Numerical Modeling of a Complex Aquifer System, Stringfellow Superfund Site, Riverside County, CA

The Stringfellow Superfund Site in Riverside, California, overlies an aquifer system comprising alluvium, weathered bedrock, and unweathered bedrock.  Perchlorate impacts to groundwater extend approximately 5 miles downgradient from a box canyon containing the source area through an alluvial paleovalley towards the Santa Ana River.  A three-dimensional numerical model of groundwater flow (MODFLOW) and perchlorate transport (MT3D) in this system downgradient of the source area was developed in 2003 and has subsequently been updated and refined to provide predictions of solute transport and evaluation of remedial alternatives.

Existing remedial systems installed to control the migration of volatile organic compounds comprise pump-and-treat extraction wells along the plume path.  Following detection of perchlorate in 2001, remedial investigations have focused on control of this compound, although additional perchlorate sources besides the Stringfellow Superfund Site have also been identified as impacting regional groundwater.

Several remedial alternatives that have been tested in the model in support of the site feasibility study include no action, continuing current pump-and-treat remediation, modifying existing pump-and-treat systems, and in-situ bioremediation.  Predicted cleanup times for perchlorate to the target background cleanup level range from 14 to 35 years; this background value is about twice the regulatory threshold due to the presence of other sources of perchlorate.  Ongoing modeling will further evaluate the feasibility of remediating perchlorate from Stringfellow to below background as well as supporting a technical impracticability evaluation for bedrock impacts.