No description available.

A two-dimensional finite element model was developed to predict water flow and nitrogen species transport and transformation in variably saturated soil. In the finite element analysis various numerical algorithms were examined to evaluate methods of improving the efficiency of the traditional finite element approach. Results indicate that when undeformed linear rectangular elements or linear triangular elements were used, the method of influence coefficients is more efficient than traditional numerical integration method in evaluating the element matrices while maintaining accuracy. Also with moderately nonlinear flow problems, use of a fourth order Runge-Kutte method produced improved efficiency over fixed or variable time step schemes for time integration. Hysteretic simulations with air entrapment showed that effects of hysteresis are greatly enhanced by the presence of air entrapment due to large differences in water contents between different saturation paths. Flux-controlled boundary conditions produced negligible hysteretic effects while maximum effects were caused under potential type boundary conditions. The magnitude of hysteretic effects are also affected by the definition of initial condition. Heterogeneity in the porous medium tends to reduce hysteretic effects. The finite element model developed to solve the convective-dispersive equation for nitrogen transport utilized an upstream weighting scheme to reduce numerical oscillation associated with low dispersion coefficients. The accuracy and validity of the model were evaluated using both published and field data. The results indicate overall predictions of various nitrogen fractions are quite sensitive to the first order nitrification rate. However, use of kinetic constants based on published results in the literature, especially in the absence of detailed field investigations, may still be sufficient to provide dependable results through simulations. Hysteresis greatly affects the transport of nitrogen species under potential type boundary conditions, but these effects are reduced substantially in the presence of high intensity line sources. Under such circumstances, it is sufficient to neglect hysteresis and use either the main drainage or the main wetting branch of the pressure-saturation relationship for simulations. / Ph. D.

Ground water contamination from agricultural nitrates poses potential adverse health effects to a large segment of the rural population of the United States. Contamination is especially prevalent in livestock intensive areas, which produce large quantities of animal waste with substantial nitrogen content. In this study, potential management strategies for reducing nitrate contamination of ground water from agricultural sources were examined using an economic-physical model of a representative dairy farm in Rockingham County, Virginia. A mixed integer programming model with stochastic constraints on nitrate loading to ground water and silage production was used to simulate the impacts of various nitrate loading reduction strategies on estimated farm level net returns over variable costs. A survey of all dairy operations in the county was conducted to assist in specifying the mathematical programming model, identify current nutrient management and quality issues, and gauge farmers’ attitudes toward ground water quality and agricultural chemical use. Results of the model indicate that substantial reductions in current nitrate loadings are possible with relatively minor impacts on farmers’ net returns through the use of currently practiced approaches of cost sharing for manure storage facility construction and nutrient management planning. Greater loading reductions are achievable through presently untried policies of land use restrictions, bans on purchase of commercial fertilizer, and imposition of standards on loadings to ground water. These reductions are achieved, however, at higher costs in terms of reduced net returns. Study results indicate that a wide range of policy options exist for reducing nitrate loading to ground water; these reductions, while varying in cost, do not appear to come at the expense of eliminating the economic viability of the county dairy sector. Model results indicate that reductions in nitrate loading of 40 to 70 percent (on average) could be achieved with reductions in farmers’ net returns of one to 19 percent, respectively, when cost sharing for manure storage construction was provided. Explicit consideration was given to the annual variability in nitrate loading due to weather and other factors. The result was higher policy costs than when average loadings alone were considered. / Ph. D.

Interest in halocarbons has been sparked by experimental evidence suggesting that these compounds are potential carcinogens and teratogens. The U.S. EPA started a nationwide program in 1970 to identify and quantify trace organic compounds in public water supplies and sewage effluents. To date no detailed large-scale areal ground-water surveys delineating concentrations of these halocarbons have been reported in the literature at the part per trillion level and below. The objectives of this dissertation are twofold: (1) identification, quantification, and detailed areal mapping of several halocarbon species in ground water near the Santa Cruz River northwest of Tucson, Arizona; and (2) development of halocarbon techniques for age dating recently recharged (0-40 years old) ground water. The halocarbon compounds considered are: trichlorofluoromethane (CC1₃F), dichlorodifluoromethane (CC1₂F₂), carbon tetrachloride (CC1₄), chloroform (CHC1₃), trichloroethylene (C₂HC1₃), methyl chloroform (CH₃CC1₃), and tetrachloroethylene (C₂C1₄). Objective (1) includes discussions of the areal halocarbon distributions detected in the ground water, their sources, and a qualitative comparison of the distributions to ground-water quality and land use patterns adjacent to the Santa Cruz River. Objective (2) utilizes the exponential atmospheric concentration buildup of CCl₂F₂, CC1₃F, and CCl₄, and the ratios of CCl₂F₂ to CCl₃F and CCl₄ to CCl₃F. Water samples from wells and the Santa Cruz River were collected in glass syringes and brought to the laboratory for analysis. An electron-capture gas chromatograph coupled to a gas stripping/concentration unit was used to quantify the halocarbons. Four high concentration areas were delineated, each probably associated with a different source: the CWUA area which was irrigated with sewage effluent during the 1960's; Rillito Creek (near its confluence with the Santa Cruz) which is a major ground-water recharge source; the Ina Road treatment plant/landfill/oxidation ponds area; and the Roger Road treatment plant "sewer farm" which is irrigated with sewage effluent. The CCl₂F₂ to CCl₃F ratio distribution indicates that most ground water in the study area is at least partially mixed with recharge less than 30 years old. The ground water adjacent to the Rillito has an apparent age of less than 10 years, in agreement with the CCl₃F distribution. Ground water in the Cortaro area has an apparent age of about 25 years, correlating with the start of irrigation in the area.

