In situ remediation of contaminated groundwater often requires the introduction of a treatment solution into the aquifer in order to promote contaminant degradation reactions. A significant challenge for in situ remediation is the inherent difficulty of mixing in porous media. Without sufficient mixing of the treatment solution and the contaminated groundwater, the degradation reactions required to achieve in situ remediation cannot occur. This project tests the hypothesis that in situ remediation of contaminated groundwater can be enhanced through strategic operation of wells, installed near the contaminant plume, whose goal is to promote stretching and folding of the interface between the treatment solution and contaminated groundwater, thereby increasing the opportunity for degradation reactions to occur. The project involves numerical simulation and optimization to investigate well operation strategies that will maximize contaminant degradation. The optimization considers the number of wells, their locations and the rates at which each well extracts or injects fluid as a function of time. The study investigates both dissolved contaminants and those sorbed to the aquifer solids in homogeneous and heterogeneous aquifers using a suite of modern groundwater models. The interplay of the well placement, pumping schemes, and aquifer properties will be characterized by a series of dimensionless numbers that can be used for remediation system design.

Contamination threatens important groundwater resources that provide the water supply for numerous municipal water utilities and domestic water wells. Already, billions of dollars have been spent on remediation of contaminated groundwater in the U.S., and yet at many locations, the remediation efforts have not met cleanup targets. This project investigates a method for improving the cleanup of contaminated groundwater by using injection and extraction wells (an existing technology) in a novel way to promote in situ groundwater remediation. This project provides an initial theoretical exploration of the enhanced mixing achievable by these novel methods that will be compared with existing methods in terms of contaminant degradation completeness, cost, and groundwater quality improvement. Additionally, this study provides a new link between established chaos theory and groundwater flow that will lead to new insights into subsurface contaminant transport. The project will also create a physical demonstration apparatus and education module that will be used to engage pre-college students in learning about groundwater by allowing users to manipulate plumes by injection and extraction. After completion, the apparatus will be available through an established teaching laboratory collection at the University of Colorado at Boulder.

Project Report

Groundwater is an essential natural resource, but vulnerable to contamination, so we spend billions of dollars annually on groundwater remediation. One approach to groundwater remediation is injecting solutions that treat contaminants in place, called in-situ remediation. This report summarizes a research study on in-situ remediation, funded by the National Science Foundation’s Hydrologic Sciences Program and conducted at the University of Colorado Denver (in collaboration with the University of Colorado Boulder) from 2011-2014. Anyone who has taken chemistry will remember adding reagents to a beaker and stirring, which rapidly mixes the reagents for quick reaction. In-situ remediation also calls for quick reactions, but because the fluids reside in the tiny pore spaces between soil grains or rock fractures, one cannot impose the turbulence that provides mixing in beakers. Instead, because groundwater moves very slowly, its viscosity (i.e., resistance to flow) is extremely important. Groundwater flows like honey, such that the injected fluid plume (i.e., a parcel of one fluid within another fluid) does not rapidly spread into the groundwater. The difficulty in spreading injected solutions into contaminated groundwater is a widely recognized, fundamental problem in groundwater remediation. This research program attacked this fundamental problem by borrowing a simple idea from the field of complex systems science: When turbulent mixing is not possible, the best way to spread one fluid into another is by chaotic flow, called chaotic advection. Chaotic advection (and chaos in general) is defined by sensitive dependence on initial conditions, meaning that neighboring fluid particles separate rapidly. This rapid separation corresponds to good plume spreading. Practically speaking, chaotic advection results from flows that combine stretching and folding, analogous to the operation of a saltwater taffy machine. This research demonstrated that one can impose stretching and folding in groundwater by injecting and extracting water from a group of wells in a deliberate, prescribed manner called engineered injection and extraction. Research was conducted primarily using computer simulations, with preliminary laboratory experiments to demonstrate proof-of-principle. Key findings were as follows: First, we demonstrated that stretching and folding does indeed generate chaotic advection, which promotes spreading (Mays and Neupauer 2012). Stretching and folding was also shown to be possible without extracting and re-injecting fluid, which is advantageous practically and legally (Mays and Neupauer 2013). Moreover, the engineered injection and extraction was revealed to have a bifurcation structure, such that slight changes in pumping led to large, qualitative changes in the resulting flow. Second, we simulated plume stretching and folding in groundwater remediation. These simulations predicted faster contaminant degradation (Piscopo et al. 2013). We also investigated how geologic complexity impacts stretching and folding, specifically heterogeneity and sorption. Heterogeneity is the idea that geologic media (soils and fractured rocks) are not uniform, so water will seek out the path of least resistance, making nonuniform flows that also generate spreading. We showed that heterogeneous media not only enhance plume spreading, but also qualitatively change the nature of the chaotic advection (Neupauer et al. 2014). Sorption is the tendency for certain contaminants to stick to soils or rocks, which retards their speed as they move in groundwater. For sorbing contaminants, we found that engineered injection and extraction is still beneficial, but with a qualitatively different pumping scheme (Neupauer and Mays 2014). Third, we conducted preliminary laboratory experiments to demonstrate plume stretching and folding in a physical model. The figure shows results for two dimensional flow of viscous silicone oil between acrylic plates separated by one tenth of a millimeter, using a plume of suspended pigment. The left panel shows the position of the plume after the 12-step pumping scheme described in our first paper (Mays and Neupauer 2012). The right panel superimposes the theoretical image in red, revealing qualitative similarity between experiment and theory, but also demonstrating the importance of dispersion around the edges of the plume. Dispersion is the subject of a related study awarded in 2014 (EAR-1417005 and EAR-1417017). Broader impacts comprise publications, presentations, training, and outreach activities. Publications include five refereed journal articles, plus seven other non-refereed conference papers. Presentations include seven conference talks (including two invited), plus ten additional poster presentations, including one international conference (Padova, Italy, June 2013). Training was provided for three undergraduate students (Louis Dankovich, Matthew Foster, and Ryan Tigera) and two graduate students (Matt Jones and Amy Piscopo), including two military veterans. Outreach activities included two presentations at the Colorado Ground Water Association and the development of a graphical user interface (GUI) for testing pumping schemes for stretching and folding. The GUI has been designed to make stretching and folding accessible for anyone from the general public to professional researchers, and will support parallel outreach efforts at the University of Colorado Boulder. This GUI, its user manual, and a tutorial on making GUIs in Matlab have been uploaded to

National Science Foundation (NSF)
Division of Earth Sciences (EAR)
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Thomas Torgersen
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University of Colorado Denver
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