Contaminants can enter groundwater through the unsaturated zone as dissolved solutes. To predict the location and extent of these contaminants, transport parameters such as pore water velocity y and dispersion coefficient D are required. These parameters are often obtained through transport experiments. The goal of this study is to determine y and D using time domain reflectometry (TDR) technique. Using TDR for transport experiments under unsaturated conditions, we investigated the effects of volumetric water content θᵥ, distance of flow path, and draining-wetting history on D. TDR was used to measure θᵥ, and salt concentration in twenty-one unsaturated column experiments. The 105 cm-long column was homogeneously packed with silica sand (particle size: 53 to 425 pm). Ten TDR probes at ten depths were used to obtain in situ breakthrough curves and a chloride electrode was used to measure effluent breakthrough curves at the bottom of the column. A 35 mM NaC1 (sodium chloride) was used as the tracer with 20 mM NaC1 as background solution. We developed a three-parameter expression relating θᵥ, to measured dielectric constant Kₐ: θᵥ =aKₐᵅ + b. This calibration expression fits as closely or better than the "universal polynomial" and is also consistent with the well-known mixing model. For an isotropic soil with homogeneous water distribution, this expression is further simplified to two parameters by taking α = 0.5. The effects of temperature, porosity, soil solid and bound water can be taken into account by varying a and b of the two-parameter expression. TDR measurements have been shown to be sensitive to bound water and not particular sensitive to the other factors. To calculate y and D from breakthrough curves of step-input experiments, a new moment analysis method has been developed. The transport parameters obtained from this new method show a little difference from the parameters determined from the convection-dispersion equation using the CXTFIT model (a published computer program for estimating solute transport parameters from observed breakthrough curves). Our results demonstrated that D is dependent on measurement methods and concentrations of experimental solutions.

Sandy soils are among the least productive soils because of their inability to store adequate water for plant growth. Their high percolation rate not only allows water to move quickly beyond the root zone, but also washes nutrients below the reach of plant roots. High evaporation occurs from the soil surface. Many acres of these soils around the world are left out of crop production. This study is a contribution to bring these soils into production by increasing their ability to hold more water in the root zone. Several promising methods of enhancing these soils were simulated, surface mulch, buried barrier layer, and a combination of both. The effects of varying texture and thickness of these layers and varying evaporative demand were investigated. The impact of such modifications on solute distribution in the soil was also simulated. A simulation model of water and solute transport in layered soils was developed for this purpose. The Richards equation for one-dimensional water transport in unsaturated soils was modified to account for the water jump between the layers. The solute transport equation was also modified by implementing the same theory of water infiltration in layered soil to the solute convective transport. The Crank-Nicolson scheme was used to solve the transport equations with the help of the Newton-Raphson iteration method. The results of the simulation show that the proposed methods increase water content in the sandy soil by up to 45%. The combination of barriers, which decreases leaching and evaporation was the most effective in conserving water. Most of the contribution came from the influence of the mulch layer in suppressing water losses by evaporation. The combination method traps solute in the root zone, and this decreased solute leaching from the soil may limit plant growth in saline soils. / Graduation date: 1993

Radon transport mechanisms in soils were studied to determine the dominant transport mechanism for Delaware county soils. In modeling the soil, it was assumed that is was homogenous and moisture-free. Two transport mechanisms were investigated, the transport of radon in the soil by molecular diffusion (assumed to be governed by Fick's law) and transport by pressure-induced flow or convection (assumed to be governed by Darcy's law). Following the previous work of W. E. Clements, a general transport equation was described which incorporated both diffusion and convection. In steady-state conditions, a closed-form solution was obtained for the concentration of radon in the soil interstices as a function of depth. Similarly, solutions were examined for transport by diffusion alone. Representative soil parameters were assigned and the diffusion fraction (the ratio of concentration due to diffusion to the concentration due to both diffusion and convection) was calculated. Referring to the work of A. B. Tanner, a radon availability number (RAN) was determined for the soils; the RAN value was a measure of the activity of radon per unit area. Analyses were also performed to determine the significance of pressure variations on calculated diffusion fractions and RAN values. For 99% of the acreage in Delaware county, the diffusion fraction was 0.95 or greater. Therefore, it was concluded that molecular diffusion is the dominant transport mechanism for the soils of Delaware county. / Department of Physics and Astronomy

A laboratory study of the mobility of Malathion in the soil environment was conducted. / Master of Science