esm222_08_9_movement

esm222_08_9_movement - ESM 222 Movement of Pollutants In...

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1 © Arturo A. Kel er ESM 222 Movement of Pollutants 2 © Arturo A. Kel er Movement of Pollutants ± In addition to transferring from one phase to another, pollutants may move in the environment within the phase they are in. ± Examples: ± PCBs emitted to the atmosphere will travel with particles along wind currents and deposit in some cases hundreds of km away from source ± dissolved pollutants (e.g. gasoline spill) may move through the subsurface with ground water 3 © Arturo A. Kel er Movement of Pollutants ± There are two main physical processes that transport the contaminants away from the source: ± Advection ± Diffusion and dispersion Rate of mass accumulation = Rate of mass flow in - Rate of mass flow out ± Rate of transformation 4 © Arturo A. Kel er Advection ± Moving along with air in the atmosphere or water in a water body (e.g. rivers, lakes, ocean, groundwater) ± Pollutant tags along at velocity of flowing air or water ± If pollutant transfers to another phase (e.g. aerosols or particulate matter in water) then its direction of movement will be altered 5 © Arturo A. Kel er Advection v = velocity (m/s) t 1 t 2 L = length (m) 6 © Arturo A. Kel er Advection ± Time required to move a given particle from point 1 to point 2 is: t 2 -t 1 = t = ± We can also refer to a volumetric flow rate, Q in (m 3 /s), as the velocity that a given surface moves through the “channel” L v
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7 © Arturo A. Kel er Advection v (m/s) t 1 t 2 L (m) Q = v A =[m 3 /s] A = area (m 2 ) 8 © Arturo A. Kel er Advection ± If the channel gets narrower or shallower (or both), the velocity at which the water flows must increase to conserve mass. v upstream v downstream v downstream > v upstream Q downstream = Q upstream 9 © Arturo A. Kel er Advection ± Advective Flux vC F adv = z F y F x F t C z , adv y , adv x , adv + + = If the velocities are constant or zero: z C v y C v x C v t C z y x + + = 10 © Arturo A. Kel er Advection ± Water moving through soil (or fractured rock) cannot go through the entire area since it is partially blocked by rock. ± It can only go through the pore space, which means a rather tortuous path… ± Actual area available for flow is: total flow nA A = 11 © Arturo A. Kel er Advection ± Why does water flow in the subsurface? ± think of it as a container, which fills in one end by rainfall and empties on the other to a stream, river, lake or ocean Water body Flow 12 © Arturo A. Kel er Advection Water (or an organic liquid) flows from a high point to a low point: h 1 h 2
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13 © Arturo A. Kel er Advection ± The driving force is the difference in heights: h = h 1 -h 2 ± Volumetric flow rate is similar to flow rate for open channels, but with a “correction” factor to account for the reduced path for water in soil: (Darcy’s law) A L h K Q = 14 © Arturo A. Kel er Advection ± Slope of water table, i.e. how fast it is dropping per distance traveled, is called gradient in hydraulic head: gradient = dh dx = [ ] m m ± K, the hydraulic conductivity, is a property of
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This note was uploaded on 08/06/2008 for the course ESM 222 taught by Professor Keller during the Spring '08 term at UCSB.

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esm222_08_9_movement - ESM 222 Movement of Pollutants In...

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