Search

Your search keyword '"Wraith, J. M."' showing total 84 results
Start Over Author "Wraith, J. M."
84 results on '"Wraith, J. M."'

5. Preferential Flow through Intact Soil Cores: Effects of Matric Head

Author

Langner, H. W., Gaber, H. M., Wraith, J. M., Huwe, B., and Inskeep, W. P.

Subjects
Soil research -- Case studies, Soil moisture -- Research, Earth sciences
Abstract

Continuous soil pores may act as pathways for preferential flow depending on their size and water status (filled or drained), the latter being largely controlled by the soil matric head (h). The literature contains a wide range of proposed minimal pore sizes that may contribute to preferential flow. The objective of this study was to examine the relationship between h (and corresponding pore sizes) and preferential solute transport in a naturally structured soil. Tracer ([sup.3][H.sub.2]O and pentafluorobenzoic acid, [PFBA]) miscible displacement experiments were performed at several h values in intact soil cores (15-cm diameter, 30-cm length) using an apparatus especially suited to maintain constant h while collecting large effluent volumes. To test for the occurrence of preferential flow, observed breakthrough curves (BTCs) were evaluated for physical nonequilibrium (PNE) using a comparison between fitted local equilibrium (LE) and PNE models. Fitting results of the observed BTCs indicated absence of PNE in all solute transport experiments at h [is less than or equal to] -10 cm. Experiments at h [is greater than or equal to] -5 cm consistently exhibited PNE conditions, indicating the presence of preferential flow. These results suggest that soil pores with effective radii of 150 [micro]m and smaller (water-filled at h = -10 cm) do not contribute to preferential flow. Observed pore water velocities were not indicative of the presence or absence of preferential flow conditions. Continuous measurements of soil water content ([Theta]) using time domain reflectometry (TDR) revealed that at h = -10 cm, [is less than] 2% of the soil volume (1-5% of [Theta] at saturation) had drained.

Published
1999

44. Physical constraints affecting bacterial habitats and activity in unsaturated porous media – a review

Author

Or, D., Smets, B. F., Wraith, J. M., Dechesne, A., and Friedman, S. P.

Abstract

The immense diversity of microbial life found in the vadose zone reflects the extremely heterogeneous and highly dynamic aquatic and chemical environments formed within soil pore spaces. The notion of planktonian free swimming microbes is unrealistic under most unsaturated conditions. Experimental and theoretical evidence suggests that surface attachment is the prevailing lifestyle, where bacterial colonies are embedded in biosynthesized extracellular polymeric substances (EPS). This strategy represents a successful adaptation to the variable and unpredictable hydration conditions near the earth surface. The EPS matrix serves as the interface with the environment; it enhances hydration and transport properties in the immediate vicinity of microbial cells, and dampens effects of highly transient fluctuations in water and nutrient fluxes. The primary effect of soil pore geometry and hydration status is on diffusion pathways to and away from stationary microbial colonies. Microbial dependency on diffusion processes occurs at all scales, but is particularly important at the colony scale. We illustrate the critical role of diffusion pathways with their complex spatial and temporal patterns in promoting coexistence and diversity. We review specific features and adaptations of microbial life to the particular conditions of terrestrial soil environments. The physical and related chemical conditions that shape microbial habitats and govern key processes in unsaturated soils are reviewed in a quantitative framework. Key physiological adaptations and biological responses to challenges presented by unsaturated conditions are discussed. Finally, we discuss potential impacts of microbial activity on properties and characteristics of the host porous medium. This review is an attempt to establish an interdisciplinary dialogue between hydrologists and microbiologists towards a quantitative integration of the role of hydrologic conditions on microbial activity and the role of microbiology in controlling macroscopic fluxes within this important compartment of the biosphere.

Catalog

Books, media, physical & digital resources
See catalog results