Thesis (Ph. D.)--University of Texas at Austin, 2005. / Supervisors: David M. Hillis and Dean Hendrickson. Vita. Includes bibliographical references.

Groundwater-derived phosphorus has often been dismissed as a significant contributor towards surface water eutrophication, however, this dismissal is unwarranted, making the quantification of phosphorus concentrations in groundwater systems immensely important. Machine learning models have been employed to quantify the concentrations of various contaminants in groundwater, but to our best knowledge have never been used for the quantification of groundwater phosphorus. The goal of this research was to use a boosted regression tree framework to produce the first believed machine learning model of phosphorus variability in groundwater, with the High Plains aquifer serving as the study area. Results display a boosted regression tree model that was not capable of explaining and predicting the statistical variance of phosphorus throughout the aquifer under standard conditions, however important variable correlation data that can potentially be incorporated into future studies that aim to further understand phosphorus dynamics in groundwater was obtained from this research.

There are many circumstances where lateral flows in the upper soil layers above the regional groundwater table are important for hillslope and catchment hydrology, and in particular for the transport of contaminants. Perched water tables frequently occur in Quaternary drift sequences, reducing rates of recharge to the underlying aquifers and altering contaminant migration pathways; recent experimental and modelling studies have demonstrated the potential importance of lateral flows in the unsaturated zone, even in homogeneous soils; and lateral interflow at the hillslope scale, and its role in generating storm runoff, is the subject of intense current debate amongst hydrologists. A numerical model for simulating transient three-dimensional variably-saturated flow in complex aquifer systems (the Variably-Saturated Subsurface flow, or VSS, model), capable of representing these conditions, is presented in this thesis. The VSS model is based on the extended Richards equation for saturated as well as unsaturated conditions, and also includes capabilities for modelling surface-subsurface interactions, stream-aquifer interactions, prescribed head and flow boundary conditions, plant and well abstractions, and spring discharges. A simple but novel approach is taken to solving the three-dimensional non-linear Richards equation on a flexible-geometry finite-difference mesh, using Newton-Raphson iteration and an adaptive convergence algorithm. The VSS model is implemented as a module of the catchment flow and transport modelling system, SHETRAN. The reliability of the full SHETRAN modelling system is demonstrated using verification and validation tests, including comparisons against analytical solutions for simple cases, and simulations of storm runoff in a small Mediterranean catchment. Simulations of flow and contaminant transport in complex sequences of Quaternary drift deposits demonstrate the full capabilities of the modelling system under real-world conditions.

This dissertation is composed of three parts of contributions. Systems of a fully penetrating pumping well in a confined aquifer near a fully penetrating stream with and without streambeds are discussed in Chapter II. In Chapter III, stream-aquifer systems with a fully penetrating pumping well in a confined aquifer between two parallel fully penetrating streams with and without streambeds are discussed. Stream depletion rates in Chapter II are solved using Laplace and Fourier transform methods, and stream depletion rates in Chapter III are solved using the potential method. Chapter II presents analytical solutions in the Laplace domain for general stream depletion rates caused by a pumping well and caused by stream stage fluctuations. For seasonal case, the stream stage is a function of time. For an individual flood wave, the stream stage is a function of time and distance along the stream. Semi-analytical solutions of seasonal stream depletion rates in time domain, using a cosine function to simulate stream stage fluctuations, are presented. The stream depletion rate caused by pumping is solved analytically, while the stream depletion rate caused by stream stage fluctuations is solved numerically. Various parameters affecting stream depletion rates, such as flood period and streambed, are analyzed. For a short-term case, the pumping rate is assumed to be constant, and a Gaussian function is used as an example of floodwaves. This part is solved using the same method as used in the seasonal case. Early time and late time approximations of the stream depletion rates are also presented. This approximation leads to an interesting finding that the stream depletion rate caused by seasonal stream stage fluctuations can be neglected if the stream aquifer system has a long time to equilibrate. In Chapter III, analytical stream depletion rates caused by a pumping well between two parallel streams with and without streambeds are presented. In this chapter, stream stage is assumed to be constant. Capture zone delineations were analyzed in the case without streambed. For the case with streambed, streambed conductance, which is an important factor controlling stream depletion, is analyzed. All solutions discussed in this dissertation can be used to predict stream depletion rates and to estimate parameters controlling stream depletion rates, which is crucial for water management. In addition to the stream depletion, the derived semi-analytical solutions in the Laplace-Fourier domain can also be used to predict drawdown in the aquifer near the stream. The derived solutions may also be used inversely to find the streambed and aquifer parameters if the stream stage fluctuation can be well described.

