Water Beneath the Surface

Vocabulary Zone of saturation Groundwater Water table Porosity Permeability Aquifer Aquitard Spring Geyser Well Cone of depression Artesian well Cavern Travertine Stalactite Stalagmite Karst topography Sinkhole

Main Topics Question Answer How does water move underground? How does a spring form? What is a well? What are some environmental problems associated with groundwater? What kind of terrain do Karst areas have?

Groundwater Collects in the pores between grains of soil and sediment, and in the joints and fractures in bedrock. These spaces add up to an immense volume of tiny openings where water collects underground and moves. 50% of the drinking water in the U.S., 40% of the irrigation water, and 25%+ of water used by industry.

Distribution Depends on the steepness of slopes, the nature of the surface materials, the intensity of rainfall, and the type and amount of vegetation Some water soaks in but doesn’t travel far, being held in place by molecular attraction on soil particles. This near-surface zone is the BELT OF SOIL MOISTURE. Roots, worm and animal burrows, and voids left by decayed roots help rainwater seep into the soil.

Distribution Much of the water in the soil seeps downward until it reaches the ZONE OF SATURATION, the area where water fills all the open spaces in sediment and rock. The upper limit of this zone is the WATER TABLE. The area above the water table is called the ZONE OF AERATION. Wells cannot pump water from the zone of aeration, only from the zone of saturation.

Groundwater Movement Groundwater moves by twisting and turning through interconnected small openings, which depend on the subsurface material’s porosity and permeability. The amount stored depends on a material’s porosity (% of total volume of a rock or sediment that is pore spaces). These can be spaces between sediments, joints, faults, and cavities (from dissolving of soluble parts) in rocks. Permeability is the ability to transmit water through connected pore spaces and depends upon the size of the pore spaces.

Clay has high porosity, but it is impermeable because its pore spaces are too small for water to move through them. Aquitards are impermeable layers that get in the way of or prevent water movement. Aquifers are permeable rock layers or sediments that transmit groundwater freely.

Springs A spring is a flow of groundwater that emerges naturally at the ground surface. A spring forms wherever the water table intersects the ground surface and an aquitard blocks downward movement of groundwater, forcing the water to flow laterally.

Hot Springs Warmer than body temperature or surrounding area More than 1000 in the U.S., 95% in the West Most due to groundwater circulating at depths where there is cooling igneous rock or other geothermal heating

Geysers An intermittent hot spring or fountain in which a column of water shoots up with great force 30 to 60 meters, followed by a column of steam with a great roar. Underground water heats in a chamber, but because it is under pressure, it superheats instead of boiling. The water expands and some flows out, reducing the pressure on the remaining water underground. When the pressure drops enough, the water boils and turns to steam, making the geyser erupt. The cycle is repeated when cool groundwater again seeps into the underground chamber.

Wells A well is a hole bored into the zone of saturation. The single greatest use of well water in the U.S. is for agriculture. The level of the water table varies throughout the year, based on periods of rainfall. An artesian well is any formation in which groundwater rises on its own under pressure. Drawdown is the reduction of the water table near a well where water is pumped out, resulting in a cone of depression. Artesian well at Oasis Ranch, Roswell, NM

Environmental Problems Associated with Groundwater Overuse and contamination threatens groundwater supplies in some areas. Treating it as a renewable resource when it can take thousands of years to recharge Land subsidence caused by its withdrawal as the weight of the overburden packs below ground sediment grains together Contamination from septic tanks, farm wastes, and inadequate or broken sewers. Also fertilizers, pesticides, highway salt, industrial materials.

Subsistence One half of the San Joaquin Valley in California has suffered severe subsistence due to groundwater withdrawal for irrigation in the 52 years between 1925, when withdrawal started, and 1977 when this picture was taken. 1925→ 1955→ 1977→

Contamination

Caverns A cavern is a naturally formed underground chamber. A common name is cave. Erosion forms most caverns at or below the water table in the zone of saturation. Usually formed in limestone rocks by acidic groundwater (carbonic acid as a result of carbon dioxide from the air and decaying plants). Travertine is a form of limestone that is deposited by hot springs or as a cave deposit. Dripstone is what cave deposits are usually called.

Dripstone Features Formed from the deposition of calcite as dripping water evaporates. Stalactites are icicle-like stone pendants that hang from the ceiling of a cavern. As water drips down, it forms hollow limestone tubes called soda straws. Can also form flowing sheets. Stalagmites are formations growing upward from the floor (or ground). Because they grow from the dripping water which splashes on the cavern floor, they don’t have a central tube but are usually more massive. Stalactites and stalagmites can grow together to form a column.

Karst Topography Irregular terrain with many depressions formed by dissolving rock at, or near, Earth's surface Common features: Sinkholes—surface depressions that form when bedrock dissolves and caverns collapse. Can be sudden or gradual. Caves and cavern Area lacks good surface drainage (streams). After rains, most water flows into sinkholes, then through caverns until it reaches the water table.