Aquifer and aquitard are terms used to characterize hydrogeologic systems. When an aquifer is bounded by the water table on the top, the aquifer is called an . A natural underground area where large quantities of ground water fill the An aquifer overlain by one or more layers of impermeable rock or soil (aquitard/. The volume of groundwater is a equivalent to a 55 meter thick layer spread out an aquifer is confined between layers of impermeable strata (aquitards). as the difference in elevation between two points on the water table.
Water content and Soil moisture Groundwater can be found at nearly every point in the Earth's shallow subsurface to some degree, although aquifers do not necessarily contain fresh water.
The Earth's crust can be divided into two regions: Unsaturated conditions occur above the water table where the pressure head is negative absolute pressure can never be negative, but gauge pressure can and the water that incompletely fills the pores of the aquifer material is under suction. The water content in the unsaturated zone is held in place by surface adhesive forces and it rises above the water table the zero- gauge-pressure isobar by capillary action to saturate a small zone above the phreatic surface the capillary fringe at less than atmospheric pressure.
This is termed tension saturation and is not the same as saturation on a water-content basis.
Types of aquifers | EARTH Water: Science and Society
Water content in a capillary fringe decreases with increasing distance from the phreatic surface. The capillary head depends on soil pore size. In sandy soils with larger pores, the head will be less than in clay soils with very small pores. The normal capillary rise in a clayey soil is less than 1. The water table is the level to which water will rise in a large-diameter pipe e.
Aquifers versus aquitards[ edit ] Aquifers are typically saturated regions of the subsurface that produce an economically feasible quantity of water to a well or spring e. A completely impermeable aquitard is called an aquiclude or aquifuge. Aquitards comprise layers of either clay or non-porous rock with low hydraulic conductivity.
In mountainous areas or near rivers in mountainous areasthe main aquifers are typically unconsolidated alluviumcomposed of mostly horizontal layers of materials deposited by water processes rivers and streamswhich in cross-section looking at a two-dimensional slice of the aquifer appear to be layers of alternating coarse and fine materials. Coarse materials, because of the high energy needed to move them, tend to be found nearer the source mountain fronts or riverswhereas the fine-grained material will make it farther from the source to the flatter parts of the basin or overbank areas—sometimes called the pressure area.
Since there are less fine-grained deposits near the source, this is a place where aquifers are often unconfined sometimes called the forebay areaor in hydraulic communication with the land surface. Hydraulic conductivity and Storativity Confined versus unconfined[ edit ] There are two end members in the spectrum of types of aquifers; confined and unconfined with semi-confined being in between.
Unconfined aquifers are sometimes also called water table or phreatic aquifers, because their upper boundary is the water table or phreatic surface. Typically but not always the shallowest aquifer at a given location is unconfined, meaning it does not have a confining layer an aquitard or aquiclude between it and the surface.
The term "perched" refers to ground water accumulating above a low-permeability unit or strata, such as a clay layer. This term is generally used to refer to a small local area of ground water that occurs at an elevation higher than a regionally extensive aquifer. The difference between perched and unconfined aquifers is their size perched is smaller.
Confined aquifers are aquifers that are overlain by a confining layer, often made up of clay. The confining layer might offer some protection from surface contamination. If the distinction between confined and unconfined is not clear geologically i. Confined aquifers have very low storativity values much less than 0. Unconfined aquifers have storativities typically then called specific yield greater than 0.
Porosity and Storativity Isotropic versus anisotropic[ edit ] In isotropic aquifers or aquifer layers the hydraulic conductivity K is equal for flow in all directions, while in anisotropic conditions it differs, notably in horizontal Kh and vertical Kv sense.
Semi-confined aquifers with one or more aquitards work as an anisotropic system, even when the separate layers are isotropic, because the compound Kh and Kv values are different see hydraulic transmissivity and hydraulic resistance. When calculating flow to drains  or flow to wells  in an aquifer, the anisotropy is to be taken into account lest the resulting design of the drainage system may be faulty. Porous versus karst[ edit ] To properly manage an aquifer its properties must be understood.
Many properties must be known to predict how an aquifer will respond to rainfall, drought, pumping, and contamination. Where and how much water enters the groundwater from rainfall and snowmelt? How fast and what direction does the groundwater travel?
