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1. Reply to comment by D. R. Lester et al. on “Plume spreading in groundwater by stretching and folding”: COMMENTARY

   https://doi.org/10.1002/wrcr.20081  

   Mays, David C.; Neupauer, Roseanna M. (February 2013, Water Resources Research)
2. Calculating lateral plume spreading with surface Lagrangian drifters

   https://doi.org/10.1002/lom3.10356  

   Kakoulaki, Georgia; MacDonald, Daniel G.; Cole, Kelly (April 2020, Limnology and Oceanography: Methods)

   Abstract This work provides a rare quantification of lateral spreading from Lagrangian measurements in a buoyant river plume by comparing four methods. Drifter motions, including along‐stream shear and rotation, can be incorrectly interpreted as lateral spreading. This work aims to improve estimates of lateral spreading by identifying additional motions in drifter trajectories. The techniques applied are first evaluated and compared using an idealized group of drifters undergoing specific types of motion, and then applied to in situ data from 27 surface Lagrangian drifters released in the Merrimack River plume (Massachusetts) under a variety of different environmental conditions. The techniques tested include two methods using the standard deviation of drifter position with respect to various interpretations of mean drifter direction and two methods using a rotating elliptical coordinate reference frame. The idealized trajectories are modeled analytically with each type of motion (i.e., spreading, rotation, and shear) separately, then in different combinations, to identify the method that best resolves and isolates lateral spreading. The idealized experiments demonstrate that three of the methods are sensitive to shear and rotational motion in various combinations. The most robust method resolving lateral spreading is the “time‐step” method, which applies a reference frame that follows the mean flow at each time step, calculated as the average direction of the drifters between two time steps. This method also successfully identifies lateral spreading in observations, which is maximized in classic bulge‐shaped plume deployments. This work is applicable to other river plume systems as well as other propagating oceanographic phenomena. 

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3. Active Spreading: Hydraulics for Enhancing Groundwater Remediation

   https://doi.org/10.1061/(ASCE)HE.1943-5584.0002167  

   Sather, Lauren J.; Neupauer, Roseanna M.; Mays, David C.; Crimaldi, John P.; Roth, Eric J. (May 2022, Journal of Hydrologic Engineering)
4. Comparison of Effective Active Spreading Designs for In Situ Groundwater Remediation

   https://doi.org/10.1061/9780784484258.013  

   Neupauer, R. M.; Mays, D. C.; Ye, M.; Greene, J. A. (January 2022, 2022 World Environmental and Water Resources Congress)

   During in-situ remediation of contaminated groundwater, a chemical or biological amendment is introduced into the contaminant plume to react with the contaminant. Reactions occur only where the amendment and contaminant are in contact with each other, so active spreading has been proposed to increase the contact area between the two reactants. With active spreading, wells are installed in the vicinity of the contaminant plume and are operated in a pre-defined sequence of injections and extractions to create a spatio-temporally varying flow field that changes the shapes of the reactant plumes, generally leading to an increase in contact area and therefore an increase in reaction. The design of the active spreading system depends on the reaction chemistry of the contaminant. This study considers active spreading scenarios for contaminants with three different types of reactions: (1) non-sorbing aqueous contaminant, A, that degrades irreversibly to a benign chemical, C, through reaction with a non-sorbing aqueous amendment, B; (2) sorbing contaminant, A, the degrades irreversibly to a benign chemical, C, through reaction with a non-sorbing aqueous amendment, B, where sorption of A is independent of the concentration of B; and (3) contaminant, A, that exhibits reversible equilibrium surface complexation with concentrations in the mobile and immobile phases dependent on the concentration of the amendment, B. We compare the active spreading strategies for these three types of reactions and identify the characteristics each strategy that lead to enhanced removal of groundwater contaminants. 

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5. Numerical and Experimental Investigation of Active and Passive Spreading for Groundwater Remediation

   https://doi.org/10.1061/9780784482964.010  

   Neupauer, R. M.; Sather, L. J.; Roth, E. J.; Mays, D. C.; Crimaldi, J. P. (January 2020, 2020 World Environmental and Water Resources Congress)

   During in situ remediation of contaminated groundwater, a chemical or biological amendment is introduced into a contaminant plume to react with and degrade the contaminant. Since the degradation reactions only occur where the amendment and the contaminant are sufficiently close, the success of in situ remediation is controlled by the degree to which the amendment spreads into the contaminant plume. Spreading is defined as the reconfiguration of the plume geometry, which occurs as a result of spatially and temporally varying flow fields. Spreading can occur passively due to spatially varying velocity caused by aquifer heterogeneity. Spreading can also occur actively by inducing spatially and temporally varying flow fields through injections and extractions of clean water in wells surrounding the contaminated site. We used coupled numerical investigations and laboratory experiments to explore the effects of active spreading and passive spreading on contaminant degradation. We report here on the effects of passive spreading on contaminant degradation. We analyze the features of the flow field and plume geometry that encourage spreading contaminant degradation, so that the results from the numerical investigation and experiments can be used to design active spreading pumping sequences for other aquifers with other heterogeneity patterns. 

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