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Groundwater . Groundwater . Groundwater . In the U.S. we use: 400 million gallons of water a day to drink 450 billion gallons of water a day in our homes, agriculture and industry Groundwater provides: >50% of our drinking water 40% of irrigation water 25% of industrial water
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Groundwater In the U.S. we use: 400 million gallons of water a day to drink 450 billion gallons of water a day in our homes, agriculture and industry Groundwater provides: >50% of our drinking water 40% of irrigation water 25% of industrial water Groundwater is not usually in the form of lakes or rivers, it mostly occupies the small pore spaces in rocks and between sediment Similar to water in a sponge.
Groundwater • We withdraw groundwater from reserves that have been accumulating over thousands of years. • We pull it out faster than it gets in there = many reserves are decreasing and will eventually run out • Critical issues related to groundwater: • Where do we find it? • How do we keep it clean? • Who owns it? • We need to understand: • How it gets into the Earth? • Where is it stored? • How does it travel? A typical oasis in Oman
Groundwater • We withdraw groundwater from reserves that have been accumulating over thousands of years. • We pull it out faster than it gets in there = many reserves are decreasing and will eventually run out • Critical issues related to groundwater: • Where do we find it? • How do we keep it clean? • Who owns it? • We need to understand: • How it gets into the Earth? • Where is it stored? • How does it travel? Rainwater or snowmelt = infiltration into ground, runoff or evaporation
Groundwater • Infiltration • (Recharge of groundwater) • Controlled By: • A. Surface materials • 1. Easy to get in: loose soils, unconsolidated sediment, highly fractured rock • 2. Hard to get in: clay, unfractured rock
Groundwater • Infiltration • (Recharge of groundwater) • Controlled By: • B. Vegetation • 1. Some plants are good because roots provide pathways for water • 2. Too much vegetation - hard for water to get to ground, stuck in trees • C. Topography • 1. High infiltration: gentle slopes • 2. Low infiltration: steep slopes
Groundwater • Infiltration • (Recharge of groundwater) • Controlled By: • D. Precipitation • 1. Moderate precipitation good, not too much, not too little • 2. Droughts: ground gets too hard, impermeable • 3. Heavy rainfall: ground gets saturated with surface runoff
II. Groundwater Distribution • Where do we find groundwater? • Gravity pulls water downward • Zone of aeration (unsaturated zone) • Pore spaces in sediment and rocks are water and air • Some water bonds to clay particles, or is taken up by roots • Other water is moving downward toward water table
II. Groundwater Distribution • Where do we find groundwater? • Gravity pulls water downward • D. Capillary Fringe • Small volume of water rises upwards against gravity • Capillary action—attraction of water molecules to mineral surfaces • Similar to meniscus in a graduated cylinder or paper towel in water
II. Groundwater Distribution Where do we find groundwater? Gravity pulls water downward B. Zone of saturation Water fills every available pore space Usually only extends to ~1000 meters because pressure too high below that - no pores C. Water Table Upper surface of the zone of saturation (separates the two).
II. Groundwater Distribution • Unsaturated zone: • As water begins to seep into the ground, it enters a zone that contains both water and air. • Below lies an intermediate zone, followed by a saturated capillary fringe, which results from the attraction between water and rocks. • As a result of this attraction, water clings as a film on the surface of rock particles.
II. Groundwater Distribution • Saturated Zone: • All the interconnected openings between rock particles are filled with water. • It is in the saturated zone that "ground water" exists.
Groundwater • II. Groundwater Distribution • Where do we find groundwater? • Gravity pulls water downward • E. Location of the Water Table • Generally mimics the surface topography, but subdued
Groundwater II. Groundwater Distribution Moist Climates: Less than few meters Deserts: 10’s meters Varies from season to season Also changes during usually wet or dry spells
Groundwater • III. Availability of Groundwater • Porosity • Determines how much water the material can hold • The volume of pore space compared to material • Percentage of open space • well rounded = more porosity • well sorted = more porosity • no cement = more porosity
Groundwater III. Availability of Groundwater B. Permeability The capability of a substance to allow the passage of a fluid Controlled by amount of pore spaces and whether they’re connected Larger pores = higher permeability Sand is much better than clay Basalt usually has many fractures so water flows through basalt
Groundwater • IV. Groundwater Flow • Flows under influence of gravity, high ground to low ground • A. Flow Rate dependent on: • 1. Hydraulic conductivity • Reflects sorting, roundness, and sizes of grains • Depends on porosity and permeability • High hydraulic conductivity: well sorted, well rounded, coarse grains • Low hydraulic conductivity: poorly sorted, angular, fine-grained
Groundwater • IV. Groundwater Flow • Flows under influence of gravity, high ground to low ground • Flow Rate dependent on: • 2. Hydraulic gradient • Essentially the slope of the water table • Steep hydraulic gradient faster flow • Gentle hydraulic gradient slower flow Groundwater velocity • Depends on permeability and hydraulic gradient (slope of water table) • Ranges from 100 m/day to mm/day • A good round number: 1 ft/day
Groundwater IV. Groundwater Flow Flows under influence of gravity, high ground to low ground B. Speed: 0.2 to 0.6 cm per day in permeable rock Tens of cm per year in unfractured igneous rock C. Discharge Water table intersects the surface = streams, lakes, swamps, springs (confined)
