_{A. Hot Springs and Geysers}

• _{by definition, the water in "hot springs" is 10-15 degrees F warmer than the mean annual air temperature for the localities where they occur}

• _{in the U.S. there are 1,000 such springs}

• _{mineral explorations over the world have shown that temperatures in deep mines and oil wells usually rise with an increase in depth below the surface}

• _{temperatures in such situations increase and average of ~1 degree F per 100 ft.}
• _{therefore, when groundwater circulates at great depths, it becomes heated, and if it rises to the surface, the water emerges as a hot spring}

• _{the water of some hot springs in the Eastern U.S. is heated in this manner}

• _{however, the great majority (over 95%) of the hot springs (and geysers) in the U.S. are found in the West}

• _{in the West, the hot springs result from cooling igneous rocks -- it is the Western U.S. where igneous activity has occurred most recently}

• _{"geysers" are intermittent hot springs or fountains in which columns of water are ejected with great force at various intervals, often rising 30-60 meters}

• _{after the jet of water ceases, a column of steam rushes out, usually with a thundering roar}

• _{perhaps the most famous geyser in the world is Old Faithful in Yellowstone National Park, which erupts ~ once per hour}

• _{geysers are also found in other parts of the world, including New Zealand and Iceland, where the term "geyser" (meaning "spouter") originated}

• _{geysers occur where extensive underground chambers exist within hot igneous rocks}

• _{as relatively cool groundwater enters the chambers, it is heated by the surrounding rock}

• _{at the bottom of the chambers, the water is under great pressure because of the weight of the overlying water}

• _{the great pressure prevents the water from boiling at the normal surface temperature of 100 degrees C}

• _{(e.g., water at the bottom of a 300 meter water-filled chamber must attain a temperature of nearly 230 degrees Celsius before it boils)}

• _{the heating causes the column of underground water to expand, with result that some is forced out at the surface}

• _{the loss of water reduces the pressure on the remaining water in the chamber}

• _{the reduced pressure lowers the boiling and a portion of the water deep inside the chamber turns to "steam", and causes the geyser to erupt}

• _{following the eruption, cool groundwater again seeps into the chamber and the cycle begins anew}

• _{when groundwater from hot springs and geysers flows out at the surface, material in solution is often precipitated, producing an accumulation of chemical (inorganic) sedimentary rock}

• _{the material deposited at any given place commonly reflects the chemical makeup of the rock through which the water circulated}

• _{when the water contains dissolved silica, a material called "siliceous sinster" or "geyserite" is deposited around the spring}

• _{when the water contains dissolved calcium carbonate, a form of limestone called "travertine" or "calcareous tufa" is deposited}

• _{in addition to containing dissolved silica and calcium carbonate, some hot springs contain sulfur, which gives water a poor taste and unpleasant odor -- e.g., "Rotten Egg Spring", Nevada}

• _{the most common method for removing groundwater is the well}

• _{a "well" is a hole that is dug, bored, or drilled into the zone of saturation}

• _{wells serve as reservoirs into which groundwater moves and from which it can be pumped to the surface}

• _{the use of wells dates back many centuries and continues to be an important method of obtaining water today}

• _{by far, the single greatest use of this water in the U.S. is irrigation for agriculture}

• _{more than 65% of the groundwater each year is used for this purpose; industrial uses rank a distant second, followed by the amount used in city water systems and rural homes}

• _{the water table level may fluctuate considerably during the course of a year, dropping during the dry season and rising following periods of rain}

• _{to insure a continuous flow of water, a well must penetrate below the water table}

• _{whenever water is withdrawn from a well, the water table around the well is lowered}

• _{this effect is termed "drawdown", and decreases with distance from the well -- the result is a depression in the water table, roughly conical in shape, known as the "cone of depression"}

• _{since the cone of depression steepens the hydraulic gradient near the well, groundwater will flow more rapidly toward the opening}

• _{for most domestic wells the cone of depression is negligible}

• _{however, when wells are used for irrigation or industrial purposes, the withdrawl of water can be great enough to create a very wide and steep cone of depression that may substantially lower the water table in an area and cause nearby shallow wells to become dry}

• _{when subsurface materials are heterogeneous, the amount of water the well is capable of providing may vary a great deal over short distances}

