calcite: (1) A mineral, calcium carbonate (CaCO3).
(2) Also limestone. A sedimentary rock consisting primarily of the mineral calcium carbonate.
calcium carbonate: (1) A sedimentary mineral made up of calcium, carbon and oxygen (CaCO3) that was formed from skeletal remains and secretions of organisms precipitated from water.
(2) A major component in hard water and forms a tenacious crust on water-handling equipment.
caliche: A general term for a prominent layer of secondary carbonate accumulation in surficial materials in warm, subhumid to arid areas. NSSH.
California Doctrine: A legal doctrine retaining aspects of both riparian rights and principles of prior appropriation. CSU.
call: The placing of a call by a senior priority to the water commissioner to stop or diminish junior diversions so that the requested amount of water may be passed to the downstream senior diversion. In such cases, junior priorities are curtailed or "called out".
capillarity: The net imbalance between surface attractions (adhesion) at the interfaces between liquids and solids, and surface tension (cohesive force) of liquids. Capillarity provides the net attraction or repulsion for liquids to rise or be depressed in small pores, voids, or interstices. See also wettability.
If the liquid is water and the water tends to wet the solid surfaces of soil and rock surrounding fine pores and interstices, then the molecular adhesive attraction between water and rock surfaces tends to be greater than the intermolecular cohesive attraction within the water, and there will be a positive pressure causing the water to penetrate the fine pores of the rock.
In soils and aquifers, where water is attracted to and wets the solid surfaces, the adhesive attractions are the predominant force, and water is retained in fine pores with great tenacity. Water held by capillarity in an aquifer cannot be produced and is part of the irreducible water.
capillary fringe: See aerated zone.
caprock: (1) The impermeable rock or shale overlying an aquifer that serves as a barrier to prevent water in a permeable aquifer from moving upward in the formation, therefore helping to maintain the pressure in the aquifer.
(2) The hard weather-resistant rock layer composed of sandstone, limestone, or lava that overlies any softer layer of rock.
carbonaceous: Containing carbon or coal derived from buried organic matter.
(2) The negatively charged ion cluster or radical CO3 in a solution containing dissociated ions of a salt such as sodium carbonate (Na2CO3). See ions.
casing: In water wells it is a steel or PVC pipe that is installed in the borehole to keep the borehole from collapsing and to protect drilled aquifers from contamination. Encloses tubing and pump in completed wells. Also see conductor pipe, surface casing, intermediate casing, production casing, protection casing.
casinghead: The top of the protective casing at the surface, or first casing string, that has the appropriate means for attaching various fittings and equipment assemblies.
casing point: The depth where the bottom of the casing is to be set. Generally the bottom of the hole and the casing point are planned to be the same depth.
casing pressure: The pressure in the annulus between the casing and the tubing. Might or might not be at atmospheric pressure.
cement bond: Is related to the quality that cement adheres to either or both the casing and the formation wall. In water well completions, the annular space between the casing and the drilled formation wall is intentionally filled with cement to isolate aquifers and stabilize the casing. The quality of adherence or bonding of the cement with the casing and/or with the formation wall determines the effectiveness of the cementing operation. The bond is defective where the cement bond is weak, or where cement is absent or missing. The void in the annular space then might constitute a channel or might allow a channel to form between aquifers or between aquifers and the level where water production takes place. See channel and channeling.
cementation: A term not used in relation to the completion process of cementing casing in a well. This term is used to refer to the precipitation from interstitial waters, and the mineral growth of material that binds sand grains or other rock materials together to become a consolidated rock. Typical cementing materials are authigenic crystals of quartz and carbonates.
certification: The process whereby a permit to appropriate water is finalized based on the completion of the diversion work and past application of water to the proposed use in accordance with the approved water-right application. A certified water right has a legal, State-issued document that establishes a priority date, type of beneficial use, and the maximum amount of water that can be used annually. GWAC.
change of water right: Any change in a way a water right is used. Can be changed in type, place, time of use, point of diversion, adding points of diversion, etc. Changes of water rights must be approved by the water court to assure that no injury occurs to other water rights. CSU.
channel: (1) A defect in the cement quality in a water well where the cement, intended for isolation of aquifers, does not fully occupy the annulus between the casing and formation wall. This defect can provide communication and can serve as a conduit for water to crossflow from one aquifer to another with subsequent loss of integrity and pressure; or, in a producing water well, can allow water to crossflow from an unauthorized aquifer to the level where water is admitted to the casing and withdrawn by the pump, thus allowing depletion of the unauthorized aquifer. See crossflow.
(2) Of a river bed or stream bed.
