On March 22, 2014, a major landslide occurred near Oso, Washington. Death toll currently at 30, with 15 still missing. Groundwater Before and After Swipe http://bit.ly/pen1jt N. Fork Stillaguamish River Thunder River, Grand Canyon Oct 2006 Groundwater: Outline 1. Groundwater intro and activity 2. Water table and groundwater flow 3. Darcy s Law and the driver for groundwater movement 4. Groundwater storage: aquifers & aquitards 4. Groundwater problems (e.g., contamination, pumping, etc.) The Water Table Top of the saturated zone Configuration similar to ground surface Water level of most lakes and rivers corresponds to the water table Varies with time (season, rainfall,... Gradient: change in water level elevation/change in distance Groundwater Movement Groundwater Movement Recharge: the infiltration of water into any subsurface formation Water table mimics topography, it is not flat Its elevation varies with time Discharge: the opposite of recharge; the exit of groundwater to the surface 1
Draw the water table How might you draw the water table in this region? What drives groundwater flow? Hydraulic gradient? Horizontal hydraulic gradient Friday s in-class activity 610 ft h 1 =550 ft Well 1 Well2 60 ft 100 ft d h (depth L to water) 610 ft h 2 =500 ft 200 ft Hydraulic gradient = water level difference / distance d h /L = (550 500) / 200 = 0.25 Flow from well 1 to well 2 Groundwater: Outline 1. Groundwater intro and activity 2. Water table and groundwater flow 3. Darcy s Law and the driver for groundwater movement 4. Groundwater storage: aquifers & aquitards 4. Groundwater problems (e.g., contamination, pumping, etc.) How fast does groundwater move? Rate controlled by 2 things: 1. Driving force gradient of potential energy think of the water table s slope 2. permeability How easily water moves through subsurface Rates extremely variable < 1 cm/year to km s/year 2
Porosity - the percentage of rock or sediment that are voids or openings A measurement of a rock s ability to hold water Porosity - the percentage of rock or sediment that are voids or openings Permeability rocks ability to transmit fluid degree of interconnectedness of pore spaces A measurement of a rock s ability to hold water Loose sand may have 30-50% porosity Compacted sandstone may have only 10-20% porosity Low in metamorphic and igneous rocks Permeability rocks ability to transmit fluid Depends on grain size/arrangement interconnectedness of pore spaces High permeability: gravels, sands, limestones with dissolution channels, fractured rocks Low permeability: clays, unfractured metamorphic and igneous rocks, limestones without dissolution Clicker question Porosity (and permeability) can be increased by fracturing and dissolution - secondary porosity Vesicular rhyolite is likely to be. A. Both porous and permeable. B. Porous but not very permeable. C. Permeable but not very porous. 3
How fast does groundwater move? Rate controlled by 2 things: 1. Driving force gradient of potential energy think of the water table s slope 2. permeability How easily water moves through subsurface Rates extremely variable Darcy s Law Q = K (h 1 h 2 ) L Flow rate Q = discharge (per unit area) (m/day) K = coefficient of permeability (m/day) h 1 = beginning height (m) h 2 = ending height (m) L = distance (m) < 1 cm/year to km s/year L Groundwater: Outline 1. Groundwater intro and activity 2. Water table and groundwater flow 3. Darcy s Law and the driver for groundwater movement 4. Groundwater storage: aquifers & aquitards 4. Groundwater problems (e.g., contamination, pumping, etc.) How does most groundwater exist within the subsurface? A. As large pools of water within large pockets (caverns) in rocks. B. Within cavernous regions occupied by rivers of water that flow underground. C. Within the small open spaces between the grains of rocks and sediments. D. Both A and B E. None of the above Aquifer: geologic unit that stores and is capable of transmitting significant water, must be both porous and permeable Aquitard: low permeability rock formations that retard (or slow down) the flow of water to or from an adjacent aquifer Unconfined Aquifer At shallow depth Water table is the top boundary Recharged by precipitation infiltrating Confined Aquifer Deeper Confined on top by a less permeable layer Recharged slowly 4
An aquifer must be: a. both permeable and porous. b. neither permeable nor porous. c. permeable, but not necessarily porous. d. porous, but not necessarily permeable. Perched water table Above the main water table In unsaturated zones Due to lenses of less permeable rocks (e.g., shales or clays) Artesian (confined) aquifer: under high pressure Water in wells rises above the top of the aquifer. City water systems can be considered artificial artesian systems. Well # is a flowing artesian well and well # is a well that must be pumped from an unconfined aquifer? (Assume that only the very bottom of the well is open) Spring Location where the water table intersects the surface or where groundwater is discharged in general A) 1, 2 B) 2, 3 C) 3, 4 D) 1, 4 E) 2, 4 Marble Canyon Grand Canyon National Park 5
Groundwater: Outline 1. Groundwater intro and activity 2. Water table and groundwater flow 3. Darcy s Law and the driver for groundwater movement 4. Groundwater storage: aquifers & aquitards 4. Groundwater problems (e.g., contamination, pumping, etc.) Problems Associated with Groundwater Overuse Subsidence Contamination - Salt water intrusion - Hard water, pollutants Drawdown due to Over Pumping Friday s in-class activity 6
Subsidence: when water is removed, sediment compacts, lowering ground surface. San Joaquin Valley subsidence Denver Basin Cross Section Denver Basin Aquifers West As of Feb 2001: ~34,000 water wells were drilled in the Denver Basin East Non-renewable resource Barkmann, 2004 Barkmann, 2004 7
Ogallala Aquifer: Largest in the U.S. Setting of the Ogallala Aquifer First used for irrigation in the late 1800s 1950s: large-scale exploitation Today: ~170,000 wells used for irrigation Unconfined aquifer; water levels dropping in south; some parts may run dry (graphs below) Water table ~200 lower in some areas! 17.11.a Ogallala Water Supplies and Usage 17.11.b-c Water Quality and Groundwater Movement More rain in east Thickest in north Water table decline greatest in south (Kansas & Texas) Groundwater Contamination Sources: Pesticides/herbicides/fertilizers Landfill pollutants Sewage Acid mine drainage Radioactive waste Oil and gasoline Salt water intrusion Saltwater Intrusion 8
Question: Why is ground water flow direction changing here? A) Pumping lowers water table so that it is tilted towards community well B) Bacteria expand and push water away from septic tank C) Drought causes water table to become lower D) Septic wastes always flow towards wells open to the air 9