• _{e.g., when two nearby wells are drilled to the same level, and only one is successful, it may be caused by the presence of a perched water table beneath one of them}

• _{massive igneous and metamorphic rocks provide another example}

• _{in most wells water doesn't rise on its own}

• _{if water is first encountered at 30 meters depth, it remains at that level, fluctuating perhaps a meter or two because of rainfall variations}

• _{however, in artesian wells, water rises, sometimes overflowing at the surface}

• _{many people think the term artesian is applied to any well drilled to great depths -- that notion is incorrect}

• _{the term artesian may be applied correctly to any situation in which groundwater under pressure rises above the level of the aquifer -- this does not always mean a free flowing surface discharge}

• _{for an artesian system to exist, two conditions must be met}

_{(2) aquicludes, both above and below the aquifer, must be present to prevent the water from escaping}

• _{when such a layer is tapped, the pressure created by the weight of the overlying water, will force the water to rise}

• _{if there were no friction, the water in the well would rise to the level of the water at the top of the top aquifer -- however, friction reduces the height of this pressure surface}

• _{the greater the distance from the recharge area (where water enters the inclined aquifer), the greater the friction and the less the rise of water}

• _{if the pressure surface is below ground level, a "nonflowing artesian well" results}

• _{if the pressure surface is above the ground and a well is drilled into an aquifer, a "flowing artesian well" results}

• _{not all aretesian systems are wells -- artesian springs also exist -- here, groundwater reaches the surface by rising through a natural fracture rather than through an artificially produced hole}

1. _{Problems Associated withGroundwater Withdrawl}

• _{the tendency for many natural systems is to establish or attempt to establish a condition of equilibrium -- the groundwater system is no exception}

• _{the water table level represents a balance between the rate of infiltration and the rate of discharge and withdrawal}

• _{any imbalance will either raise or lower the water table}

• _{long-term imbalances can lead to a significant drop in the water table if there is either a decrease in the groundwater recharge, such as occurs during a prolonged drought, or an increase in the rate of groundwater discharge or withdrawal}

• _{in some regions groundwater has been and continues to be treated as a nonrenewable resource}

• _{surface subsidence can result from natural processes related to groundwater}

• _{however, the ground may also sink when water is pumped from wells faster than natural recharge processes can replace it}

• _{this effect is particularly pronounced in areas underlain by thick layers of unconsolidated sediments}

• _{as the water is withdrawn, the water pressure drops and the weight of the overburden is transferred to the sediments}

• _{the greater pressure packs the sediment grains tightly together and the ground subsides}

_{(2) Saltwater Contamination}

• _{in many coastal areas the groundwater resource is being threatened by the encroachment of salt water}

• _{since fresh water is less dense than salt water, it floats on the salt water and forms a large, lens-shaped body that may extend to considerable depths below sea level}

• _{when excessive pumping lowers the water table, the base of the freshwater zone will rise considerably, and the result may be saltwater contamination of wells}

1. _{The Geologic Work of Groundwater}

• _{recall that groundwater dissolves rock -- a fact that is key to understanding how caverns and sinkholes form}

• _{because soluble rocks, esp. limestone, underlie millions of square kilometers of Earth's surface, it is here that groundwater carries on its important role as an erosional agent}

• _{recall that limestone is nearly insoluble in pure water, but is quite easily dissolved by water containing small quantities of carbonic acid}

• _{recall that carbonic acid is formed as carbon dioxide is dissolved into rainwater}

• _{when the groundwater comes in contact with limestone, the carbonic reacts with the calcite in the rocks to form calcium bicarbonate, a soluble material that is then carried away in solution}

• _{the most spectacular results of groundwater's handiwork are limestone caverns}

• _{in the U.S. alone, there are over 17,000 caves}

• _{e..g., Carlsbad Caverns in southeastern New Mexico, and Mammoth Cave in Kentucky -- more than 540 km of interconnected passages}

• _{most caverns are created just at or just below the water table in the zone of saturation}

• _{here acidic groundwater follows lines of weakness in the rock, such as joints and bedding planes}

• _{as time passes, the dissolving process slowly creates cavities and gradually enlarges them into caverns}