(3) A river bed or stream bed that has become buried by overburden and has become a channel, finger, conduit, or other connecting part of an aquifer.
channeling: In water wells, it is water crossflowing behind casing from one depth to another. The water will crossflow from an aquifer at higher water pressure to another at lower pressure, or crossflow from one depth to another where the opposing pressure is lower. See crossflow.
charge: To load, to fill, to add to energy or supply. A generic term. Compare recharge.
chlorite: A magnesium, iron, aluminum silicate. Exhibits intermediate to high radioactivity level of the clays. Density 2.76 g/cm3 Found as authigenic, pore-lining clusters and rosettes of leafy crystals standing on end.
clastic: Sedimentary rock formed from the degradation or disintegration products of preexisting rock which have been moved individually from their place of origin.
clay: A major constituent in most shales. Fine hydrous silicate minerals with several different crystalline forms depending on the mineral. Some with platelets and leaves, some with fibers. Each clay mineral exhibits its own radioactivity level. Compare shale.
clay shale: A clay shale is a shale wherein the major mineral constituent is clay. Usually has high porosity of 35 to 45 percent. Occupied by water closely related to connate water. Compacts readily under stress of overburden while expelling a fresher component of water. See shale and compaction.
coefficient of storage: See storativity.
COGCC: Colorado Oil and Gas Conservation Commission. See also www.colorado.gov/cogcc.
coliform bacteria: A type of bacteria that is found in the intestinal tract of all animals including humans. Levels of these bacteria are used as an indicator of water-well cleanliness. CSU.
Colorado Water Quality Control Act: Legislation to prevent injury to beneficial uses made of state waters to maximize the beneficial uses of water and to achieve the maximum practical degree of water quality in Colorado. GWAC.
communication: Any condition, action or means that allows water to flow and/or water pressure to be transmitted from one location to another. Sometimes a condition that hydraulically connects two or more aquifers. Sometimes for the purpose of seeking relief. See relief and compare isolation.
community water system: See central water system.
compact: (1)An agreement between states apportioning the water of a river basin to each of the signatory states. CSU.
(2) The act of compacting. To compress. See compaction.
compact call: The requirement that an upstream state cease or curtail water diversions from the river system that is the subject of the compact so that downstream states’ compact entitlements may be met. CSU.
compaction: (1) A natural occurrence. It is the normal compaction of the overburden and underlying beds that takes place as sedimentation continues and overburden increases in weight. When sediments initially are deposited, the water-filled interpore void space is at its greatest. As the sedimentary process continues, the increasing weight of the overburden progressively squeezes water out of all underlying sediments, resulting in a continually decreasing porosity. As long as the expelled water can find relief, a normal pore pressure gradient will be established and observed. See abnormal pore pressure and normal pore pressure.
(2) Related to producing aquifers. When the pore pressure of an aquifer is reduced by water production, the framework of the sedimentary materials surrounding the pore space is subjected to an increased proportion of the weight of the overburden. In this event, there is a strong likelihood that compaction will resume as pore pressure diminishes.
In the following relationship, after Rubey, William W. and Hubbert, M. King, Bull. Geol. Soc. Am., Vol. 70,(1959), an equilibrium exists.
In this equality, the stress of overburden (geostatic load) is supported by the grain-to- grain stress (lithostatic load) + pore pressure (resulting from hydrostatic load), all expressed in units of pressure. In this balance, depressurization in the pore space continues as long as water production continues; pore pressure is reduced and the proportion of the geostatic load supported by the structural framework of the clay shales and aquifers increases correspondingly to maintain the equality. The increase in the compactive force borne by the rock framework might or might not be sufficient to support the overburden. If not, grains and particles of shales, sands, and other minerals will undergo further readjustment, deformation, distortion, and compression. If this happens, additional water will be expelled from the clay shales, total pore volume will be reduced by the additional compaction within the clay shales and aquifers. Because nearly incompressible formation water actually expands as pore pressure diminishes (see bulk modulus), the compaction of the structural framework, particularly of the clay shales, is accompanied by the expulsion of water, and the compaction process will continue all the while maintaining the state of equilibrium in the equality above. It is possible that some measure of subsidence of the ground surface will occur. See also drainage (2) and bulk modulus(3) and (4).
Shales and clay shales have the most serious role in the compaction process and resulting subsidence. For the most part, clay shales and the aquifers they surround have been subjected to the stress of overburden over the span of geologic time from the time of deposition to the present. By far the greatest compaction the shales will undergo, for this depth and geostatic load, has already taken place. Further compaction, and therefore recognizable subsidence, cannot take place unless additional dewatering takes place.
And, this additional dewatering cannot take place unless water pressure is further decreased by additional water production from the aquifers.
In the clay shales overlying unconfined aquifers, from the ground surface to the depth of the water table there will be no further compaction resulting from any behavior within the underlying aquifers. Below the water table, in confined aquifers, changes in water pressure in the aquifer and in the adjoining clay shales can be affected by excessive water production. As water production continues, water pressure within the aquifers decreases. All waters in communication with the waters in the producing aquifers will be subject to the same decompression, and the framework materials surrounding the pore spaces will undergo increased compactive stress, in conformance with the pressure balance shown above. The structural framework supporting the clay shales will resume compaction and there might be alteration of the pore space within the aquifers, as well. Additional water is squeezed from the compacting clay shales, and, pore space in the clay shales and aquifers decreases in total volume. The events found on the right side of the equilibrium balance equation above are not sequential, but occur simultaneously while obeying the fundamentals of equilibrium. The resulting effect of this behavior is an addition to aquifer water supply by water expelled from the surrounding shales, and an artificial maintenance of pressure within the aquifers. This artificial decrease in the pressure decline rate could be misleading and erroneously could be attributed to natural recharge. If subsidence of the overlying ground surface is measurable, then only part of the pressure maintenance is due to natural recharge. Also see rebound.
