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Groundwater and Contaminant Transport Modeling for Environmental Site Remediation Decision-Making
thesisposted on 01.05.2021, 00:00 by Gary Richard Johnson
The thesis document explores the use of groundwater and contaminant transport modeling as a mechanism for forecasting and improving decisions during the environmental remedial process. The advent and technological development of visualization and predictive models over the past several years provides opportunities for the professional practitioner to improve strategic environmental decisions. This document develops the conceptual model in the form of site characterization and explores the use of a fate and contaminant transport model. The study area is an abandoned property that experienced a gasoline leak from underground storage tanks. An environmental investigation identified petroleum hydrocarbons impacting the shallow aquifer system and extending horizontally in a northerly direction for approximately ¾ of a mile. A soil venting and air sparge remediation system was installed and operated for six years until dissolved hydrocarbons achieved an asymptotic value in the shallow groundwater system. The dissolved phase of the petroleum-related hydrocarbons, in many instances, are the priority concern at leak sites after the mobile light nonaqueous liquid (LNAPL) is resolved and endures as the long-term focus of the contaminant transport model. The spatial distribution of the dissolved petroleum hydrocarbons components of concern – benzene, toluene, ethylbenzene, xylene isomers (BTEX) has been monitored since April 2008 and continued periodically during the environmental assessment and mitigation process – most recent analysis in 2019. The conceptual site model and site characterization describe a shallow aquifer impacted by petroleum hydrocarbons in gaseous, adsorbed, aqueous, and non-aqueous (LNAPL) phases. The study area was remediated to a low dissolved, and partially sequestered residual hydrocarbons state. Two distinct BTEX plumes were identified at the study area and identified as a “source” near MSG-7 and a “downgradient” area near MW-17. Between these two source areas, the contaminant pathway was remediated to non-detectable dissolved BTEX values forming two source conditions at the site. The fate and contaminant transport characteristics of residual dissolved components remaining at the study area were evaluated to quantify the migratory and natural attenuation potential of the contaminants at the site. The groundwater flow and contaminant transport model employ the graphical user interface of Visual Modflow Flex 6.1 to interact with the underlying MDOFLOW, MT3DMS, and RT3D codes to solve complex subject numerical model issues. The MODFLOW and MODPATH codes identify migratory patterns, hydraulic characteristic outcomes, and the relationship to hydrocarbons contaminants at the study area. The Modular Three-Dimensional Multi-Species (MT3DMS) Transport model uses a single species BTEX parameter to evaluate the fate and transport of the BTEX dissolved plume using no kinetic reaction parameters. The reaction transport three-dimensional (RT3D) code simulates contaminant fate and transport model of BTEX plume sources using seven pre-programmed reaction modules to evaluate different reactive contaminants and compares the outcome to the MT3DMS model. Multiple simulations of the RT3D model were used to estimate the active remediation and post-remediation predictive conditions for the study area.