Recent invasions of woody plants into semiarid grasslands are a world-wide phenomena with potential ramifications for global-scale carbon cycling. An understanding of how biological and non-biological processes within ecosystems influence water loss to the atmosphere is important to evaluating the consequences of woody plant encroachment on carbon and water cycling in semiarid lands. Accordingly, evapotranspiration in a Chihuahuan Desert grassland was partitioned into its component fluxes for the 2005 summer growing season using a combination of microlysimeters, to quantify soil evaporation, and eddy covariance, to quantify evapotranspiration and net ecosystem exchange of CO2 (NEE). While some of the results of this study (e.g., the ratio of T to ET) are expected to be highly dependent on the particular characteristics of the 2005 summer rainy season, many of them reveal a more general picture about the timing and magnitude of the biological and non-biological water and carbon cycling responses for a warm-season semiarid grassland. This will be important for trying to understand what happens to the carbon and water cycling processes as grasslands are invaded by shrubs.

<p> Early thermal breakthrough is an issue of concern in the geothermal industry, especially with regard to engineered geothermal systems (EGS). Determination of the specific surface area (SSA) of the fluid/rock interface in an EGS is vital to predicting early thermal breakthrough. An approach to this problem involving the application of tracers with different sorption properties (lithium, bromide, and deuterium) is presented. Upon injection into a reservoir, these tracers react along the fluid/rock interface to varying degrees. The resulting breakthrough separation at the extraction well can be used to derive SSA by applying a modified form of the advection-dispersion equation. For proof of concept, field tests were conducted in a sandstone bedding-plane fracture near Chazy, New York. Results showed minimal breakthrough separation, indicating insufficient sweep of the fracture area, and that lithium was not an ideal tracer in this medium. However, a relative measure of SSA can still be derived. </p>

<p> Karst carbonate aquifers are traditionally difficult to model due to extreme permeability heterogeneities and non-Darcian flow. New modeling techniques and test applications are needed to improve simulation capabilities for these complex groundwater systems. This study evaluates the coupled continuum pipe-flow framework for modeling groundwater flow in the Madison aquifer near Rapid City, South Dakota. The Madison carbonate formation is an important source of groundwater underlying Rapid City. An existing equivalent porous medium (EPM) groundwater model of the Madison aquifer was modified to include pipe networks representing conduits. In the EPM model, karstified portions of the aquifer are modeled using high hydraulic conductivity zones. This study hypothesized that the inclusion of conduits would allow for a simpler hydraulic conductivity distribution and would improve modeled fits to available data from a 10-year monitoring period. Conduit networks were iteratively fit into the model based upon available environmental and dye tracer test data that approximated major karst pathways. Transient simulation results were evaluated using observation well hydraulic heads and estimated springflow data. In a comparison to the EPM model, the new modeling results show an improved fit to the majority of observation well targets, and negligible impact to springflow data. The flow dynamics of the aquifer model were significantly altered, with the conduit networks acting as gaining or losing subsurface features, behaving as regional sinks during dry periods and flowpath heterogeneities during wet periods. The results of this study demonstrate that the coupled continuum pipe-flow modeling method is viable for use within large regional aquifer models.</p>

<p> This study uses DC Electrical Resistivity Tomography (ERT) to delineate hydrostratigraphic units within the lower South Platte River alluvial aquifer. The geophysical investigation was conducted at Tamarack Ranch State Wildlife Area in northeastern Colorado, where the South Platte River is artificially recharged via pumping to surface recharge ponds and groundwater flow through the underlying unconfined alluvial aquifer system. Twenty-seven ERT profiles collected within a 4.2 km² study area on the south bank of the South Platte River define 3 different electrostratigraphic units. The ERT data was correlated with drilling logs and laboratory resistivity measurements to develop a hydrostratigraphic model and confining bedrock surface map. Results indicate 7-25 m thick eolian sand deposits (50-800 ohm-m) serve as infiltration zones and do not readily store groundwater. These eolian deposits form up to 15 m high sand hills in the southern half of the study area, and underlie recharge ponds that are used as water sources for artificial recharge of the river. The underlying alluvium (20-3890 ohm-m) varies from 10-70 m thick and functions as the primary groundwater storage unit. A 10-20 m thick intermittent conductive zone (25-80 ohm-m) occurs within the upper part of the alluvial layer that underlies the sand hills, and is interpreted to be caused by clay deposits that potentially influence initial groundwater flow paths emanating from the recharge ponds. The alluvium is underlain by highly conductive siltstone and claystone bedrock formations (1-60 ohm-m) that confine the aquifer system. The bedrock surface is complexly eroded (1055-1110 m.a.s.l.) and is characterized by prominent large-scale paleo-topographic lows (at typical scales of 700 m wide, 35-40 m deep and 700 m wide, 20-25 m deep) that occur on the northern bank of an incised paleo-channel. These features are interpreted to represent a paleo-topographic surface formed by groundwater outflow in the form of piping and sapping networks. The rugged bedrock topography establishes a previously unrecognized first order control on groundwater flowpaths within the unconfined alluvial aquifer.</p>

