Transport of suspended participate matter is widely recognized to occur in subsurface environments. Field data indicate that viruses, bacteria, and clay minerals can migrate considerable distances and that small particles and macromolecules are implicated in the transport of organic contaminants and radio-nuclides. Furthermore, media permeability can be significantly altered by changes in aqueous chemistry through particle release and capture. Quantitative models for predicting particle transport are available within the water filtration literature that account for the mechanisms of particle-media collisions and the conditions for attachment. Predictions from the filtration models are used to analyze particle migration through porous media at typical groundwater flow velocities. As particles accumulate within media pores, available models become less predictive because of the coupling between particle retention and permeability reduction. An examination of filtration data reveals that retention of a relatively small solid volume within media pores can reduce media permeability by orders of magnitude. The fact that contaminants adsorbed to particles are mobile has important implications in understanding and predicting contaminant transport. The design of laboratory experiments and the collection of field samples often neglect contaminants transported by suspended colloids and particles.

Particle transport through porous media

Work of the last 10 years has demonstrated that oceanic particle size distribution by volume tends to be flat at mid-water depths (equivalent to a cumulative particle number distribution with a slope of −3) and is peaked in nepheloid layers with active resuspension and in surface waters with active biological production. The observed loss of fine peaks from the suspensions to yield flat distributions requires aggregation of the material, as the fines settle slowly. Mechanisms leading to particle collision are examined; for interactions between particles of similar size, Brownian motion dominates below 1.5 to 8 μm. However, if large particles (such as ‘marine snow’) are present at realistic concentrations, they become important in the removal of fine particles by shear-controlled coagulation. The coagulation times calculated for shear are too long for steady state to be presumed while the size distributions evolve under the influence of coagulation mechanisms. Therefore suggestions that the flat size distributions are quasi-stationary results of shear-controlled coagulation are rejected, and the notion that there is sub-equal production of particles at different points in the spectrum is favoured. Such production and the subsequent scavenging of small particles by large settling ones confers great importance on components of biological origin in both providing elements of the total size spectrum and determining the distribution and sedimentation of others of lithogenic origin. In surface waters, filtration rates by zooplankton indicate that aggregation rates of particles above submicron sizes are biologically determined.

Size spectra and aggregation of suspended particles in the deep ocean

The biological clogging of natural porous media, often in conjunction with physical or chemical clogging, is encountered under a wide range of conditions. Wastewater disposal, artificial groundwater recharge, in situ bioremediation of contaminated aquifers, construction of water reservoirs, or secondary oil recovery are all affected by this process. The present review provides an overview of the techniques that are used to study clogging in the laboratory, or to monitor it in field applications. After a brief survey of the clogging patterns most commonly observed in practice, and of a number of physical and chemical causes of clogging, the various mechanisms by which microorganisms clog soils and other natural porous media are analyzed in detail. A critical assessment is also provided of the few mathematical models that have been developed in the last few years to describe the biological clogging process. The overall conclusion of the review is that although information is available on several aspects of the biological clogging of natural porous media, further research is required to predict its extent quantitatively in a given situation. This is particularly true in cases that involve complicating factors such as predation or competition among organisms.

Environmental impact and mechanisms of the biological clogging of saturated soils and aquifer materials

