05.3.waterpollt3.aos104f08.sld9

05.3.waterpollt3.aos104f08.sld9 - Groundwater Pollution To...

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Groundwater Pollution 1 Water Pollution III Types of Aquifers Groundwater Flow Physics ! Darcy’s Law and Hydraulic Gradient ! Movement of Underground Plumes ! Cleanup Stormwater Flows ! Contamination Sources ! Impacts, Solutions 2 1980 Comprehensive Environmental Response, Compensation and Liability Act (CERCLA), more commonly known as Superfund. 300,000 – 400,000 sites need remediation. Many can be cleaned up for ~$100,000, but some require millions. Total ~0.5–1 trillion dollars. ! The annual Gross Domestic Product (GDP) is currently 10 trillion dollars. Groundwater Pollution Facts 3 • To date, roughly 10 billion has been spent on superfund site remediation. • 1984 Resource Conservation and Recovery Act (RCRA). This legislation hopes to avoid more superfund sites. 4 Groundwater Structure 5 Unconfined aquifers: typically bounded below by a non-porous layer but are not bounded above. Confined aquifers: sandwiched between two rock layers. 6 The quantity of water in an aquifer is determined by its porosity. ! Fails to account for the amount of water that will be retained due to surface tension at the solid interfaces. • The specific yield tells the amount of water that can actually be withdrawn (of the total volume). Porosity ! ( ) = volume of voids volume of voids and solids 7 Material Porosity (%) Specific Yield (%) Clay 45 3 Sand 34 25 Gravel 25 22 Gravel and Sand 20 16 Sandstone 15 8 Limestone or Shale 5 2 Quartz or Granite 1 0.5 For an aquifer of sand with cross sectional area of 1 m 2 and depth of 2 m, the volume of water is: Volume of water = Porosity ( ) Vol. of material ( ) = 0.34 ! 2 m 3 = 0.68 m 3 Yield of water = Specific yield ( ) Vol. of material ( ) = 0.25 ! 2 m 3 = 0.5 m 3 8 Groundwater Flow The hydraulic gradient is defined as: Where h i are the vertical heights L is the horizontal distance. The gradient is dimensionless, and can be expressed as: Hydraulic gradient = h 2 ! 1 L Hydraulic gradient = dh dL 9
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Darcy’s Law for flow through porous media Q = flow rate (m 3 /day) K = hydraulic conductivity (m/day) A = cross-sectional area (m 2 ) Material Hydraulic Conductivity (m/ Clay 0.0004 Sand 41 Gravel 4100 Gravel and Sand 410 Sandstone 4.1 Limestone, Shale 0.041 Quartzite, Granite 0.0004 *Note that the actual values can vary by several orders of magnitude, and aquifers are highly non-homogeneous. Q = KA dh dL 10 Example A confined aquifer 20 m thick has two wells spaced 500 m apart, and the difference in the water levels at the two wells is 2 m. The hydraulic conductivity is 50 m/day (assume an arbitrary width of 1 m). What is the flow rate? = = 50 m day ! " # $ % 1 m ( ) 20 m ( ) 2 m 500 m = 4 m 3 day (per meter of width) 11 From Darcy’s Law, we can get the Darcy Velocity: = v = A 12 Darcy’s Law Velocity is for flow through the total porous media per m 3 of aquifer volume ! Underestimates the actual groundwater
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This note was uploaded on 09/24/2009 for the course AO 104 taught by Professor Jeffery during the Spring '09 term at UCLA.

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05.3.waterpollt3.aos104f08.sld9 - Groundwater Pollution To...

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