Lecture41-43(2008)

Lecture41-43(2008) - Solute Trans port and Salinity Chapter...

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Unformatted text preview: Solute Trans port and Salinity Chapter 9: Environmental Soil Ph ysics Soil Water • Liquid water in soil is not chemically pure. • Water flowing in soil carries chemicals and biological materials with it. • As water moves through soil chemicals desorbs from the soil particles and go into solution. • The desorbed chemicals are transported with the flowing water. • Some desorbed chemicals can adsorb onto soil particles that come in contact with the water, and others move with the water to the groundwater. • Solutes (chemicals or substances dissolved in water or other fluid) move with soil water and/or in response to concentration gradients. • Chemicals applied to the soil are in high concentrations at the soil surface - -with the general direction of chemical movement being downward. Solute Trans port • Chemicals move through the soil by convection and/or diffusion . • Chemicals that are transported through the soil by water are convectively transported . • Chemicals at high concentrations undergo random thermal motion or agitation resulting in repeated collisions and deflection of solute molecules in water. • The thermal motion results in spontaneous movement of the solute (chemical) from regions of high to one of low concentrations ( diffusive transport) . 1. Convection -- solutes carried by water, J c , is described by the following equation: J c = C K ) H/ ) L where C is the concentration of solute, K is the hydraulic conductivity, and ) H/ ) L is the hydraulic gradient. = qC 2. Diffusion -- random thermal motion and repeated collisions and deflections of molecules in water. • Solutes diffuse from where their concentration is higher to where it is lower. • Solute diffusion rate, J d , for solute in water where the water is not moving is: J d = D o ) C/ ) L where D o is the diffusion coefficient for the solute diffusing in bulk water (water not moving) ) C/ ) L is the concentration gradient. (Note: similar to hydraulic gradient). Diffusion in soil, D s : -- Soil is a porous medium. • Soil contains a solid matrix with porous sections. • Water and solute flow in soil must take place within the porous space....
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Lecture41-43(2008) - Solute Trans port and Salinity Chapter...

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