deep Well (ADW-1) was drilled on the banks of Oro Grande Wash (OGW) following a Pilot Recharge Project conducted by the Mojave Water Agency. The Drill site was located about 1.0 miles north of the California aqueduct and near the location of the original recharge project. This area was in an optimal location because of its proximity to extraction wells owned by the local water districts. It was also identified as a possible location by the USGS, due to its proximity to the OGW and subsurface geology. Prior to the recharge test, a cone penetrometer survey indicated that the soils down to 100 feet would be conducive to recharge. A resistivity survey conducted three months after the recharge indicated that the recharge plume had migrated north and appeared to be perched on fine-grained material at a depth of 300 feet.This area is located on the Victorville fan which is composed of a stratigraphie sequence that fines northward away from the original source terrain of the San Gabriel Mountains. These units are exposed in the In-Face Bluffs of Cajon Valley and are composed of porous coarse grained material. In addition both Shoemaker Gravel and underlying Harold Formation contain cemented layers which may cause water to become perched. Based on Gravity models the drilling should have encountered bedrock at 2800 feet below ground surface (bgs) instead, it was reached at 1753 bgs. Also the coarse grained units of the bluff sediments which were expected to continue northward were manifested as a few thin aquifer layers surrounded by mostly finer silt and clay units.