This dissertation is composed of three parts of contributions. Systems of a fully penetrating pumping well in a confined aquifer near a fully penetrating stream with and without streambeds are discussed in Chapter II. In Chapter III, stream-aquifer systems with a fully penetrating pumping well in a confined aquifer between two parallel fully penetrating streams with and without streambeds are discussed. Stream depletion rates in Chapter II are solved using Laplace and Fourier transform methods, and stream depletion rates in Chapter III are solved using the potential method. Chapter II presents analytical solutions in the Laplace domain for general stream depletion rates caused by a pumping well and caused by stream stage fluctuations. For seasonal case, the stream stage is a function of time. For an individual flood wave, the stream stage is a function of time and distance along the stream. Semi-analytical solutions of seasonal stream depletion rates in time domain, using a cosine function to simulate stream stage fluctuations, are presented. The stream depletion rate caused by pumping is solved analytically, while the stream depletion rate caused by stream stage fluctuations is solved numerically. Various parameters affecting stream depletion rates, such as flood period and streambed, are analyzed. For a short-term case, the pumping rate is assumed to be constant, and a Gaussian function is used as an example of floodwaves. This part is solved using the same method as used in the seasonal case. Early time and late time approximations of the stream depletion rates are also presented. This approximation leads to an interesting finding that the stream depletion rate caused by seasonal stream stage fluctuations can be neglected if the stream aquifer system has a long time to equilibrate. In Chapter III, analytical stream depletion rates caused by a pumping well between two parallel streams with and without streambeds are presented. In this chapter, stream stage is assumed to be constant. Capture zone delineations were analyzed in the case without streambed. For the case with streambed, streambed conductance, which is an important factor controlling stream depletion, is analyzed. All solutions discussed in this dissertation can be used to predict stream depletion rates and to estimate parameters controlling stream depletion rates, which is crucial for water management. In addition to the stream depletion, the derived semi-analytical solutions in the Laplace-Fourier domain can also be used to predict drawdown in the aquifer near the stream. The derived solutions may also be used inversely to find the streambed and aquifer parameters if the stream stage fluctuation can be well described.

This dissertation is composed of five chapters and three major contributions are presented in Chapter II, III and IV. Chapter I provided a review of studies on solute transport in aquifer-aquitard system. If the aquitard is considered, two categories of methods address the diffusive flux between the aquifer and aquitard: the old method treats the diffusive flux as a volumetric source in the governing equation of the solute transport in the aquifer; the new method treats the aquifer-aquitard boundary as a strict physical boundary with the requirement of continuity of solute concentration and the vertical flux. The new method is adopted throughout this study. In Chapter II, a review of numerical techniques on Inverse Laplace Transform is provided. By careful comparison between several popular algorithms, the multiple precision Stehfest algorithm is chosen as the method to inverse out solutions on solute transport in Laplace domain throughout this dissertation. In Chapter III, solutions were obtained for two dimensional solute transport in an aquifer-aquitard system with a divergent radial flow field, which can treat different types of solute input function and advection, longitudinal and transverse dispersion in the aquifer, vertical diffusion in the aquitard, retardation and radioactive decay in the aquifer and aquitard are taken into account. Mass exchange via diffusion between the aquifer and aquitard are investigated. The effects of hydrologic properties of the aquitard on solute transport are analyzed. Comparisons were made between the results from this study and those from previous studies. The diffusion along the aquifer-aquitard boundary was treated as a volumetric source term, and proved these solutions yield more accurate solute concentration, while those from previous studies tend to overestimate solute concentration in the aquitard, and underestimate the concentration in the aquifer. In Chapter IV, solutions were derived for the transport of radioactive isotopes in an aquifer-aquitard system with regional flow field. This study focused on the effects of different solute transport processes on the results of groundwater age dating using radiometric techniques. Chapter V summarized the remaining problems in this study and directions for future researches.

