Plume spreading in groundwater by stretching and folding

Abstract

[1] This paper proposes a new approach to the hydraulics of in situ groundwater remediation. In situ remediation promotes reactions between an injected treatment solution and the contaminated groundwater, but without a hydraulic mechanism to promote spreading, the laminar flows characteristic of porous media will keep the two fluids in approximately the same relative configuration as they travel through the aquifer, limiting the opportunity for reactions to occur. To address this fundamental limitation, this paper borrows a key result from the fluid mechanics literature : Spreading in laminar flows is optimized by chaotic advection. Previous studies have applied this result to groundwater remediation using the pulsed dipole model, but that model depends on reinjection of fluid, which presents a number of theoretical and practical limitations. Accordingly, this paper proposes a new conceptual model for plume spreading by chaotic advection, using an engineered sequence of extractions and injections of clean water at an array of wells, which generates plume spreading by stretching and folding the fluid interface between the injected treatment solution and the contaminated groundwater but does not require reinjection. The paper includes an overview of the analytical techniques-Poincaré sections, periodic points, stable and unstable manifolds, heteroclinic points, and Lyapunov exponents-used to demonstrate chaotic advection in the limiting case in which diffusion is negligible. Numerical simulations show that spreading by stretching and folding is complimentary to spreading resulting from aquifer heterogeneity.

On the importance of diffusion and compound-specific mixing for groundwater transport: An investigation from pore to field scale

“…The limiting role of mixing to the overall transformation rates of contaminants in the subsurface is increasingly recognized as well as the importance of mixing enhancement by natural flow and transport processes (e.g., Rolle et al, 2009;Werth et al, 2006) and engineered approaches (Kitanidis and McCarty, 2012;Mays and Neupauer, 2012;Trefry et al, 2012). Moreover, there is a clear need to distinguish between true mixing processes, depending on aqueous diffusion, and processes such as advective heterogeneities, which may lead to squeezing, stretching and meandering of a solute plume but do not directly contribute to physical mixing.…”

Organic contaminant transport and fate in the subsurface: Evolution of knowledge and understanding

“…The results showed that the effects of aquifer heterogeneity were insignificant under the forced recirculation scheme [ Gandhi et al ., ]. Enhanced mixing by stretching and folding can be induced with pulsed injection and extraction of clean water outside of a plume [ Mays and Neupauer , ]. This mixing promotes reaction between an injected treatment solution and the contaminant [ Piscopo et al ., ].…”

Resurgence of Pump and Treat Solutions: Directed Groundwater Recirculation

“…Sites where aquifer hydraulics vary less tend to have long, thin, and relatively uniform plumes; conversely, sites with highly varying hydraulic conditions tend to have wider plumes with more irregular contaminant distribution. Researchers in chaos theory describe this element of transport as plume stretching and folding (Bagtzoglou and Oates ; Mayes and Neupauer ; Ottino ; Piscopo et al ).…”