In a critical review of pathogen and indicator-organism transformations and transport from land areas receiving organic wastes, microbial die-off was described assuming first-order kinetics. First-order die-off rate constants (k) were calculated from the literature data for various pathogens and indicator organisms. For indicator organisms average die-off.rates were 1.14 day-' (0.08-9.1) for fecal coliforms, and 0.41 day-' (0.05-3.87 day-') for fecal streptococci. For pathogens, the average die-off rates were 1.33 day-' (0.21-6.93) for Salmonella, 0.68 day-' (0.62-0.74 day-~) for Shigella sp., and 1.45 day-' (0.04-3.69 day-') for viruses, respectively. Die-off rates increased approximately two times with a 10°C rise in temperature ($-30°C). Microbial die-off increased with decrease in soil moisture and was minimum in a pH range of 6-7. Correction factors were presented to adjust the k values for the changes in temperature, moisture, and pH. Retention of pathogens and indicator organisms by soil particles was described as. suming a linear isotherm. Retention of microorganisms increased with an increase in clay content of the soil. Major transport processes reviewed were leaching and surface runoff for land areas receiving animal wastes, and pastures and rangeland watersheds where animals distribute waste directly on the land. Some of the important research needs identified include (i) mechanisms involved in the retention of bacteria and viruses by the soil; (ii) measurement of retention coefficients for some important pathogens and indicator organisms, as a function of physico-chemical properties of soil; (iii) processes involved in the transport of bacteria and viruses ' Paper no. 6448 of the Journal Series of the North Carolina Agric. Res. Serv., Raleigh.
This textbook integrates classic principles of flow through porous media with recently developed stochastic analyses to provide new insight on subsurface hydrology. Importantly, each of the authors has extensive experience in both academia and the world of applied groundwater hydrology. The book not only presents theories but also emphasizes their underlying assumptions, their limitations, and the potential pitfalls that may occur as a result of blind application of the theories as "cookiecutter" solutions. The book has been developed for advanced-level courses on groundwater fluid flow, hydraulics, and hydrogeology, in either civil/environmental engineering or geoscience departments. It is also a valuable reference text for researchers and professionals in civil/environmental engineering, geology, soil science, environmental science, and petroleum and mining engineering.
Predicting contaminant migration within the vadose zone, for performance or risk assessment, requires estimates of unsaturated hydraulic conductivity for field soils. Hydraulic conductivities, K, were experimentally determined as a function of volumetric moisture content, 0, for Hanford sediments. The steady state head control method and an ultracentrifuge method were used to measure K(0) in the laboratory for 22 soil samples. The van Genuchten model was used to fit mathematical functions to the laboratorymeasured moisture retention data. Unsaturated conductivities estimated by the van Genuchten-Mualem predictive model, using the fitted moisture retention curve and measured saturated hydraulic conductivity, K s, were compared to those obtained by a scaled-predictive method that uses a single K(0) measurement as a match point near the dry regime. In general, the measured K values and those predicted from van Genuchten-Mualem relationships showed considerable disagreement. This suggests that the use of laboratory-measured K s results in an inadequate characterization of K(0) for the desired range of moisture content. Deviations between the measured and predicted K were particularly severe at relatively low moisture contents; for some samples, there were differences in excess of 2 orders of magnitude at low 0. However, use of the same moisture retention curve-fitting parameters and a single steady state head control-based K(0 ) measurement near the dry regime resulted in considerable improvement. In fact, for the coarse-textured soils considered in this study, results indicate that a K(0) measurement near the dry regime must be used to obtain reliable estimates of unsaturated K at low 0. The study provided important insight on application of two different experimental techniques of measuring unsaturated conductivities.
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