Stringfellow Superfund Site – Characterization, Remediation, and Modeling of Groundwater Impacts

The Stringfellow Superfund Site in Riverside, California, overlies an aquifer system comprised of alluvium, weathered bedrock, and unweathered fractured bedrock.  Impacts to groundwater extend approximately 5 miles downgradient from a box canyon containing the source area, through an alluvial paleovalley, and ultimately toward a river.  In the source area where over 34 million gallons of industrial waste were deposited, groundwater impacts are present primarily within bedrock.  This condition persists within the canyon, with transport transitioning primarily to the alluvial paleovalley downgradient of the canyon.  However, even within bedrock, transport appears to be associated with fracturing controlled by antecedent structural components that initiated the formation of the canyon and paleovalley.

Remediation to control migration of VOCs and perchlorate includes pump-and-treat systems in each of the aquifer units; recent enhancements to these systems exhibit likely improvements in plume control.  Following detection of perchlorate in 2001, remedial investigations for the Stringfellow Site have focused on control of this compound, although additional perchlorate sources (e.g., Chilean fertilizer and quarry blasting chemicals) have also been identified. Isotopes are being used to attempt source differentiation.  Bench and pilot scale in-situ bioremediation testing for perchlorate reduction has also been performed.  Overall effectiveness was not well supported in bedrock, in part due to competing electron acceptor pathways and potential difficulty in distribution of electron donor.  A bromide tracer test performed with this study was used to evaluate transport velocities.

A three-dimensional numerical model of groundwater flow and perchlorate transport in this system downgradient of the source area was developed in 2003 and has subsequently been updated and refined to assist in predictions of solute transport and evaluation of remedial alternatives.  In addition, a three-dimensional site model has been developed to depict the aquifer system and help elucidate the complex hydrogeologic interactions and transport between these units.