How much water leaves the ground as springs and evaporation? How much water can be sustainably pumped out? How quickly will a contamination incident reach a well or spring? Computer models can be used to test how accurately the understanding of the aquifer properties matches the actual aquifer performance. Porous aquifer properties depend on the depositional sedimentary environment and later natural cementation of the sand grains.
The environment where a sand body was deposited controls the orientation of the sand grains, the horizontal and vertical variations, and the distribution of shale layers. Even thin shale layers are important barriers to groundwater flow. All these factors affect the porosity and permeability of sandy aquifers. Groundwater flow directions can be determined from potentiometric surface maps of water levels in wells and springs. Aquifer tests and well tests can be used with Darcy's law flow equations to determine the ability of a porous aquifer to convey water.
A groundwater flow rate of 1 foot per day 0. Karst[ edit ] Water in karst aquifers flows through open conduits where water flows as underground streams Karst aquifers typically develop in limestone.
Surface water containing natural carbonic acid moves down into small fissures in limestone. This carbonic acid gradually dissolves limestone thereby enlarging the fissures. The enlarged fissures allow a larger quantity of water to enter which leads to a progressive enlargement of openings.
Abundant small openings store a large quantity of water. The larger openings create a conduit system that drains the aquifer to springs. These conventional investigation methods need to be supplemented with dye tracesmeasurement of spring discharges, and analysis of water chemistry. Geological Survey dye tracing has determined that conventional groundwater models that assume a uniform distribution of porosity are not applicable for karst aquifers.
Locating a well in a fracture trace or intersection of fracture traces increases the likelihood to encounter good water production. For example in the Barton Springs Edwards aquifer, dye traces measured the karst groundwater flow rates from 0.What is an Aquifer?
This may occur in eroded limestone areas known as karst topographywhich make up only a small percentage of Earth's area. More usual is that the pore spaces of rocks in the subsurface are simply saturated with water—like a kitchen sponge—which can be pumped out for agricultural, industrial, or municipal uses. If a rock unit of low porosity is highly fractured, it can also make a good aquifer via fissure flowprovided the rock has a hydraulic conductivity sufficient to facilitate movement of water.
Porosity is important, but, alone, it does not determine a rock's ability to act as an aquifer. Areas of the Deccan Traps a basaltic lava in west central India are good examples of rock formations with high porosity but low permeability, which makes them poor aquifers. Similarly, the micro-porous Upper Cretaceous Chalk Group of south east England, although having a reasonably high porosity, has a low grain-to-grain permeability, with its good water-yielding characteristics mostly due to micro-fracturing and fissuring.
Groundwater movement is slow relative to that in surface streams. This is because it must percolate through pore openings and is further slowed by friction and electrostatic forces.
Aquifer - Wikipedia
For comparison, typical rates of flow are as follows: Local — Shallow flow occurs over short times and distances, whereas, deep long distance flow occurs over time scales of centuries. The rate at which groundwater moves through the saturated zone depends on the permeability of the rock and the hydraulic head.
The hydraulic head is defined as the difference in elevation between two points on the water table. The hydraulic gradient is the hydraulic head divided by the distance between two points on the water table.
The velocity, V, is of groundwater flow is given by: If we multiply this expression by the area, A, through which the water is moving, then we get the discharge, Q. It simply states that discharge is proportional to the hydraulic gradient times the permeability. Note that like stream discharge, Q has units of volume per time i.
Springs A spring is an area on the surface of the Earth where the water table intersects the surface and water flows out of the ground. Some springs occur when an aquitard intersects an aquifer at the surface of the Earth.
Such juxtaposition between permeable and impermeable rock can occur along geological contacts and fault zones see figure The waters are usually rich in dissolved minerals that often precipitate around the springs.
They develop in two settings: Hot springs are distinctive geological features.
If the surface through volcanic ash they become a viscous slurry called mudpots. If they precipitate dissolved minerals on cooling, they can form deposits like travertine made of calcite.
Hot springs can also produce a wide range of colors due to thermal sensitive bacteria that metabolize sulfur minerals. Geysers form when hot water erupts to the surface. They are caused by boiling of the water at depth which causes vapor bubbles to rise and reduce the pressure.
