chapter  7
42 Pages

Effects of Microbiological Factors on Water Flow in Soils

I. Typical Change in Permeability ................................ 260 II. Microbiological Effects on Permeability.................... 261

A. Direct Effects....................................................... 261 B. Indirect Effects .................................................... 263

1. Effects of Gaseous Production...................... 263 2. Effects of Ferrous Iron.................................. 266

III. Effects of Microbiological Factors on Water Flow in Natural Soils .......................................................... 269 A. Microorganisms in Soils...................................... 269 B. Microbiological Effects on Permeability

of Natural Soils ................................................... 273 IV. Effects of Microbiological Factors on Water

Flow in Clay Liners of Landfill Sites ........................ 277 V. Microbial Clogging Models......................................... 280

A. Microbial Clogging in Soils ................................. 280 B. Microbial Clogging Models.................................. 282 C. Colony-Enveloping Model ................................... 285

VI. Microbial Growth and Substrate Transport Model... 288 A. Microcolony Concept ........................................... 288 B. Conservation and Production of Biomass

in a Microcolony .................................................. 290 1. Flow Rate Chart of Material ........................ 290 2. Monod Equation............................................ 291 3. Fluxes in Diffusion Layer............................. 293 4. Advection-Dispersion Equations ................. 294 5. Solution of Advection-Dispersion

Equations ...................................................... 296 References............................................................................ 298

I. TYPICAL CHANGE IN PERMEABILITY

It is generally known that when a soil is submerged and the hydraulic conductivity of the soil is measured every day, a systematic change of saturated hydraulic conductivity is obtained, as shown in Figure 7.1. Allison (1) explained this change in permeability as consisting of the following three phases:

1. Initial decrease attributed to a structural change. The decrease in this permeability is probably due either to structural changes resulting from swelling and dispersion of the dry soil upon wetting or due to dispersion resulting from a decrease in electrolyte

content of the soil solution as any salts present are removed in the percolate. On highly permeable soils this initial decrease is small, or nonexistent, but for relatively impermeable soils, it may be appreciable and continue for 10 to 20 days before the second phase of increase is apparent.