Low-Energy Alternatives for Removing Contaminant Plumes in Groundwater
Paul F. Hudak Abstract
A “no-action” and three low-energy groundwater remediation alternatives were evaluated with a numerical mass transport model. The low-energy alternatives included a permeable reactive barrier, non-pumped wells with filter media, and an extraction-injection well pair. Components of each alternative occupied a linear transect located 5 meters (m) downgradient of a contaminant plume and perpendicular to the regional hydraulic gradient. The model identified the shortest barrier, least number of non-pumped wells, and lowest pumping rates necessary to contain the plume onsite. The plume moved offsite in the no-action alternative, but was successfully contained with each low-energy alternative. Within 910-950 days (d), each alternative removed the contaminant plume; however, the well pair was most effective, requiring less infrastructure and pumping only 2.6 m3/d. Results of this study suggest that low-capacity extraction-injection well pairs may be viable alternatives to more costly permeable reactive barriers and non-pumped wells in some settings.
Introduction
Low-energy alternatives for cleaning contaminated ground- water have become increasingly popular over the past 20 years. A common example is permeable reactive barriers located in the pathways of migrating contaminant plumes, that remove
or decompose contaminants without pumping groundwater. [1] At field sites in several countries, permeable reactive bar- riers have treated metals,[2] metalloids,[3] hydrocarbons,[4]
nutrients,[5] chlorinated solvents,[6] and radioactive solutes. [7] Several modeling studies also have shown the capability of permeable reactive barriers.[8-12] However, permeable reac- tive barriers require specialized trenching equipment and are costly to install and maintain, especially for installations more than 20 m deep that operate for many years.
In some environments, multiple wells with removable filter cartridges instead of submersible pumps are alternatives to permeable reactive barriers.[13] Media inside the cartridges immobilize or transform contaminants. Cartridges are easily removed and replaced following chemical breakthrough.[13] Drilled wells can reach much greater depths than trenches; however, non-pumped wells also apply to shallow aquifers. Sufficiently dense arrays of non-pumped wells may function like a permeable reactive barrier, potentially at much lower installation and maintenance cost.[14] A possible limitation of non-pumped arrays is that contaminant plumes may migrate between adjacent wells and possibly offsite.[15]
A third low-energy alternative is an extraction-injection well pair operating at a low pumping rate, creating a hydraulic
barrier downgradient of a contaminant plume.[16] The well pair occupies a transect downgradient of the plume and approxi- mately perpendicular to the regional hydraulic gradient. The extraction well removes contaminated water, which is treated above ground and then injected back into the aquifer. This alternative tends to be less expensive to install and has more predictable long-term performance than a permeable reactive barrier.[16] In addition to creating a hydraulic barrier with the extraction well, the injection well dilutes contaminant concentrations in groundwater. In some cases, well pairs requiring low pumping rates can be operated with solar power in favorable climates.
Natural attenuation processes augment any remediation alternative; for example, if a contaminant plume is allowed to migrate and attenuate within an onsite buffer zone, hydro- dynamic dispersion can further lower contaminant concentra- tions.
While others[16] evaluated the viability of injection-extrac- tion well pairs based upon advection and flow-line distri- butions, this study examines their relative performance considering advection and hydrodynamic dispersion. A no- action alternative was compared with three active alterna- tives: a permeable reactive barrier, non-pumped wells with filter media, and an extraction-injection well pair.
Materials And Methods
The computer program MT3DMS[17] was applied to a hypothetical unconfined aquifer (Figure 1). MT3DMS is a modular three-dimensional multi-species transport model. This numerical groundwater flow and mass transport simula- tor utilizes a block-centered finite-difference grid, in this case comprising 195 rows (east-west), 400 columns (north-south), and one layer. Adjacent nodes were 0.25 m apart along rows and columns. The water table had an elevation of 10.000 m and 9.003 m at the western and eastern boundaries of the model domain (respectively), yielding an average regional hydraulic gradient of approximately 0.01 eastward. The aquifer’s base had an elevation (datum) of 0 m. The lateral northern and southern boundaries, and the lower boundary of the model domain were set as no-flow boundaries. Hydraulic conductiv- ity and the effective porosity of the simulated aquifer were 2 m/d and 0.25, respectively.
The model initially produced a flow field and contaminant plume from a 1.6 m2 source area with a constant concentration of 100 mg/L located near the western boundary of the model domain (Figure 1). After 410 d, the plume was 45 m from the eastern boundary, establishing initial contaminant concentra- tions for subsequent remediation trials. In mass transport simulations, longitudinal dispersivity was 1.0 m, transverse
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