Date of Award

8-2018

Document Type

Thesis

Degree Name

Master of Science (MS)

Department

Environmental Engineering and Earth Sciences

Committee Member

Dr. Ronald Falta, Committee Chair

Committee Member

Dr. David Freedman

Committee Member

Dr. Lawrence Murdoch

Abstract

Pump and Treat technology remains one of the most common approaches for groundwater remediation at contaminated sites. Despite its prevalence, the effectiveness of the Pump and Treat approach is limited by the chemical properties of the contaminant, heterogeneity, and cost (Mackay and Cherry, 1989; NRC, 1994). Pump and Treat systems are often in operation for long periods of time, at which point their environmental footprint, in addition to cost, needs to be considered. Greenhouse gas emissions from electricity generation to power pumps and the disruption of ecosystems over time are undesirable environmental effects that can be a cause of concern at sites that employ Pump and Treat (Ellis and Hadley, 2009; USEPA, 2009). A potentially more sustainable alternative to Pump and Treat remediation is source zone isolation. Source zone isolation can be achieved by surrounding a contaminant source with a material of contrasting permeability. The construction of an impermeable barrier, such as a sheet pile or slurry wall, is one technique that can be used to contain a contaminant source. However impermeable barriers must typically be excavated to a confining layer or bedrock and require nearly perfect construction to be effective. Impermeable barriers may also create a scenario where groundwater mounding takes place. A permeable hydraulic barrier, such as a French drain (Davis and Stansfield,1984) or constant head trench (Ankeny and Forbes, 1997), can provide a cheaper and more robust means for source zone isolation than an impermeable barrier. Clean, upstream groundwater would preferentially flow into, through, and exit the permeable barrier without contacting the source zone and contributing to an existing plume. This study utilizes groundwater flow and contaminant transport modeling to assess the influence of various permeable hydraulic barrier design parameters such as thickness, effective hydraulic conductivity, depth, and the inclusion of wells on source zone isolation. Additionally, different hydrogeologic scenarios for a prospective site are explored to see how they impact the performance of a permeable hydraulic barrier. Sensitivity analyses were performed on different design parameters in the presence of preexisting plumes generated from a continuous source in a homogeneous aquifer conceptual model and in a heterogeneous one as well. The simulation results for a homogeneous aquifer conceptual model suggests that permeable hydraulic barrier designs should fully penetrate an aquifer in order to achieve optimal results. Increases in the effective hydraulic conductivity and trench thickness are shown to increase hydraulic isolation. Simulation results of a sensitivity analysis performed on a heterogeneous aquifer conceptual model indicate that the presence of heterogeneity may enhance the performance of a hydraulic barrier. Designs such as a gravel trench, a gravel trench with pipe, and a gravel trench with pipe and source zone pumping were simulated for aquifer models of a prospective site where different hydrogeologic scenarios are considered. Calibrated groundwater models were constructed for scenarios such as an aquifer overlying intact bedrock, an aquifer intersected by a highly conductive layer and overlying intact bedrock, and an aquifer overlying fractured bedrock. A highly heterogeneous aquifer model was also constructed and considered for a case where the bedrock is intact and where the underlying bedrock is fractured as well. Each design considered reduced plume mass by at least 79% for each hydrogeologic scenario. The inclusion of pipe in the gravel trench outperformed the gravel trench design by 1% to 7%. Aquifer scenarios that have intact bedrock were generally more responsive to the inclusion of pipe in the hydraulic barrier. For scenarios that considered an aquifer overlying fractured bedrock, the difference between the performance of a gravel trench with pipe and a gravel trench designs was less and source zone pumping was needed to improve the performance of the permeable hydraulic barrier designs.

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