This results in rapid boiling which sends the water to the surface as a geyser. The cycle then repeats after the empty chamber is refilled with water and is heated to the boiling temperature. Hot springs and geysers are common in active volcanic regions, notably Yellowstone Park in Wyoming.
Wells A well is human-made hole that is dug or drilled deep enough to intersect the water table. Wells are usually used as a source for groundwater. If the well is dug beneath the water table, water will fill the open space to the level of the water table, and can be drawn out by a bucket or by pumping.
Fracture systems and perched water bodies can often make it difficult to locate the best site for a well. A special kind of confined aquifer is an artesian system, shown below. In an artesian system, the aquifer is confined between aquitards and is included so that the pressure inside the aquifer can push the water from a well or spring upward to nearly the same level as the top of the water table.
Artesian systems are desirable because they result in free flowing artesian springs and artesian wells. Changes in the Groundwater System When discharge of groundwater exceeds recharge of the system, several adverse effects can occur. Most common is lowering of the water table, resulting in springs drying up and wells having to be dug to deeper levels. If water is pumped out of an aquifer, pore pressure can be reduced in the aquifer that could result in compaction of the now dry aquifer and result in land subsidence.
In some cases withdrawal of groundwater exceeds recharge by natural processes, and thus groundwater should be considered a non-renewable natural resource. Water Quality and Groundwater Contamination Water quality refers to such things as the temperature of the water, the amount of dissolved solids, and lack of toxic and biological pollutants. Water that contains a high amount of dissolved material through the action of chemical weathering can have a bitter taste, and is commonly referred to as hard water.
Hot water can occur if water comes from a deep source or encounters a cooling magma body on its traverse through the groundwater system. Such hot water may desirable for bath houses or geothermal energy, but is not usually desirable for human consumption or agricultural purposes.
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Most pollution of groundwater is the result of biological activity, much of it human. Among the sources of contamination are: Sewers and septic tanks Waste dumps both industrial and residential Gasoline Tanks like occur beneath all service stations Biological waste products - Biological contaminants can be removed from the groundwater by natural processes if the aquifer has interconnections between pores that are smaller than the microbes.
For example a sandy aquifer may act as a filter for biological contaminants. Agricultural pollutants such as fertilizers and pesticides. Salt water contamination - results from excessive discharge of fresh groundwater in coastal areas. Groundwater contamination can result from a point source where the contaminant plume emanates from 1 spot.
Concentrations of the contaminant are highest near the source and decrease away from the source. Or, from a widespread source where the pollution is introduced over a wide area and diffused throughout the groundwater over a broad region. Nonpoint source contaminants are difficult to identify and address. Groundwater contaminant plumes change over time. They grow in length with groundwater flow.
They grow in width by diffusion and dispersion. Large plumes pollute large areas and affect many people. Remediation of Groundwater Contamination Problems In order to begin remediationcontaminant characterization is first done. Monitoring wells are installed to assess flow behavior. This allows for chemical testing to quantify the amount of and character of the contaminants. Strategies are then designed to reduce health risks. Remediation is usually quite expensive.
Most strategies include removing the source of the contaminant, then pumping the groundwater out and treating it. Sometimes heat is pumped in to volatilize the groundwater or steam is pumped in to clean out the containments. Newly developed techniques uses bacteria to clean the groundwater in a process called bioremediation. Prevention of Groundwater Contamination Contamination is best prevented by managing land uses. Landfills now require lining the bottom of the landfill with impermeable clay and plastic liners.
Underground storage tanks require double-lining to prevent leakage. Still the best practice is to require that contaminants not be allowed into the groundwater system. Dissolution - Recall that water is the main agent of chemical weathering. Groundwater is an active weathering agent and can leach ions from rock, and, in the case of carbonate rocks like limestone, can completely dissolve the rock.
Chemical Cementation and Replacement - Water is also the main agent acting during diagenesis. It carries in dissolved ions which can precipitate to form chemical cements that hold sedimentary rocks together.
Groundwater can also replace other molecules in matter on a molecule by molecule basis, often preserving the original structure such as in fossilization or petrified wood. Caves and Caverns - If large areas of limestone underground are dissolved by the action of groundwater these cavities can become caves or caverns caves with many interconnected chambers once the water table is lowered.