Presentation on theme: "Ground Water: Water Flowing Underground. A Spring: Ground Water Becomes Surface Water."— Presentation transcript:
Ground Water: Water Flowing Underground
A Spring: Ground Water Becomes Surface Water
Why study ground water? Remember – Only about 3% of Earth’s total water budget is fresh water. And – Ground water is about 95% of Earth’s “unfrozen” fresh water.
Why study ground water? Where is ground water found? How does ground water flow? How fast does ground water move? What is the composition of ground water? How does ground water shape the landscape?
In the U.S. we use ~314 billion liters (83 billion gallons) of ground water every day. About 40% of our drinking water in the U.S. comes from ground sources (Las Vegas ~10%). About 66% of our ground water is used for agriculture. It represents ~95% of Earth’s unfrozen fresh water. It influences the shape of surface landscapes. It is a critical resource for sustaining human life. Why study ground water?
A hamburger requires water to raise wheat for the bun, to grow hay and corn to feed the cattle and to process the bread and beef. Together with french fries and a soft drink, this all-American meal uses about 1,500 gallons of water—enough to fill a small swimming pool. To grow cotton for a pair of jeans takes about 400 gallons. A shirt requires about 400 gallons. To produce the amount of finished steel in a car has in the past required about 32,000 gallons of water. Why study ground water?
Where is ground water found? Ground water is water that soaks into soil and rock at the surface, moves through fractures and pores to some depth below the surface, and resides there for periods of up to thousands of years. Ground water may reside in alluvium (loose sediment) or rocks which have porosity (e.g., clastic sedimentary rocks). Porosity - pores, fractures, joints, and other open spaces in rock or alluvium. It is simply the percentage of open space. Permeability – a measure of the interconnectedness of pore space. High permeability equals ability of water to flow through the pore spaces of the rock or alluvium.
Porosity varies according to the type of pore space and material. Generally, pores are small (a few mm or less) and found in between unconsolidated grains, and as fractures in rock. Where is ground water found?
Calculation of porosity: Two containers are filled to an equal level, one with sediment, the other with water. The water is poured in until it reaches the top of the sediment. The amount of water that fits into the spaces within the sediment is the amount of porosity. If the volume of water poured in is ~50% of the sediment volume, then the porosity of the sediment is ~50%. Where is ground water found?
The Concept of a Water Table Where is ground water found?
The water table is simply the top surface of the saturated zone. The water table is a surface that generally follows the topography of the ground surface above.
Where the water table intersects the ground surface there will be a body of water. It may be a stream or other body of water such as a lake. Where is ground water found?
Periods of high rainfall – ground water may move into streams and lakes. The water table is high. Where is ground water found? Periods of low rainfall – water from streams and lakes may move into the ground water. The water table has lowered. Periods of drought – water from streams and lakes may move into the ground water. The water table has dramatically lowered and may be disconnected from surface water.
Ground water flows through the pore space. Its ability to flow is a measure of the permeability. Permeability is literally the ability for water to flow through a porous material. The connectedness of the pore space is the primary control, and to a lesser extent the size of the pore spaces. Where is ground water found?
The water table separates the saturated zone from the unsaturated zone (or the zone of aeration) The saturated zone is the region below the water table where all pores are completely filled with water. The unsaturated zone, above the water table, contains some air in the pores. Where is ground water found?
A perched water table is formed where an impermeable layer of rock (an aquitard, e.g. shale or mudstone) stops the downward flow of water to the deeper, main water table. Springs at the surface are often connected to perched water tables. An aquifer is simply an underground body of water. Where is ground water found?
Some aquifers are very large. Others are quite small by comparison. For example, the aquifers beneath the Las Vegas valley are only ~10 miles in lateral extent.
Aquitards may control and confine ground water and form aquifers. Water moves downhill under the influence of gravity, often from higher elevation recharge areas in mountainous regions (where rain occurs). Where is ground water found?
(A) Change in ground water level with rainfall for a shallow well in NV. (B) Ground water depletion caused by increased withdrawal over time for wells in NM. How the water table may change over time.
17.2 Why and how does ground water flow? Fig 11.12 Subsidence caused by ground water withdrawal. Compaction of pore space when water is removed causes surface elevation decrease and fissure formation.
Ground water flows downwards under the influence of gravity from higher areas of recharge to lower areas, where it may be either stored in aquifers, or discharged into streams. In ground water systems deeper = slower movement = longer residence time. How does ground water flow?
Downward moving ground water may discharge into streams, enter and reside in unconfined aquifers, or become trapped in confined aquifers.
Pollution in ground water: Once contaminants enter the ground water they may remain there for thousands of years, or be discharged into streams. How does ground water flow?
How fast does ground water move? Can ground-water flow velocity be measured? We can measure the transit time of water using a “tracer” chemical. In this experiment on Cape Cod, a bromide chemical was injected into a well in a moving aquifer and monitored over the course of 18 months until it reached another test well and was detected.
656 detection wells and 3 injection wells were used to determine flow rate and direction. How fast does ground water move?
Experiments and isotopic dating give us information on movement rates of ground waters and on how long they remain in aquifers. Generally movement of ground water is a few centimeters per day. Water in some aquifers may be as old as 10,000 years or more. Combined, this information tells us that if we deplete aquifers by pumping they will not recharge naturally in our lifetimes. In other words, they are in some cases a non- renewable resource like crude oil.
What is the composition of ground water? Water-mineral reactions: Occur constantly in the saturated zone. Slow-moving water, long travel paths, and increased temperature at depth all increase reactions. Cementation Occurs where mineral precipitation takes place. Slows water down even more, so more reaction potential.
Hard water = high concentration of dissolved material: Minerals from ground water precipitate under the right conditions. This may be in between mineral grains, or in your water pipes. These are usually Ca or Mg salts from calcite or dolomite. They decrease effectiveness of soaps and clog showerheads, irons, and coffee makers. Minerals can be removed using water softeners, but soft water leaches ions from metal pipes. What is the composition of ground water?
Some economic minerals are produced by hot, circulating ground water—Cu, Pb, and Zn ores. These (and other metals) dissolve easily in warm water. They precipitate as sulfide minerals such as galena, chalcopyrite, and sphalerite. What is the composition of ground water?
How does ground water shape the landscape? Soluble minerals such as calcite in limestone dissolve in ground water. Dissolved material leaves open spaces behind. Caves are formed this way. These spaces can become large enough to be expressed on the surface as: Subsidence Sinkholes Regions exhibiting such features are said to have karst topography.
Development of karst features in a landscape. How does ground water shape the landscape?
The formation of caves involves both erosion and redeposition of soluble minerals.