Capping and Remedial Extraction of Groundwater in a Shallow Marine Shoreline Sequence at a Former MGP Site in Florida

Groundwater extraction wells were installed and tested to support a “hydraulic containment system” as part of an approved Interim Remedial Action Measure at a former MGP site which operated into the 1950s in Jacksonville, Florida. Site characterization indicate the presence of three sand layers separated by clay layers which compose a marine shoreline stratigraphic sequence. Groundwater contaminants (COCs) were delineated both vertically and horizontally within the three sand units which overly a relatively thin weathered limestone unit (not contaminated) encountered at a depth of 45 to 50 feet. COCs include VOCs, PAHs, arsenic, and cyanide.

Published hydraulic conductivity parameters for the three zones were unavailable for the three zones. Therefore aquifer testing of each sand zone was performed to estimate the hydraulic parameters for capture design. Based on aquifer test data collected from an array of observation wells, a transmissivity of 287 feet²/day (hydraulic conductivity of 31.8 feet/day) and a transmissivity of 464 feet²/day (hydraulic conductivity of 77.3 feet/day) were calculated for the intermediate and deep zones, respectively. The deep zone was determined to be hydraulically separated from the overlying layers, whereas the shallow zone appears hydraulically connected to the intermediate sand zone

Radius of capture analyses demonstrated that the extraction wells will contain the contaminant plumes for all zones. A pumping rate of 2-3 gpm for the intermediate zone extraction well and a pumping rate of 5-6 gpm for the deep zone well should be sufficient to contain the plumes on the on-site and off-site. The extracted groundwater is pre-treated with an oil/water separator, permitted and discharged into the local POTW, and the system is operated using PLC, transducers, pump controllers, and supporting equipment. The remedial system is currently operational and the site will be capped to limit recharge and further migration of contaminants