Dye Tracer Study—Tried and True Method Yields Surprising Results

The use of tracer dyes is a technically valid and cost-effective method for characterizing contaminant fluxes and hydraulic properties in complex hydrogeologic systems. Dye tracing methods were successfully employed at a site in New Jersey to evaluate the effectiveness of the groundwater containment system and to update the conceptual site model (CSM). The data has driven a reevaluation of the groundwater containment system and CSM, including a review of interim alternative technologies to increase efficiency while a new approach capping the remedial action timeframe at 15 years is tested and implemented. Uncertainty with regard to the persistence of constituents in downgradient monitoring wells and the influence of long-term pumping from the interconnected overburden and basalt bedrock aquifers led to the evaluation of methods that would both address multiple hypotheses on contaminant flux and update the CSM. The property contains several distinct features that add to its complexity, including a former surface impoundment underlain by alluvial sediments and fractured bedrock, and the immediate presence of water bodies. The fluorescent dyes fluorescein, eosine, and rhodamine WT were selected for the Dye Tracer Study (DTS) and individually injected at three locations following baseline sampling. The injection locations considered the presence of source material (former surface impoundment), hydraulic properties of the aquifers (pumping induced gradients, travel times, heads from adjacent water bodies), and parent/daughter compound concentration relationships. The DTS was conducted over 14 months and involved the collection of grab and composite samples from monitoring and extraction well networks, and along an adjacent brook. A single dye, fluorescein, was identified over the course of the DTS. The fluorescein was injected in the former surface impoundment and travelled south at an approximate rate of ten feet/day. The DTS illuminated flow pathways that were unexpected in terms of speed of groundwater migration and extent laterally and vertically