Published on January 17, 2008
Groundwater: Groundwater Lecture 20 Copyright © 2007 Joe Marx. Topics for This Lecture: Topics for This Lecture What is groundwater? How does groundwater accumulate and flow? What causes cold and hot springs? What is the relationship between groundwater and caves? What are sinkholes? ◄ Groundwater flooding, Frederickson, Washington, 1997. Hydrologic or Water Cycle: Hydrologic or Water Cycle Continuous circulation of water among a set of natural reservoirs at or near Earth’s surface. Pore Space: Pore Space Porous sandstone Pore Small opening within regolith or bedrock. Porosity Percentage of volume occupied by open space, including pores, fractures and dissolved hollows. Permeability: Permeability Capacity for transmitting water through interconnected openings. Joints and faults account for much of the permeability of bedrock. The sand on the left has a moderate porosity and a high permeability. The clay on the right has a high porosity but a low permeability. ► Infiltration: Infiltration Movement of water downward from the surface into the regolith or bedrock. Underground Zones: Underground Zones Unsaturated Zone Region immediately below the ground where both air and water fill the pore space. Saturated Zone Subsurface region, below the unsaturated zone, where water fills all connected pores. Groundwater: Groundwater All of the water in the saturated zone. Water Table Upper surface of the saturated zone. Capillary Fringe Region next to the water table where groundwater wicks back into the unsaturated zone. Underground Water Storage: Underground Water Storage ◄ Relative groundwater storage capacities of regolith and bedrock at increasing depth below the surface. Capacities of Earth’s freshwater reservoirs. ► Rock Permeability: Rock Permeability Aquifer Rock or sediment that readily conducts water. Confining Bed Rock or sediment that is relatively impermeable. Confined Aquifer Aquifer cut off from the surface by a confining bed. Water Table Topography: Water Table Topography The shape of the water table is a subdued version of the overlying ground surface. Groundwater Flow: Groundwater Flow Schematic of local and regional groundwater flow. Rates of Groundwater Flow: Rates of Groundwater Flow Compared to surface water, groundwater moves very slowly—generally from 12 to 1,800 feet per year. Perched Groundwater: Perched Groundwater Unconfined groundwater separated by an unsaturated zone from a deeper, larger body of groundwater. Groundwater and Rock: Groundwater and Rock Groundwater contains ions dissolved from the rock and the sediment through which it flows. Ions sometimes precipitate from groundwater, helping to cement sediment or to fill veins. ◄ Fracture-controlled groundwater staining in granodiorite, Sierra Nevada, California. Spring: Spring Big Spring, Carter County, Missouri. Natural opening through which groundwater flows onto the surface or into a body of surface water. Geothermal Region: Geothermal Region Area where magma or very hot rock is situated relatively near the surface and heats up the groundwater. Geothermal area at Rotorua, New Zealand. Hot Spring: Hot Spring Hot spring along Hot Creek, California. Emits water hotter than human body temperature. Normally, the near-surface groundwater of a locale has a temperature close to the mean annual air temperature. Geyser: Geyser Hot spring that intermittently erupts a jet of hot water and steam. ◄ Geyser at Yellowstone National Park, Wyoming. ▲ Icelandic geyser at the start of eruption. Mudpot: Mudpot Hot spring that emits a viscous slurry formed when groundwater mixes with volcanic ash and clay. Mud Caldron in Wyoming's Yellowstone National Park Mud pot, Bumpus Hell, Lassen Volcanic National Park, California. Cave: Cave Naturally-formed underground cavity large enough for a human adult to enter. Usually due to dissolution of limestone, dolomite or gypsum bedrock by groundwater. If the water table drops, air replaces the water within the cave, ending its growth. ◄ Entrance to Gregory’s Cave, Great Smoky Mountains National Park, Tennessee. Underground Cavities: Underground Cavities In addition to caves, countless smaller fissures and tubes usually exist in soluble bedrock. The pitted surface at Burren, Ireland, suggests that many underground cavities lie below Schematic of a small cave surrounded by other underground cavities. Cave Networks: Cave Networks Interconnected chambers and passages, sometimes at multiple levels. The solubility of beds and joints determine the location and geometry of caves and cave networks. Speleothem: Speleothem Any mineral deposit, usually travertine, precipitated from water moving through a cave. Infiltrating water drips from cave ceilings and flows down walls. Speleothem in a cave in central Missouri. ► Soda Straw: Soda Straw Tubular, hanging speleothem about the diameter of a drop of water. Soda straws in Round Springs Cave, Missouri. Conical Speleothems: Conical Speleothems Carlsbad Caverns National Park, New Mexico. Stalactite Hangs from the ceiling. Stalagmite Rises from the ground. Karst Landscape: Karst Landscape Topography formed on dissolving bedrock. Solution Opening: Solution Opening Produced when dissolution enlarges pre-existing openings in bedrock. Solution openings in upstate New York. ► Sinkhole: Sinkhole Topographic depression, roughly circular, caused by the subsurface removal of bedrock and regolith. Fresh sinkhole in Frederick County, Maryland. Sinkhole Formation: Sinkhole Formation Aligned sinkholes in the Kaibab formation, near a tributary of the Little Colorado River, Arizona. Sinkholes form when the roof collapses into a subsurface cavity or when precipitation continuously deepens and widens a solution opening. Mid-Atlantic Karst: Mid-Atlantic Karst ◄ Karst regions in the Mid-Atlantic: RED = Relatively large cavities in metacarbonate rock. BLUE = Relatively large cavities in tilted sedimentary carbonate rock. YELLOW = Relatively small cavities in metacarbonate rock. LIGHT GREEN = Relatively small cavities in tilted sedimentary carbonate rock. DARK GREEN = Relatively small cavities in flat-lying sedimentary carbonate rock.