The PGA Superfund Site hydrostratigraphy is generally described in three subunits:

Subunit A

Interbedded sands, silty sands, and clayey sands with localized sand and gravel sequences. Extends from the ground surface to approximately 110 feet below ground surface (bgs) near PGA-South and generally deepens northerly. Localized, well-developed caliche has been encountered above and below the groundwater level.  In 2016, groundwater was encountered at 70 feet bgs in PGA-South. However, PGA-North groundwater depth is variable across the plume footprint. Groundwater depth varies from 90 feet bgs to 140 feet bgs, depending upon the location along the elongated north-to-south plume footprint. Historically, the PGA-North Subunit A groundwater flow direction was north-northwesterly toward regional groundwater depressions created by irrigation water extraction. Changes in irrigation and municipal extraction practices and locations during the early- to mid-2000’s pulled the groundwater plume northeasterly (north of Interstate-10). To counteract the eastern component, more recent remediation extraction wells and subsequent plume-edge clean water injection (creating hydraulic barriers) have created some localized groundwater flow direction shifts toward the west/northwest. PGA-South overall Subunit A groundwater flow direction is southwest. The contaminant plume flows from the east side of the former Loral property to the south-southwest under the airport property and is contained by extraction and re-injection of treated water at the southwest plume boundary.

Subunit B

Unconsolidated silt and clay deposits with interbedded lenses of fine to coarse sand with depths extending from approximately 110 to 160 feet bgs near PGAS, and 160 to 230 feet bgs at PGAN. Integrity varies across the site. Where not developed as an aquitard, Subunit B may allow contaminant leakage to Subunit C. Groundwater movement is generally considered to be non-continuous laterally and, where developed, an aquitard without a consistent horizontal groundwater flow.

Subunit C

Unconsolidated and interbedded mixtures of silty sands, clayey sands, and fine- to coarse-grained sands. Extends from approximately 160 to 310 feet bgs at PGAS, and 200 to 350 feet bgs at PGAN. Groundwater movement is generally north to northwest.