A New Perspective on Flow, Transport, and Achievable Endpoints in Large Plume Restoration: Removing the Diffusion Road Block

Designers of remediation systems for cleaning up contaminated groundwater plumes are faced with the challenge of developing reliable estimates of the time to meet metrics. Common questions from responsible parties, regulators, and stakeholders include: 1) how long will it take to achieve performance goals? 2) how much less time would it take to clean up 70% or 80% of the plume foot print? 3) how much time to reduce the plume concentrations by 90% or 99%? or simply 4) when can we turn the remedy off? While engineers may struggle to answer these questions, diffusion has become the prumary technical basis for uncertainty. This presentation analyzes the movement of groundwater and contaminants by focusing on permeability contrasts to define primary, secondary, tertiary flow pathways. This approach, combined with high resolution plume delineation, allows us to distinguish between slow advection and diffusion, and identify more effective remedial approaches.

The division of flow through aquifers by order-of-magnitude contrasts in hydraulic conductivity defines the conceptual three-compartment model of flow and transport. The primary flow pathways defined by the most permeable facies within an aquifer are advective pathways that convey 90% of groundwater flow; 9.0% of groundwater flow occurs through secondary pathways where advection is slower and the effects of storage begin to emerge; while the tertiary pathways represent only 1% of flow and contaminants in true storage zones. Primary advection zones are the most conducive to remediation, reagent delivery, and treatment; the secondary pathways are slower advection zones where clean-up is possible but will take more time; while storage zones need to be assessed for restoration and the potential to recontaminate advective pathways. This concept is applied to data from multiple sites to provide new insights on projects that were less successful and to identify strategies for future approaches to be more robust.