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THE HYDROLOGIC CYCLE 2: GROUNDWATER Slide 2 The Hydrologic Cycle - Fresh Water Storage Reservoir % of Total Fresh Water Glaciers (Frozen)76% Groundwater22% Rivers & Lakes < 2% Groundwater largest liquid fresh water storage reservoir. Slide 3 The Groundwater System Slide 4 Gaining Streams Losing Streams Slide 5 Slide 6 The Water Table Mirrors Surface Topography Slide 7 Groundwater Flows From High Pressure To Low Pressure Areas Slide 8 Usually, this means Down Gradient Slide 9 So, To Understand Groundwater Flow We Need To Know: 1)Depth of the water table. 2)Gradient (slope) of the water table. 3) Characteristics of the rocks that the groundwater is traveling through. Slide 10 Depth and Gradient: Gradient determines the direction of groundwater flow. Slide 11 Gradient = (h 1 - h 2 )/L Where: h 1 and h 2 are the elevations of the water table at two locations, and L is the horizontal distance between them h2h2 h1h1 Slide 12 Rock Characteristics: 1)Porosity How much water the rock will hold. 2)Permeability The ability of a rock to transmit water. Depends on size and connectivity of pores. Slide 13 Porosity A) Sediments Soil 55% by volume Clay 45 Sand 35 Gravel 20 B) Rocks Shale 5% Limestone 5 Sandstone 15 Granite 1 Fractured Limestone 20 Permeability Clay Shale Solid Limestone Sandstone Fractured Limestone Gravels & Sands High Low Slide 14 The first person to study these characteristics of the groundwater system was a geologist named Henri Darcy. Darcy noted that the velocity of groundwater flow was proportional to the gradient of the water table. 1) V (h 1 - h 2 )/L but we need to account for permeability 2)V = k (h 1 - h 2 )/L where k is a permeability variable, and 3)Q (discharge) = Ak (h 1 - h 2 )/L where A is a measure of cross sectional area Slide 15 Darcy Law Q (discharge) = Ak (h 1 - h 2 )/L = AV Pore Remember, when calculating cross sectional area the water is only flowing through the open pores (unlike a river channel). So A (area) = width X depth X porosity. Slide 16 Problem 1)What is the gradient of the water table whose elevation is 500 at well 1, 300 at well 2, and the wells are 2000 apart? 2)If the aquifer has a width of 40 feet, and a depth of 40 feet, a porosity of 5%, and a k = 0.01 ft/day, what is the groundwater velocity and discharge? Slide 17 1) Gradient = (h 1 - h 2 )/L = 500 300/2000 = 0.1 2) Velocity = k (h 1 - h 2 )/L = 0.01ft/day x 0.1 =.001ft/day 3) Q = Ak (h 1 - h 2 )/L = 40 x 40 x.05 x.001 =.08 ft 3 /day Slide 18 2) Groundwater Contamination Human activity Produces Many Sources of Waste! Slide 19 Contaminant Plume Slide 20 Contaminant Plume in Map View