Reactive Barriers: Hydraulic Performance and Design Enhancements

Abstract

The remediation of contaminated ground water is a multibillion-dollar global industry. Permeable reactive barriers (PRBs) are one of the leading technologies being developed in the search for alternatives to the pump-and-treat method. Improving the hydraulic performance of these PRBs is an important part of maximizing their potential to the industry. Optimization of the hydraulic performance of a PRB can be defined in terms of finding the balance between capture, residence time, and PRB longevity that produces a minimum-cost acceptable design. Three-dimensional particle tracking was used to estimate capture zone and residence time distributions. Volumetric flow analysis was used for estimation of flow distribution across a PRB and in the identification of flow regimes that may affect the permeability or reactivity of portions of the PRB over time. Capture zone measurements extended below the base of partially penetrating PRBs and were measured upgradient from the portion of aquifer influenced by PRB emplacement. Hydraulic performance analysis of standard PRB designs confirmed previously presented research that identified the potential for significant variation in residence time and capture zone. These variations can result in the need to oversize the PRB to ensure that downgradient contaminant concentrations do not exceed imposed standards. The most useful PRB design enhancements for controlling residence time and capture variation were found to be customized downgradient gate faces, velocity equalization walls, deeper emplacement of the funnel than the gate, and careful manipulation of the hydraulic conductivity ratio between the gate and the aquifer.

Constructal design of permeable reactive barriers: groundwater-hydraulics criteria

“…2 Corresponding notations in [23] are (k, k b , α, Q 0 , h, θ b ) → (k a , k b , γ e , V 0 , d a , m b ) where in [23] h and θ b are the vertical thickness of the barrier-aquifer and PRB porosity(= m b in our notations). extrema to the functional (14). These extrema date back to the classical brachistochrone problem and Fermat's principle of fastest light-beam refraction through a heterogenous optically resistive medium.…”

“…Hydraulics of groundwater plays an important role in the design of PRBs (e.g., [8][9][10][11][12][13][14]). In particular, the position of the PRB with respect to the vector V 0 (the magnitude of which is V 0 ), the shape of the interface CME (denoted L) and the pair (m b , k b ) should be designed in such a manner that the PRB extends hydraulically across (i.e., transversal to the ambient flow) the targeted contamination plume, intercepts as much of the plume as possible (as quantified in the flow rate Q passing through the PRB), and retains the intercepted water in contact with the reactive material as long as possible.…”

The problem of flow by-pass at permeable reactive barriers

“…While a given reactor cell may or not possess impermeable side walls, the treated portion of the groundwater flow can be increased by deploying impermeable cutoff walls extending from the reactor sides into the aquifer, which is referred to as the funnel-and-gate (F&G; Starr and Cherry [3]) system. Velocity equalization walls (VEWs; Painter [4]) have been proposed to achieve a more uniform flux distribution entering the reactor. Furthermore, draining trenches can be used in the drain-and-gate (D&G; Bundesministerium für Bildung und Forschung [5]) configuration, which act as collectors of contaminated groundwater up-gradient of the reactor and as distributors of clean water on the down-gradient side.…”

Capture flows of funnel-and-gate reactive barriers without gravel packs

“…1(a); [3]), impermeable funnel arms are deployed laterally extending into the aquifer in order to increase [4]) may be used to achieve more uniform contaminant residence time distribution across the reactor by avoiding flow singularities (i.e., the blow-up of the magnitude of the hydraulic gradient) near the reactor and, thus, providing largely uniform flow into, across and out of the reactor. While numerical studies are more abundant in literature, e.g., [3,4], previous work by the authors, [5][6][7], investigates different hydraulic aspects of these PRB configurations in a two-dimensional (horizontal) analytical framework by applying the theory of holomorphic functions, in particular, the conformal mapping technique. However, for arbitrary reactor conductivities results are valid only in the presence of highly permeable gravel packs at the up and down-gradient faces of the reactor, which provide for constant hydraulic head distributions throughout the gravel packs and, thus, strictly uniform flow across the reactor.…”