John C. Tracy

Beneficial effects of plants in the remediation of soil and groundwater contaminated with organic materials

Abstract The use of plants in remediation of soil and unconfined groundwater contaminated with organic materials is appealing for a variety of reasons: (1) plants provide a remediation strategy that utilizes solar energy; (2) vegetation is aesthetically pleasing; (3) plant samples can be harvested and tested as indicators of the level of remediation; (4) plants help contain the region of contamination by removing water from soil; (5) rhizosphere microbial communities are able to biodegrade a wide variety of organic contaminants; and (6) many plants have mechanisms for transporting oxygen to the rhizosphere. However, before effective plant remediation strategies can be developed, an understanding is needed of the physical, biological, and chemical relationships that determine the fate of each organic contaminant in the rhizosphere. This review presents an overview of some factors required to understand and model the complex processes that determine the fate of the organic contaminants in plant remediation ...

Prairie Wetland Complexes as Landscape Functional Units in a Changing Climate

The wetland complex is the functional ecological unit of the prairie pothole region (PPR) of central North America. Diverse complexes of wetlands contribute high spatial and temporal environmental heterogeneity, productivity, and biodiversity to these glaciated prairie landscapes. Climatewarming simulations using the new model WETLANDSCAPE (WLS) project major reductions in water volume, shortening of hydroperiods, and less-dynamic vegetation for prairie wetland complexes. The WLS model portrays the future PPR as a much less resilient ecosystem: The western PPR will be too dry and the eastern PPR will have too few functional wetlands and nesting habitat to support historic levels of waterfowl and other wetland-dependent species. Maintaining ecosystem goods and services at current levels in a warmer climate will be a major challenge for the conservation community.

EXPERIMENTAL TECHNIQUES FOR MEASURING PARAMETERS DESCRIBING WETTING AND WICKING IN FABRICS

Once capillary pressure and permeability are determined for saturations ranging from near zero to 100%, liquid transport related to both wicking and wetting behavior can be described by Darcys equation. The purpose of the work reported here is to assess and develop experimental techniques that allow capillary pressure and per meability to be measured over a wide range of saturations. Cotton and polypropylene fabrics are the test materials. Capillary pressure head is measured as a function of saturation for cotton and polypropylene fabric samples using the column test, and permeability is measured as a function of saturation using the siphon test. The siphon test works for cotton but not for polypropylene. A new method using a transient measurement technique is developed to determine the permeability of both samples as a function of saturation; it works well for both samples.

Forecasting of reference crop evapotranspiration

Abstract The first step in the determination of irrigation water requirements for use in project planning, design, and operation commonly involves the prediction of the reference crop evapotranspiration. This paper presents a simple mathematical model for forecasting reference crop evapotranspiration at three stations in California that represent humid, semi-arid, and arid weather conditions, covering three extremes in the California climate. A time domain-time series model is identified for the reference crop evapotranspiration data at each of the sites and compared to other simple methods of forecasting using monthly average and yearly difference approaches. Based on the results of model comparisons it is concluded that a generalized time series model can be used to forecast reference crop evapotranspiration at each of the stations, and that the time series model can provide a reasonably accurate, yet simple method of forecasting reference crop evapotranspiration in many agricultural areas of California.

Experimental and modeling studies of the fate of organic contaminants in the presence of alfalfa plants

Abstract Experimental investigations were carried out in the laboratory to study the impact of vegetation in bioremediating soil and groundwater contaminated with hazardous organic substances. A chamber consisting of two U-shaped channels, each 1.8 m in length, 10 cm in width, and 35 cm in depth, was set up. The channels were packed with fine sandy soil collected from near a landfill. Alfalfa plants were grown in the channels under laboratory conditions for nearly two years. The water fed to the plants in one channel was contaminated with toluene solution at saturated concentrations at 26°C. Plants in the other channel were fed with water contaminated with phenol solution at 500 ppm (v/v). The contaminant concentrations in the groundwater were monitored at sampling wells located along each of the channels. The influent and effluent flow rates from each channel were recorded daily. Evapotranspiration significantly influenced the fate of the pollutants. Dispersion and adsorption processes in the channel were studied separately, by introducing bromide tracer as a broad pulse into the toluence fed channel, and by observing the washout of toluene and phenol contaminants following a feed step change to pure water. Tracer studies indicated that short-circuiting at the U-bend of the channel was quite significant. Previously developed models which described the fate of contaminants in variably-saturated soils in the presence of vegetation are employed to simulate the fate of these hazardous organic substances in the laboratory chamber.