Yuen Yuk-man. / Bibliography: leaves 117-120 / Thesis (M.Phil.) -- Chinese University of Hong Kong, 1983

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A 4.87 hectare potato field at St. Leonard d'Aston, Quebec was instrumented to measure surface runoff and tile drain flow over the 2 year growing season period, 1989-1990. The soil type was a Ste. Jude sandy loam. Several soil and water parameters and NPK concentrations in runoff were measured. The CREAMS (Chemicals, Runoff and Erosion from Agricultural Management Systems) computer simulation model was validated for the study site. / Observed N concentrations in tile drain flow exceeded the Canadian water quality guideline of 10 mg/L. Observed P concentrations were less than 0.01 mg/L; and K concentrations, for the most part, exceeded 10 mg/L. / CREAMS overpredicted event surface runoff depths, and underpredicted event percolation depths. However, total monthly surface runoff and percolation depths closely matched observed values. / CREAMS overpredicted event nitrate concentrations in tile drain flow. There was a poor match between predicted and observed event nitrate concentrations in tile drain flow (coefficient of predictability, CP$ sb{ rm A}$ = 104.95). However, predicted total monthly nitrate load closely matched observed values (CP$ sb{ rm A}$ = 0.84). Total monthly and seasonal nitrate loads in tile drain flow were underpredicted.

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A method was developed for assessing the vulnerability of groundwater to contamination from contaminant sources at the soil surface, using a numerical groundwater flow model linked to a digital map database. The method was applied using the pcARC/INFO Geographic Information System (GIS) to input, store, and manipulate base maps, resulting in a database of digital maps for the alluvial aquifer system in the Willamette Valley of western Oregon. Digital elevation maps were created by digitizing topographic maps of land surface (1:250,000 scale), water surface, and the base of the Tertiary-Quaternary sedimentary deposits (1:500,000 scales). Soil association and aquifer unit maps digitized from 1:500,000 scale map sheets were also used. Data were extracted from ARC/INFO to the SURFER software package to create a 3-D surface model for each of the digital elevation maps. An ARC/INFO point coverage was then used to store and overlay these surfaces, allowing the creation of maps of depth to water, saturated thickness, and water table gradient. These data became the input to a numerical finite element groundwater flow model. The model solves a dual formulation problem for the potential function and the stream function to calculate the time-of-travel for water to flow from the surface to the water table and laterally for 100 meters as an index of groundwater vulnerability. A cluster analysis is used to condense the data and form a training data set for a multiple regression model. The regression model is fit to the results of the finite element model with an R-squared of greater than 0.96. The simpler regression model is then used for mapping travel times for the entire study area. When properly calibrated against the finite element model and when combined with the digital map database and Geographic Information System (GIS) procedures described, the regression model can be conveniently used to assess the vulnerability of groundwater to contamination over large areas. / Graduation date: 1989

Suppose C(x,t) is the concentration at position x in Rᵈ and time t > 0 of a solute which is diffusing in some medium. If on a local scale the dispersion of the solute is governed by a constant dispersion matrix, 1/2(δ²), and a random velocity field, V(x), then C satisfies a convection-diffusion equation with random coefficients, (DIAGRAM, TABLE OR GRAPHIC OMITTED...PLEASE SEE DAI) (1). Usually V(x) is taken to be μ + εU(x) where μ ε Rᵈ, U(x) is a given stationary random field with mean zero, and ε > 0 is a dimensionless parameter which measures the variability of V(x). Hydrological experiments suggest that on a regional scale the diffusion is classically Fickian with effective diffusion matrix D(ε) and drift velocity α(ε). Thus for large scales (DIAGRAM, TABLE OR GRAPHIC OMITTED...PLEASE SEE DAI) (2) is satisfied by the solute concentration. Here τ and χ are respectively time and space measured on large scales. It is natural to investigate the relation of the large scale coefficients D and α to the statistical properties of V(x). To relate (1) to (2)--and thus to approximate D(ε) and α(ε)--it is necessary to rescale t and x and average over the distribution of V. It can then be shown that the transition form (1) to (2) is equivalent to (DIAGRAM, TABLE OR GRAPHIC OMITTED...PLEASE SEE DAI) (3) where A = (∇•δ²∇)/2 + √nμ• ∇ and B(U) = √nU(√nx) • ∇. By expanding each side of (3) estimates of D(ε) and α(ε) can be obtained. The estimates have the form (DIAGRAM, TABLE OR GRAPHIC OMITTED...PLEASE SEE DAI) (4). Both D₂ and α₂ depend on the power spectrum of U. Analysis shows that in at least the case of incompressible fluids D₂ is positive definite. In one dimensional transport α₂ < 0, hence α(k) < μ(k) through second order.