Chapter 11Ground Water: Chapter 11 Ground Water Hydrologic Cycle: Hydrologic Cycle Global Distribution of Water: Global Distribution of Water Ground Water Topics: Ground Water Topics The Water Table
Porosity and Permeability
The Movement of Ground Water
Aquifers Water Table: Water Table Ground Water Definitions: Ground Water Definitions Porosity: Volume percentage of rock that consists of voids or openings. It is a measure of the rock’s ability to hold water.
Permeability: Capacity of a rock to transmit fluid through pore space and fractures.
Rocks that allow water to flow easily are referred to as permeable, while those that do NOT are referred to as impermeable. Porosity and Permeability Table: Porosity and Permeability Table Water Table in quarry: Water Table in quarry Water Table Perched Water Tables: Perched Water Tables Causes of Ground Water Movement: Causes of Ground Water Movement Hydraulic head (h) = elevation + pressure Hydraulic gradient =
difference in head/distance Water flows from zones of high
hydraulic gradient to low
hydraulic gradient! h.g. = h/L Flow Velocity and Potential: Flow Velocity and Potential Flow is always perpendicular to the lines of equal potential. Potential = acceleration of gravity • hydraulic head = g • h ~ h Groundwater Flow Velocity: Groundwater Flow Velocity Flow velocity is governed by Darcy’s Law
Darcy’s Law states that the velocity is equal to the permeability times the hydraulic gradient, where permeability is the capacity of a porous material to transmit fluid.
This gives the equivalent flow velocity through an open pipe.
To correctly apply to real rocks, must divide by the porosity.
Ground water velocity = perm./porosity • hydraulic gradient
V = (K/n) • (h/L)
where K is hydraulic conductivity (a measure of permeability) and n is porosity. Ground Water Movement in Permeable Rock: Ground Water Movement in Permeable Rock Within a uniformly permeable rock, water table tends to mimic surface
topography. Flow usually parallels sloping water table. Aquifers: Aquifers Aquifer: body of saturated rock or sediment through which water can move easily.
Examples: Sandstone, conglomerate, well-fractured limestone, bodies of sand and gravel
Unconfined aquifer: water table is only partly filled.
Confined aquifer: water table completely filled with water under pressure.
Aquitard or aquiclude: body of rock with low permeability, which retards the flow of water across it. Confined vs. Unconfined Aquifers: Confined vs. Unconfined Aquifers Additional Ground Water Topics: Wells
Springs and Streams
Ground Water Contamination
Balancing Withdrawal and Recharge Additional Ground Water Topics Wells: Wells Well: hole drilled into the ground to access ground water. May be drilled or dug and may require pumping to bring water from depth to surface.
Cone of depression: conical feature in the water table that develops through well drawdown (removal of groundwater faster than recharge).
Artesian well: is one in which flow of water reaches surface without the need of pumping. Must tap into a confined aquifer. Good vs. Poor Well: Good vs. Poor Well Dry Well in Fractured Rocks: Dry Well in Fractured Rocks Changes in Water Table with Rainfall: Changes in Water Table with Rainfall Drawdown: Drawdown Mississippi Alluvial Aquifer System: Mississippi Alluvial Aquifer System Mississippi Alluvial Aquifer System: Mississippi Alluvial Aquifer System Major cone of depression with ~80 ft. decline
in the equipotential surface since 1920’s DeWitt Stuttgart Subsidence: Subsidence Artesian Well Controls: Artesian Well Controls Artesian Well: Artesian Well Large Spring: Large Spring Intermittent Springs/Seeps: Intermittent Springs/Seeps Spring Controls and Types: Spring Controls and Types Gaining vs. Losing Streams: Gaining vs. Losing Streams Stream type controlled by changes in the water table. Gaining stream: ground water
flow into stream Losing stream:
water flows into
saturated zone Eventually can produce
dry stream! Sources of Ground Water Pollution: Sources of Ground Water Pollution Pesticides
Household garbage and improperly installed or maintained septic systems
Industrial toxic waste Animal Waste: Animal Waste Industrial Toxic Waste: Industrial Toxic Waste Landfill’s Impact on Ground Water Quality: Landfill’s Impact on Ground Water Quality Industrial Pollution: Industrial Pollution Septic Systems: Septic Systems Pollution Problems Aggravated by Pumping Wells: Pollution Problems Aggravated by Pumping Wells Salt Water Infiltration Animation: Salt Water Infiltration Animation Effects of Ground-Water Action: Effects of Ground-Water Action Caves, Sinkholes, and Karst Topography
Hot Water Underground
Geothermal Energy Development of Karst in Limestone: Development of Karst in Limestone - Rainwater infiltration along cracks.
- Reaction with limestone causing
dissolution and acidification of
- Re-precipitation of dissolved calcite. - Lowering of water table.
H2O + CO2 + CaCO3 = Ca++ + 2HCO3- Cave development Dripstone development Cave structures: Cave structures Stalactites
(floor) Dripstone or
built by dripping
ground water. Karst Topography and Structures: Karst Topography and Structures Karst is well developed in NW Arkansas! Shale concretions: Shale concretions Geodes: Geodes Geyser Development: Geyser Development Calcite Precipitation and Formation of Travertine Terraces in Yellowstone: Calcite Precipitation and Formation of Travertine Terraces in Yellowstone Geyserite Deposits: Geyserite Deposits Steam
deposits Castle Geyser, Yellowstone Geothermal Plant: Geothermal Plant