Results of the remedial investigations performed to date indicate a complex contaminant history at the site. Notably, contaminant source areas are associated primarily with former unlined evaporation ponds. These ponds were situated in upland areas, outside the boundary of the shallow aquifer. Because of infiltration of wastewater through these pond bottoms, a perched zone was formed in the sandy-silty sediments overlying an upper clay member of the St. David Formation. Various paleogeomorphic features have been instrumental in controlling the movement of the COCs from the perched zone to the shallow aquifer. Historically, the perched zone drained laterally eastward into the shallow aquifer. Detailed examination of water quality dynamics, coupled with surficial geophysical surveys and exploratory borings, defined paleogeomorphic features on top of the clay. These features play significant roles in routing the perched zone flow and the dynamics.
In particular, a prominent buried paleofeature identified as the Molinos Creek paleodrainage, underlies the perched zone and extends beneath the alluvial plain west of the San Pedro River (Deane, 2000). Fine-grained overbank deposits associated with the Molinos Creek paleodrainage are believed to comprise a laterally confining unit (LCU) within the shallow aquifer. This LCU isolates the western portion of the shallow aquifer hydraulically from the main body of the shallow aquifer. Likewise, the LCU serves to control the flux of COCs. These aquifer divisions defined by the LCU have been distinguished as the Molinos Creek Sub-Aquifer (MCA) and the San Pedro Aquifer (SPA). Field investigations including seismic reflection, exploratory borings, piezometry, hydrogeochemical data, and contaminant spreading, as well as numerical modeling, collectively indicate that this feature apparently isolates contaminant discharges entering the shallow aquifer (MCA) from the perched zone from the rest of the shallow aquifer (SPA), and from the San Pedro River to the east. These hydrogeologic controls, combined with the potential for bioattenuation of both nitrate-N and perchlorate COCs, has permitted continued consideration of the applicability of a monitored natural attenuation remedy for that portion of the Superfund site.