Facets of Vulnerability and Risk at the Nexus of Groundwater and Unconventional Energy Resources
Date of Award
Doctor of Philosophy (PhD)
Forestry and Environmental Studies
Unconventional oil and gas (UOG) development is the extraction of fossil fuels from shale and other low permeability formations, made possible by technological advances in horizontal drilling and high volume hydraulic fracturing. Hydraulic fracturing (“fracking”) refers to the stage in UOG development where millions of gallons of water mixed with sand and chemicals are injected at high pressures to create openings in the target formation, enabling the flow of trapped hydrocarbons. UOG development has been hailed for its socioeconomic benefits, allowing the United States to become a net energy exporter in recent years and facilitating the transition away from coal towards lower CO2-emitting natural gas. With natural gas viewed as a necessary transition fuel towards renewable energy sources, the UOG industry is projected to continue to expand domestically and globally in the coming decades. Nevertheless, concerns about the risks posed by this water-intensive industry, especially on drinking water contamination and its public health implications, persist. Such questions are particularly salient in many rural communities hosting expansive UOG development colocated with local populations depending on private water wells for their daily needs.This dissertation focuses on developing physically based approaches for quantifying groundwater vulnerability to contamination from UOG development activities at multiple scales. In this work, vulnerability refers to the likelihood for contaminants to reach a groundwater receptor, such as a drinking water well. The work also aims to elucidate the hydrogeologic and geospatial controls on vulnerability. In Chapter 2, the vulnerability of individual water wells within a watershed is evaluated using a backward advective-dispersive transport formulation. This approach is expanded to the county scale in Chapter 3, where machine learning models were trained to learn general input-output relationships from physically based models. In Chapter 4, regional scale vulnerability is assessed using a forward advective transport formulation implemented within an ensemble framework. The groundwater models are calibrated to match observations of hydraulic head and discharge to ensure their accurate representation of field conditions. Potential sources of uncertainty in the physically based approaches are explored through the use of multiple alternative models and global sensitivity analysis, and uncertainties in the physics-informed machine learning approaches are explored through various data splitting and cross-validation techniques. Groundwater flow and transport model parameters that have a dominant influence on vulnerability in our study regions include the hydraulic conductivity and porosity of the aquifer and the well pumping rate. Furthermore, consistent with the source-pathway-receptor framework, vulnerability is controlled by proximity to contaminant sources and conditioned by topography and hydrogeological connectivity. Geospatial metrics describing these factors simultaneously, namely the inverse distance to nearest upgradient UOG source and the difference in elevation between a receptor and its nearest UOG source, were highly informative in predicting vulnerability in both the backward and forward transport formulations. In topographically controlled groundwater systems, the former can be viewed as an approximate measure of the advective flow path length, while the latter captures the difference in potential energy needed to drive flow. The assessment of groundwater vulnerability to contamination is shown to complement other lines of inquiry in the investigation of potential impacts of UOG development. These include contemporary and historical groundwater quality observations, as well as UOG violation reports indicating environmental releases of contaminants. The novel approaches developed in this work present a significant advancement in the evaluation of UOG contamination risks, which have traditionally been assessed based on proximity alone without incorporating physical flow and transport mechanisms. The spatially explicit approaches enable objective evaluation of the adequacy of existing setback distances, which are land zoning regulations that aim to protect critical receptors (e.g., drinking water wells) by creating physical separation from UOG activities. The proposed vulnerability assessment frameworks offer scientifically defensible platforms for integrating physical principles with stakeholder values in the design of policies to protect groundwater resources from contamination.
Soriano, Mario, "Facets of Vulnerability and Risk at the Nexus of Groundwater and Unconventional Energy Resources" (2022). Yale Graduate School of Arts and Sciences Dissertations. 525.