Statistical and continuum models of fluid-solid reactions in porous media

CHANGES IN PERMEABILITY CAUSED BY TRANSIENT STRESSES: FIELD OBSERVATIONS, EXPERIMENTS, AND MECHANISMS Michael Manga, 1 Igor Beresnev, 2 Emily E. Brodsky, 3 Jean E. Elkhoury, 4 Derek Elsworth, 5 S. E. Ingebritsen, 6 David C. Mays, 7 and Chi-Yuen Wang 1 Received 7 November 2011; revised 15 February 2012; accepted 10 March 2012; published 12 May 2012. [ 1 ] Oscillations in stress, such as those created by earth- quakes, can increase permeability and fluid mobility in geo- logic media. In natural systems, strain amplitudes as small as 10 A6 can increase discharge in streams and springs, change the water level in wells, and enhance production from petroleum reservoirs. Enhanced permeability typically recovers to prestimulated values over a period of months to years. Mechanisms that can change permeability at such small stresses include unblocking pores, either by breaking up permeability-limiting colloidal deposits or by mobilizing droplets and bubbles trapped in pores by capillary forces. The recovery time over which permeability returns to the prestimulated value is governed by the time to reblock pores, or for geochemical processes to seal pores. Monitor- ing permeability in geothermal systems where there is abun- dant seismicity, and the response of flow to local and regional earthquakes, would help test some of the proposed mechanisms and identify controls on permeability and its evolution. Citation: Manga, M., I. Beresnev, E. E. Brodsky, J. E. Elkhoury, D. Elsworth, S. E. Ingebritsen, D. C. Mays, and C.-Y. Wang (2012), Changes in permeability caused by transient stresses: Field observations, experiments, and mechanisms, Rev. Geophys., 50, RG2004, doi:10.1029/2011RG000382. INTRODUCTION [ 2 ] The permeability of Earth’s crust is of great interest because it largely governs key geologic processes such as advective transport of heat and solutes and the generation of elevated fluid pressures by processes such as physical com- paction, heating, and mineral dehydration. For an isotropic Department of Earth and Planetary Science, University of California, Berkeley, California, USA. Department of Geological and Atmospheric Sciences, Iowa State University, Ames, Iowa, USA. Department of Earth and Planetary Sciences, University of California, Santa Cruz, California, USA. Department of Civil and Environmental Engineering, University of California, Irvine, California, USA. Department of Energy and Mineral Engineering, Center for Geomechanics, Geofluids, and Geohazards, EMS Energy Institute, Pennsylvania State University, University Park, Pennsylvania, USA. U.S. Geological Survey, Menlo Park, California, USA. Department of Civil Engineering, University of Colorado Denver, Denver, Colorado, USA. Corresponding author: M. Manga, Department of Earth and Planetary Science, University of California, 307 McCone Hall, Berkeley, CA 94720, USA. (manga@seismo.berkeley.edu) material, permeability k is defined by Darcy’s law that relates the fluid discharge per unit area q to the gradient of hydraulic head h, q ¼A kgr rh; m where r is the fluid density, m the fluid viscosity and g is gravity. The permeability of common geologic media varies by approximately 16 orders of magnitude, from values as low as 10 A23 m 2 in intact crystalline rock, intact shales, and fault cores, to values as high as 10 A7 m 2 in well-sorted gravels. Nevertheless, despite being highly heterogeneous, perme- ability can be characterized at the crustal scale in a manner that provides useful insight [e.g., Gleeson et al., 2011]. [ 3 ] The responses of hydrologic systems to deformation provide some insight into controls on permeability, in par- ticular its evolution in time. For example, the water level in wells and discharge in rivers have both been observed to change after earthquakes. Because earthquakes produce stresses that can change hydrogeologic properties of the crust, hydrologic responses to earthquakes are expected, especially in the near field (within a fault length of the Copyright 2012 by the American Geophysical Union. Reviews of Geophysics, 50, RG2004 / 2012 1 of 24 Paper number 2011RG000382 8755-1209/12/2011RG000382 RG2004

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Changes in permeability caused by transient stresses: Field observations, experiments, and mechanisms

Removal mechanisms in deep bed filtration

Kinetics of the filtration of dilute suspensions

K.J. Ives Professor of Public Health Engineering University College London The filtration of liquids is a process which is widely used in industry. In some cases the object of the process is to clarify the liquid, as in the filtration of beer, or sugar solutions; in other cases the object is to recover solids suspended in the liquid, for example, ores in mineral processing, or the production of pigments such as titanium dioxide. In addition to the many industrial applications of liquid filtration, a considerable investment lies in municipal applications for the filtration of drinking water, wastewater and the sludges arising from water and sewage treatment. Indeed, in terms of volume filtered, it is likely that the municipal application exceeds that of industry. The difference between clarification of the liquid and recovery of the solids is often expressed in the division of the processes into deep bed filtration and cake filtration.

The Scientific Basis of Filtration

Capture Mechanisms in Filtration

Previous theoretical and experimental work has established the mathematical laws describing the water quality and head loss changes through the depth and during the time of operation of a filter run. The present study has evaluated how this performance will change in other filter runs operated with different sized media, or at different filtration rates, or at different water temperatures. Using a particulate suspension of plastic microspheres (diameter 1.3 microns) to conform with the mathematical idealizations, it has been found that filter efficiency varies inversely with grain size, inversely with the filtration rate, and as the inverse square of the water viscosity. Some examples of different filter operations are calculated and compared using these relationships. Other published theoretical and experimental studies have been reviewed, which also indicate inverse relationships between filter efficiency and grain size, rate, and viscosity. However, these are not all identical and it appears that each suspension may have characteristic exponents associated with the three variables investigated.

Kenneth J. Ives

Papers

Removal mechanisms in deep bed filtration

Kinetics of the filtration of dilute suspensions

The Scientific Basis of Filtration

Capture Mechanisms in Filtration

Research on Variables Affecting Filtration