Modeling heat and mass transfer in fabrics

Abstract A numerical model was developed for simulating the heat and mass transfer in fabrics during the wicking process. The model was applied to two different fabrics, cotton and polypropylene. The model shows that, as the water is wicked through the fabric specimens, two temperature zones are formed. Within each region the temperature gradient is small, but between the regions it is more significant. Unlike the temperature, the variation of the fractional saturation is continuous along the specimens. Experiments were also conducted to obtain the temperature distributions during wicking for the two fabrics and to validate the numerical model.

Development of Soil Moisture Drought Index to Characterize Droughts

AbstractA new drought index termed the “soil moisture drought index (SODI)” is developed to characterize droughts. The premise of the index is based on how much water is required to attain soil moisture at field capacity. SODI captures variations of precipitation, temperature, and soil moisture over time. Three widely used drought indices, including the standardized precipitation index (SPI), the standardized precipitation evapotranspiration index (SPEI), and the self-calibrated palmer drought index (sc-PDSI) are compared with SODI along with local hydrological variables such as streamflow, reservoir storage, and groundwater level for cross-validation. The result indicates that SODI reacts more evidently to relate changes in precipitation and temperature than SPI and SPEI by characterizing soil moisture over time. Results also show that SODI outperforms the existing drought indices in the sense that SODI can detect and quantify the extended severe droughts associated with climate variability and change. S...

Modeling moisture transfer in fabrics

Abstract By viewing the fabric as a highly porous medium, a conservation equation for moisture transfer in fabric was developed as a function of capillary pressure and permeability under isothermal conditions. The equation considers the transfer of both liquid and vapor due to the capillarity effect in the fibrous medium. This equation was solved numerically by the Galerkin finite-element method and was applied to two different fabric samples, one made of cotton and one of polypropylene. The model is capable of simulating the wicking process in general including the effect of gravity on moisture transport using parameters that can be measured readily. Results from the numerical model were compared with vertical and horizontal wicking experimental data. The model predictions agreed well with the experimental data in both cases. The vertical wicking experiment agreed better than the horizontal wicking experiment because the vertical test is easier to conduct and the fabric is not in contact with a support.

MODELING THE FATE OF TOLUENE IN A CHAMBER WITH ALFALFA PLANTS 2. NUMERICAL RESULTS AND COMPARISON STUDY

Investigations were conducted in a chamber to study the role of alfalfa plants in bioremediating toluene. Modeling and experimental results indicate that in situ bioremediation of toluene in the presence of plants is feasible and economical. This is primarily due to evapotranspiration which greatly enhances the vertical transport of dissolved contaminants from the saturated zone to the rhizosphere, thereby increasing the possibility of aerobic degradation. The fate of toluene was simulated and monitored for groundwater contaminated with toluene at saturated concentrations. FT-IR instrumentation was used to monitor toluene in the headspace gas of the vegetated chamber. Overall mass balance, based on groundwater and headspace measurements, indicated about 75% loss or biodegradation of toluene from the chamber during steady state. Evolution of significant amounts of CO 2 accounted for mineralization of toluene. Simulation results predicted toluene biodegradation in the unsaturated zone where both oxygen and toluene were present. Dispersion processes in the soil were characterized by bromide tracer analysis. Predictions from simulations were compared with the water content and toluene concentrations measured in the chamber.

MODELING THE FATE OF TOLUENE IN A CHAMBER WITH ALFALFA PLANTS 1. THEORY AND MODELING CONCEPTS

A model was developed to investigate the fate of organic contaminants in soils in the presence of vegetation. The model has two modules. The first module simulates the soil-water and root-water pressure heads under the influence of water extraction by the roots of growing vegetation. Evapotranspiration due to alfalfa plants is an outflux boundary condition at the soil surface for this model. The distributions for water and air contents and Darcy water flux are obtained from the soil-water pressure heads. The second module simulates the fate of soil constituents in the porous medium using the Darcy water flux. The constituents assumed to be present in vegetated soil were contaminant, biomass, oxygen, and root exudates. A Galerkin finite element method was used to solve the model equations in two dimensions to enable comparison with an experimental system. The domain simulating the experimental chamber was assumed to be comprised of rectangular elements with bilinear shape functions which represented the variations within each element. Convergence to solution for the non-linear equations was accomplished using the Picard iterative algorithm. The time derivative was approximated using an implicit CrankNicholson scheme.