GROUNDWATER REMEDIATION
Evaluating the effectiveness of pumping well configurations for groundwater remediation By Paul F. Hudak
L
ow-cost alternatives for remediating contaminated groundwater are becoming increasingly popular. Permeable reactive barriers can treat contaminants without costly pumping of groundwater (EPA, 2002). Placed hydraulically downgradient of contaminant plumes, pervious barriers have treated arsenic, metals, organics and other contaminants in groundwater (Guerin et al., 2002; Ludwig et al., 2002; Lai et al., 2006; Gilbert et al., 2010). However, installing permeable reactive barriers, especially deep installations, requires specialized and costly excavation equipment. Additionally, monitoring, removing, disposing of, and replacing filter media can be costly over long operational periods. At some sites, non-pumping wells equipped with removable porous filters could be used instead of permeable reactive barriers (USGS, 1999). Similar to permeable reactive barriers, filter media in non-pumped wells immobilize or transform contaminants. Conventional drilling rigs can reach much greater depths than trenching equipment; thus, non-pumped wells could be used in deep as well as shallow settings (Hudak, 2009). However, nearly adjacent placement may be necessary to keep contaminants from migrating between non-pumped wells. Installing, monitoring, and maintaining numerous closely-spaced wells may be cost-prohibitive at many sites (Hudak, 2014). Alternatively, one extraction well and one accompanying injection well, pumping at very low rates, may create a less costly, hydraulic barrier to contain contaminant plumes in some settings (Cunningham and Reinhard, 2002). A typical installation is collinear, along a transect crossgradient to the prevailing hydraulic gradient, located in front of a contaminant plume. One well extracts 40 | February 2018
Figure 1. Map of initial (top) and residual (bottom) contaminant plume contacting downgradient boundary without intervention; contours in mg/L.
contaminated groundwater, which is treated above ground and then injected into the aquifer via the other well. The injection well also dilutes contaminant concentrations in groundwater. Such dilution complements dilution by fresh groundwater in an aquifer. These processes, combined with the effects of hydrodynamic dispersion, contribute to lowering contaminant concentrations in aquifers. Additionally, low-capacity extraction and injection wells do not require excessive amounts of energy. In some settings, solar energy could power them. While well pairs may be effective at controlling contaminant plumes, using three wells offers a potential advantage. This involves an extraction well placed directly downgradient of the plume’s leading tip, with peripheral injection wells for containment. Conversely, a three-well
pattern may include a central injection well and peripheral extraction wells. Earlier studies involved flow line distributions for evaluating well pairs (Cunningham and Reinhard, 2002; Wu et al., 2008). Hudak (2015a) used modeled advection and hydrodynamic dispersion to evaluate two-well configurations in simulated homogeneous and heterogeneous aquifers. In a previous study, an injection-extraction well pair outperformed a permeable reactive barrier and non-pumped wells with filter media (Hudak, 2015b). METHODS This study examines a no-action scenario, compared with two-well and three-well alternatives for remediating a contaminated aquifer. A flow and transport model, MT3DMS
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