Setting the Stage: A Groundwater Transport Model Used to Support an Innovative MNA Approach

A groundwater extraction and treatment system began operation in 1993 to provide hydraulic capture and limit off-site migration of volatile organic compounds (VOCs) at a Superfund site in southern New Hampshire. In 2003, following 10 years of effective operation and numerous optimization measures, 1,4-dioxane was analyzed for and detected in groundwater, which led to an Explanation of Significant Differences to modify the remedial action set forth in the 1988 ROD and establish a clean-up goal of 3.0 µg/L for 1,4-dioxane. Through December 2011, 149 million gallons of groundwater were treated; however, concentrations of select VOCs and more importantly 1,4-dioxane remained above ROD levels. To address these residual levels stemming from source area depletion effects, a detailed approach to evaluating the potential for MNA was predicated (and supported) by a groundwater flow and transport model. This model included two overburden units and two bedrock zones to simulate fracture flow using data obtained from local household well drilling, borehole geophysics, and hydraulic conductivity testing. The calibrated model was incorporated into a transport model using MT3D (Zheng 1990) to develop an understanding for both total VOC and 1,4-dioxane plume development and migration over time, based on key system operational periods. A series of forward-looking simulations were then used to evaluate plume configuration and contaminant magnitudes in the absence of remedial actions and with only natural attenuation affecting the respective VOC and 1,4-dioxane plumes. The simulations were conducted by adding an extended stress period with multiple time-steps after the stress period, simulating the most recent round of sampling data. The predictive capabilities of modeling were substantiated by a series of statistical and interpolative evaluations to assess plume size retraction and stability. The potential for in situ biodegradation of 1,4-dioxane was evaluated one-step further using environmental molecular diagnostics with a favorable outcome.