UNSCRAMBLING THE EGG - A CASE STUDY IN CONTAMINANT TRANSPORT AND UNMIXING CO-MINGLED PLUMES TO DETERMINE AN APPROPRIATE REMEDIATION STRATEGY

Comingled groundwater plumes originating from different sources can pose many complex challenges for consultants as well as the responsible parties and lawyers involved. Recent work on a case in New Jersey has addressed the identification and allocation of responsibility using multiple lines of evidence and several forensic techniques on a 60-acre comingled plume of chlorinated organics and petroleum compounds. This case study clearly illustrates the importance of understanding complex groundwater flow regimes, surface water interactions, and the contaminant transport of 17 plumes. The convergence of commingled groundwater plumes into the systematic network of steeply dipping extensional fractures and bedding-parallel fractures aligned sub parallel to a fault zone created the need to better understand the structures and contaminant transport in a fractured aquifer. Through the use of hydrogeologic and transport principles, source location, concentration history, surface water impacts, and forensic analysis we defined the migration pathways of multiple VOC plumes and allocated impacts to several potentially responsible parties. Plume contaminants in this case include, but are not limited to: MTBE/TBA, PCE/TCE/DCE/VC, 1,1,1-TCA/1,1-DCE/1,1-DCA and CTC.

We approached the allocation issue by first using the Individual Plume Approach (IPA) for each source area and each aquifer (i.e. bedrock, deep overburden and shallow overburden). Fate & transport techniques included plume-front dating (travel-time calculations) and the correlation of migrating concentration peaks in groundwater through established monitoring networks. VOC-impacted bedrock groundwater is transported up into a shallow overburden aquifer over en echelon faults creating a concentrated contamination area. Forensic techniques, including VOC daughter product ratio dating methods from multiple wells in overburden and bedrock aquifers, were successfully used to establish clearly divergent release dates for plumes composed of the same VOC, but from separate sites and potentially responsible parties. Compound-specific isotope fingerprinting analysis was also used to help differentiate sources of comingled TCE.