Leakage of stored bulk phase CO₂ is of particular risk to sequestration in deep saline aquifers due to the fact that when injected into typical saline aquifers, the CO₂ rich gas phase has lesser density than the aqueous phase resulting in buoyancy driven flow of the fluids. As the CO₂ migrates upward, the security of its storage depends upon the trapping mechanisms that counteract the migration. While there are a variety of trapping mechanisms the mechanism serving as motivation for this research is local capillary trapping. Local capillary trapping occurs during buoyancy-driven migration of bulk phase CO₂ within a saline aquifer (Saadatpoor, 2009). When the rising CO₂ plume encounters a region where capillary entry pressure is locally larger than average, CO₂ accumulates beneath the region. While research is continued by means of numerical simulation, research at the bench scale is needed to validate the conclusions made from simulation work. Presented is the development of a bench scale experiment whose objective is to assess local capillary trapping. The initial step in accomplishing this objective is to understand the fluid dynamics of CO₂ and brine in a saline aquifer which is categorized as two phase immiscible buoyancy driven displacement. Parameters influencing this displacement include density, viscosity, wettability and heterogeneity. A bench scale environment created to be analogous to CO₂ and brine in a saline aquifer is created in a quasi-two dimensional experimental apparatus, which allows for observation of plume migration at ambient conditions. A fluid pair analogous to supercritical CO₂ and brine is developed to mimic the density and viscosity relationship found at pressure and temperature typical of storage aquifers. The influences of viscosity ratio, density differences, porous medium wettability and heterogeneity are observed in series of experimental sequences. Three different fluid pairs with different viscosity ratios and density differences are used to assess density and viscosity influences. Porous media of varying grain size and wettability are used to assess the influence of heterogeneity and wettability. Results are qualitatively consistent with theoretical results and those from previous works. / text

Thesis (M.S.)--University of Wyoming, 2009. / Title from PDF title page (viewed on May 7, 2010). Includes bibliographical references (p. 116-118).

Domestic water wells in Eastern Ontario were identified in potentially vulnerable fractured and karst bedrock aquifers using geologic and geochemical data. A novel methodology is presented that evaluates ¹³⁷Cs and ²¹⁰Pbₑₓ as local indicators of groundwater vulnerability. The method is designed to determine the vulnerability of a specific well. Suspended sediment samples and well-bottom sediment samples were collected from both potentially vulnerable and non-vulnerable wells. Surface soil samples were also collected from West Rural Ottawa and the Township of Alfred & Plantagenet in Ontario, Canada. Gamma spectroscopy was used to analyze the samples and quantify the presence of the radionuclides in cps and cps/g. The spectral data indicate no significant difference in the activities of ²¹⁰Pbₑₓ among samples, but a significant difference in the activities of ¹³⁷Cs was observed between surface soil samples and well-bottom sediment samples collected from vulnerable wells. The data suggest that ²¹⁰Pbₑₓ does not act as a good indicator of vulnerable aquifers because of its geogenic origin. The anthropogenic origin of ¹³⁷Cs precludes this issue, and while ¹³⁷Cs was detected in measurable quantities at the surface, its use as an indicator of vulnerable aquifers is limited by hydrologic and geologic controls that prevent infiltration in vulnerable terrains.

The Hampton Roads Sanitation District is heading a Managed Aquifer Recharge project designed to build water resiliency for the district as well as meet recent regulations concerning effluent released into the Chesapeake Bay. The Sustainable Water Initiative for Tomorrow (SWIFT) project will include five injection well fields across the Virginian Coast. The first of these fields to be implemented is the James River site, scheduled to begin in 2025. A model of the Virginia Coastal Plain region was created in 2009 and has been used to simulate the combined impact of the full-scale SWIFT project. This study estimated the change in hydraulic head in the Potomac Aquifer System caused by the proposed James River recharge well field at a regional and local level. That estimation required the use of a widely accepted model of the Virginia Coastal Plain developed in 2009 which was subjected to a limited validation using USGS monitoring well data for comparison. That model was then used to establish boundary conditions for a local scale model surrounding the James River site, after which each model was used to run four pumping scenarios with varying rates of recharge. The validation of the Virginia Coastal Plain model found it to be satisfactory for the scope of this work, and it was therefore used to interpolate boundary conditions for the developed local model. The regional and local model both showed an increase in the simulated head values of the Potomac Aquifer System. The regional model simulated a sharper initial increase than the local model, however, long term the local model simulated a greater impact to the groundwater levels from the proposed recharge. / Master of Science / The Potomac Aquifer System (PAS) is a main water source for most of eastern Virginia and high pumping rates have caused notable drawdown in several areas. The Hampton Roads Sanitation District (HRSD) has initiated the Sustainable Water Initiative for Tomorrow (SWIFT) project that is designed to alleviate the stress on the PAS by artificially recharging the PAS through injection well. A regional groundwater model, built in 2009, has been used to estimate the impact of the proposed recharge for the SWIFT project at full capacity. This work validated the use of the regional model within the region of the first proposed SWIFT well field at the James River Site. Once the validation was complete, the regional model provided a framework to develop a more detailed model on a smaller scale. That model was then used to simulate the proposed injection well field at varying rates to estimate the effect of the James River Site. This study has shown that the regional model provides an adequate framework to build local scale models. The simulations run in both the regional and local models found that the proposed recharge increases the water levels in the PAS immediately surrounding the well field and that the impact is felt to distances exceeding 50 miles after 10 years.