Groundwater contamination has become an increasingly prevalent problem. Our highly industrialized society has demanded the mass production and refinement of many toxic compounds which inevitably find there way to the subsurface where they become groundwater contaminants. The best way to control groundwater contamination is to prevent the spilling, leaking, and infiltration of these compounds into the subsurface through effective containment at the surface. However, as the need for industrial and agricultural chemicals increases, so will the incidence of groundwater contamination. Therefore, the development of effective and efficient remediation techniques is a necessity.
If the zone of contamination is relatively small enough, it may be feasible to directly excavate the contaminated stratum. This may be a viable techniques for soils contaminated with highly refractory organics such as PCBs and some pesticides (Fetter 393). This entails the employment of earth moving machinery to physically remove the contaminant source (eg. UST) and contaminant plume from the subsurface regime. However, the extent of most contamination problems is unsuitably large for such a remediation technique. A possible alternative is to employ hydrodynamic barriers around the source and plume in order to prevent any further migration of the contaminant downgradient in the groundwater system.
Hydrodynamic barriers around a contaminant source and plume can be accomplished primarily in one of two ways. Groundwater cutoff-walls (slurry walls) can be physically installed around the zone of contamination. The walls are typically composed of either a soil-bentonite mixture or concrete (Fetter 396) of sufficiently low hydraulic conductivity so as to inhibit flow through them. This prevents the groundwater from flowing through the contaminant zone and transporting it downgradient. A complete sealing of the contaminant zone using this method is typically very costly and in many cases technically infeasible (Bogacki et al. 363). Alternately, hydraulic controls can be used to isolate the zone of contaminated groundwater by modifying the local flow regime through the strategic placement of pumping and injection wells. This remediation technique is commonly referred to as hydrodynamic isolation. It finds utility in that it doesn't require the installation of enormous physical boundaries as is the case will the slurry wall technique.
Hydrodynamic isolation systems vary in complexity and effectiveness. The simplest system involves a single pumping well located at either the leading edge or centroid of the contaminant plume. Isolation systems grow in complexity as more pumping and injection wells are employed in variety of configurations to maximize the systems effectiveness. The objective of the hydrodynamic isolation technique to modify the groundwater flow pattern so as to contain the contaminant plume within the capture zone of the well(s). The capture zone of a well, or system of wells, is defined as the area contributing flow to the well (Fetter 403). With the contaminant plume isolated within the capture zone, it will eventually migrate to the well, where it may be withdrawn and treated.
Multi-well hydrodynamic isolation systems such as
the one in Figure 1 employ a pair of pumping and injection wells.
The pumping well is located at the leading edge of the plume
(much like with the single well systems). The water pumped from
this well is reinjected into the containment area via the
injection well positioned upgradient. The withdrawn water may be
appropriately treated before reinjection, or may be enriched with
nutrients to promote biodegradation within the aquifer (Fetter
399).
A double-cell hydraulic isolation system (Figure 2) can be
devised to optimize the treatment efficiency of a multi-well
system. The contaminant plume is isolated within a smaller well
capture zone, and therefore smaller volumes of water will be
required to pump the contamination. Treatment costs will
subsequently be lowered with this decreased volume (Fetter 401).
Hydrodynamic containment systems are feasible for many practical applications. While large physical flow barriers such as slurry walls are often too expensive or technically impossible to install, isolation wells are not. For these wells to be effective however, considerable surveying must be done to find the extent and location of the contaminant plume so that the wells can be positioned correctly. Isolation wells are useful in that they can effectively prevent any further advection of contaminant downgradient. They also provide a means by which to remove the contamination from the aquifer, as well as the option to treat the contamination if necessary.
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Faculty Advisor: Daniel Gallagher, dang@vt.edu
Copyright © 1998 Daniel Gallagher
Last Modified: June 7, 1998
