Estuarine wetlands and salt marshes are fundamentally driven by variations in freshwater inflow. In semi-arid salt marshes, such as the Nueces River Delta, TX, the stochastic nature of freshwater inflow events exposes resident organisms to a wide range of environmental conditions. In this study, we investigate (1) the relative importance of environmental variables on porewater salinity and (2) determination of freshwater inflow needs based on the response of emergent plants to salinity variations. Porewater salinity variations were tracked on a continuous basis with deployed conductivity sensors and on a synoptic basis with soil water extracts. We found that spatial patterns of porewater salinity were characterized by a high degree of variability in creekbank areas (23.8 ± 7.68) relative to interior marsh areas (44.2 ± 3.4). Our observations were used to test a simple model capable of predicting porewater salinities based on environmental variables. Both empirical measurements and model simulations indicated that semiannual tides play a critical role in controlling porewater flushing from precipitation and freshwater inflow events. Estimation of freshwater inflow needs for the Nueces Delta proceeded in two steps. First, we examined the response of three common emergent plants species (Borrichia frutescens, Spartina alterniflora, and Salicornia virginica) to variations in salinity. The abundance of one species in particular (S. alterniflora) was tightly coupled to salinity variations whereby salinities exceeding 25 ± 5 resulted in dramatic declines in coverage. Next, the relationship between freshwater inflow and porewater salinity was examined with respect to the salinity “tolerance” of S. alterniflora. Estimated inflow needs based on maintenance of substantial (> 20%) S. alterniflora coverage was comparable to both previous inflow needs estimates and mean annual inflows observed over the course of the study. The results of this study suggest that S. alterniflora abundance provides a reliable indicator of overall estuarine hydrological condition in the Nueces Delta. / text

<p> Systematic soil sampling and analyses provided DDT, DDE and DDD, organic and mineral matter concentrations, as well as various soil physical and hydraulic properties from three study sites at Point Pelee National Park. A soil's physical properties, soil and water management practices, and DDT application history, can affect DDT degradation and change the relative amount of its metabolites. DDD is the principal product of the dechlorination of DDT in high moisture content, reducing anaerobic soil environments. The main degradation product in soils under aerobic conditions is DDE. %DDT, %DDE and %DDD abundances used in conjunction with soil environment characterization data, can be used to indicate not only whether DDT is degrading, but what environmental factors are controlling its degradation. At the Park, DDT is primarily lost from the soil by microbial degradation to DDE and DDD. %DDT, %DDE and %DDD ratios from each study site indicated that wetter more organic-rich soil environments degraded DDT to its metabolites at a faster rate than drier less organicrich soils. Moreover, historical water level data was used to illustrate that this wetter study site was flooded for part of the year when adjacent marsh water levels were high. These conditions resulted in the accumulation of organic matter over time and the creation of alternating anaerobic/aerobic conditions in the soils resulting in an increased rate of degradation of DDT in these areas. Based on the relative %DDT in the soils at each study site, relative half-life estimates for the first order decay of DDT to its metabolites DDE and DDD were calculated. Half-life estimates for DDT at the wettest and most organic-rich study site range from 6 to 8 years; significantly lower than the two other study sites, which range from 15 to 30 years and are on the high end of the range reported in the literature.</p> / Thesis / Master of Science (MSc)

Analytical solutions for aquifer response to pumping are commonly used to infer the hydraulic properties of aquifers. This dissertation develops new analytical solutions for the analysis of pumping test data from confined and unconfined aquifer.An analytical solution for flow to a partially penetrating well of infinitesimally small radius in a compressible unconfined aquifer is developed that allows inferring its saturated and unsaturated hydraulic properties from drawdowns recorded in the saturated and/or the unsaturated zone. The effects of unsaturated zoneconstitutive parameters and thickness on drawdowns in the saturated and unsaturated zones as functions of position and time is investigated; the solution is validated against numerical simulations of drawdown in a synthetic aquifer having unsaturated properties described by the van Genuchten (1980) - Mualem (1976)constitutive model; used to analyze drawdown data from a pumping test conducted by the US Geological Survey at Cape Cod,Massachusetts; and corresponding estimates of van Genuchten - Mualem parameters are compared with laboratoryvalues obtained for similar materials in the area.Drawdowns generated by extracting water from a large diameter (e.g. water supply) well are affected by wellbore storage. An analytical solution in Laplace transformed space for drawdown in a uniformanisotropic confined aquifer caused by withdrawing water at a constant rate from a partially penetrating well with storage is developed. When the pumping well is fully penetrating the solution reduces to that of Papadopulos and Cooper (1967); to that of Hantush (1964) when the pumping well has no wellbore storage; to the solution of Theis (1935) when both conditions are fulfilled; and to that of Yang et al. (2006) when the pumping well is partially penetrating, having finite radius but lacking storage. The solutionis validated against synthetic pumping test data and used to explore graphically the effects of partial penetration, wellbore storage and anisotropy on time evolutions of drawdown in the pumping well and in observation wells.The analytical solution for unconfined aquifers is extended to the case of a finite diameter pumping well with storage. The extended analytical solution is used to investigate the effects of storage in the pumping well and delayed piezometer response on drawdowns in the saturated and unsaturated zones as functions of position and time. The solution is validated against numerical simulations of drawdown in a synthetic aquifer having unsaturated properties described by the van Genuchten (1980) - Mualem (1976) model. It is then used to analyze a seven-day pumping test conducted by University of Waterloo researchers at the Canadian Forces Base Borden in Ontario, Canada; and to compare our results with those ofMoench (2008).

Doctor of Philosophy / Department of Biological & Agricultural Engineering / Aleksey Y. Sheshukov / As the world faces an increasing demand for food due to the growing global population and the pernicious effects of land degradation, there is a need to overcome this challenge by using sustainable management practices for agricultural productions. One of the problems, which sustainable agriculture seeks to address, is the loss of topsoil due to soil erosion. Changing weather patterns also contribute to the average annual rainfall across the globe with an excess precipitation, which creates runoff and causes soil erosion. One of the significant yet less studied types of soil erosion is ephemeral gully erosion. Formed by the concentrated overland flow during intensive rainfall events, ephemeral gullies are channels on agricultural fields that can be removed by tillage operations but appear at the same location every year. Even though simplified ephemeral gully models estimate soil losses, they do not account for complicated hydrological and soil erosion processes of channel formations. The purpose of this research work is to investigate sediment sources and develop tools that can predict ephemeral gully erosion more efficiently. To achieve this goal, an experimental study was conducted on an agricultural field in central Kansas by tracking channel development, monitoring soil moisture content, and recording the amount of rainfall. Runoff and sediment loads from contributing catchment and critical and actual shear stresses were estimated by the computer model, and conclusions were made on the effect of saturation dynamics on the erosion processes. Furthermore, a two-dimensional subsurface water flow and soil erosion model was developed with the variable soil erodibility parameters which account for the subsurface fluxes and the effects on the soil detachment process. The model was applied to study the impacts of variable soil erodibility parameters on the erosion process for different soils and various antecedent soil moisture conditions. Also developed to estimate the soil losses at the field scale was an integrated spatially-distributed ephemeral gully model with dynamic time-dependent channel development. The model showed good fit by matching the experimental data. The results from this work can be used to advance the research of soil erosion prediction from concentrated flow channels and ephemeral gullies formed on agricultural fields.

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