The flow of water through variably saturated subsurface media is commonly modeled by Richards' equation, a nonlinear and possibly degenerate partial differential equation. Due to the nonlinearities, this equation is difficult to solve analytically and the literature reveals dozens of papers devoted to finding numerical solutions. However, the literature also reveals a lack of two important research topics. First, no a priori error analysis exists for one of the discretization schemes most often used in discretizing Richards' equation, cell-centered finite differences. The expanded mixed finite element method reduces to cell-centered finite differences for the case of the lowest-order discrete space and certain quadrature rules. Expanded mixed methods are useful because this simplification occurs even for the case of a full coefficient tensor. There has been no analysis of expanded mixed methods applied to Richards' equation. Second, no results from parallel computer codes have been published. With parallel computer technology, larger and more computationally intensive problems can be solved. However, in order to get good performance from these machines, programs must be designed specifically to take advantage of the parallelism. We present an analysis of the mixed finite element applied to Richards' equation accounting for the two types of degeneracies that can arise. We also consider and analyze a two-level method for handling some of the nonlinearities in the equation. Lastly, we present results from a parallel Richards' equation solve code that uses the expanded mixed method for discretization.

The availability of high-resolution LIDAR and GIS data combined with the onset of more powerful computers has made the use of distributed parameter hydrologic models more feasible. This study employs one such model, Vflo(TM), along with a widely used hydraulic model, HEC-RAS, to simulate the runoff-response of a resaca network in Brownsville, TX using newly available LIDAR data. While previous studies have attempted to simulate the runoff-response of a resaca, they were greatly limited by the lack of detailed topographic data and information describing the hydraulic structures that connect one resaca pool to the next. Floodplain mapping in the area revealed that very little flooding occurs in and around the resacas compared to the two main drainage ditches in the area. Furthermore an evaluation of the available storage in the resacas shows great potential for the use of resacas as detention reservoirs in the future for excess stormwater.

As the results of seismic reflection and ground penetrating radar (GPR) studies become more prevalent as input for quantitative groundwater and engineering studies, it is important to evaluate traditional approaches to data processing and analysis. Where conventional methods fail it is necessary to investigate and/or develop non-traditional approaches to data analysis. I present five stand-alone studies that are focused on characterizing shallow aquifers using seismic reflection and GPR data processing and analysis. Each of the projects involves a new approach to data analysis either through alternative processing strategies that are not widely applied in environmental studies or development of new processing methods and/or algorithms. The first two studies are focused on seismic reflection imaging problems that arise in the shallow environment. I first present a detailed discussion of the errors that can result from conventional normal-moveout (NMO) processing, and the application of pre-stack depth migration (PSDM) to improve image accuracy. Second, I apply dip-moveout (DMO) processing to a data set from the Puget Sound. DMO is rarely applied in environmental studies, but can improve velocity analysis and image quality where there are dipping layers or scattering events. The final three studies are focused on attribute analysis of GPR and seismic reflection data in direct detection studies. Over the past 10--15 years, direct detection has been used successfully in the energy industry to identify oil and gas reserves from exploration scale seismic reflection data, but is a new approach to the analysis of GPR and shallow seismic reflection data. Direct detection studies use reflected wave attributes such as amplitude, frequency content, and phase to estimate material properties. I first present a detailed discussion of GPR amplitude vs. offset (AVO) analysis for direct detection of free phase non-aqueous phase liquid contaminants (NAPLs). The analysis is not straightforward and requires careful consideration of electromagnetic dynamic properties. Second, I present a shallow seismic case study where a predictable AVO response was detected. Finally, I present a new wavelet decomposition and time-frequency representation, and illustrate applications in GPR attenuation analysis for NAPL detection and lithology characterization.

This study aimed to understand annual and long-term chemical and physical processes that affect the development and state of coastal lakes in the Canadian High Arctic. The first major research project studied the formation of hypersalinity in a seasonally isolated marine basin (SIMB) located near Shellabear Point, Melville Island, Northwest Territories (75'N, 113'W). To quantify the role of brine rejection from a seasonal ice pan, an annual ice-formation model was applied, which incorporated stable oxygen isotope fractionation. This model complemented seasonal sampling of ionic and isotopic composition and a full-year recording of physical lacustrine parameters. Additionally, a pilot study using radon-222 as a tracer of groundwater seepage was undertaken to quantify the brine seepage from surrounding permafrost into the hypolimnion of the SIMB. Results indicate that brine rejection from the annual formation of lake ice in a SIMB is the major driver of hypersalinity in this system, and that the current chemical constitution may have formed in less than a decade. This study contributes novel hypotheses on the chemical formation of hypersaline lakes, but also coastal meromictic lakes derived from epishelf lakes during the Holocene. The second investigation focused on the seasonal variability of limnological processes in two coastal freshwater lakes in the Canadian Arctic. East and West Lake are located at the Cape Bounty Arctic Watershed Observatory (CBAWO, 75'N, 109'W), Melville Island. From 2006 to 2009, there have been two central changes in the lake systems: 1) an enhanced ionic loading into both lakes since 2007, and 2) a significant increase in the suspended sediment concentration in the lower 15 m of West Lake in 2009. The elevated ionic concentrations are a result of increased active layer thickening during an unusually warm summer in 2007. The high turbidity in 2009 had a substantial impact on annual mixing of West Lake. The suspended sediment produced a density gradient that prevented river-generated density currents from delivering fresh oxygenated water to the bottom of the lake, and prevented thermal warming. The lack of re-oxygenation and nutrient delivery likely had a substantial impact on the benthic ecosystem in the lake. / Thesis (Master, Geography) -- Queen's University, 2010-05-24 10:26:59.208

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