In compliance with Pascal’s Law, all communicating waters experience the same changes in pressure. The clay shales underlying aquifers experience loss of water as do those above, but with moderation. As the clay shales associated with the aquifer system continue to be dewatered by the production of water from permeable aquifers, the geostatic load due to overburden continually decreases by the loss of water, thus expelling slightly less water from underlying shales. What water is expelled must move upward against gravity to find relief. This results in a small decrease in the rate that underlying shales compact.
Compaction cannot take place indefinitely. It is limited by the amount of overburden and the amount of hydrostatic load, both of which progressively increase with depth. Ultimately, but not necessarily during the span of the wells beneficial life, if pressure decline were to continue, compaction will respond less and less to the production of water as the framework gains rigidity commensurate with the lithostatic load it supports. After all compaction has taken place that will take place, for that depth of interest and that degree of compaction only, the pressure in the remaining water will decline rapidly due to the imbalance between withdrawal and recharge that caused the decline in pore pressure in the first place.
This behavior of producing aquifers and associated clay shales is predicated on the occurrence of measurable subsidence occurring at the ground surface.
(1) Running PVC or steel casing from the ground surface to the bottom of the drilled hole to prevent the collapse or caving of the borehole.
(2) Providing a means for water to enter the casing from outside the casing. This usually is provided by slots in the casing or by gun perforations.
(3) Cementing or grouting the casing in place with a cement or cement-bentonite mixture to ensure that all aquifers are effectively isolated so that crossflow or contamination cannot take place. Steps (2) and (3) can be performed in reverse order depending on the means used for putting entry holes in the casing. See crossflow, also thief level, and thief zone.
(4) A 4th part of the process is that well logs should be run in the borehole under openhole conditions, before the casing string is run in the hole, in order to evaluate the formations and strata containing the aquifers. And, well logs should be run again after the casing has been cemented in place in order to evaluate the quality of the completion work. Remedial work can be performed if necessary. See surface casing, well log, well logging, gun perforating and squeeze cementing.
(5) A final part of the preparation of a well for operation would be installation of the pump and disinfection of all downhole hardware. See disinfectant.
(6) Although there can be natural communication between permeable beds, such as by fractures, it is important to note the possible consequences in water wells when the completion of oil or gas wells is faulty. In oil and gas wells, if both the surface casing and the production casing are not completed properly, oil or gas under pressure from producing wells, or from shut-in wells, or from capped uneconomical wells, or from plugged and abandoned wells, can enter channels in the annular space between the production casing and the face of the drilled hole. In oil and gas wells, if hydraulic fracturing is employed to increase the permeability of the hydrocarbon-bearing zone and to improve the production rate, the high pressure necessary to produce fractures can break down the cement sheath in the annular space between the production casing and drilled formation wall. If this happens, it creates a path for the oil or gas to communicate with beds at shallower depths. If the formation pressure in the oil- or gas-bearing strata is great enough it can force oil, gas, or salt water, to rise to the depth of the permeable aquifers where it can communicate with and contaminate any or all unprotected aquifers.
Once a contaminating fluid has found a path to shallower depths it will seek relief to any region exhibiting a lower pressure. If no relief can be found at the wellhead, the contaminant, whether it is salt water or oil or natural gas, will seek relief in permeable aquifers where it can migrate laterally until relief is found, or until pressure equalizes. Darcy’s equation applies. In the case of a gas well where free gas is produced, or an oilwell where the oil has a high gas-to-oil ratio, gas dissolved in the oil can come out of solution when the oil enters the lower-pressured environment of the well bore, and all free gas then can rise to the level of the aquifers, and the formation pressure in the hydrocarbon-bearing beds will be transmitted to the aquifers with little change in pressure because of the low density of the gas.
This gas, under pressure from the oil- and/or gas-bearing formation, will seek relief in permeable aquifers if it cannot find an alternate path to the atmosphere. In response to the excessive pressure, the two separate volumes of water and gas tend to combine into a smaller volume in order to relieve some of the pressure. To do this, gas goes into solution. Without excessive pressure, gas and water will remain separated. This gas, both dissolved and undissolved, which has been forced to migrate within an aquifer will come out of solution , or separate from water, when the water is exposed to the prevailing atmospheric pressure. This gas is flammable.
This is a common occurrence when water wells are located amid or near the deeper oil and gas wells, and when completions in either water wells or in oil and gas wells are faulty, or when the completion has become deteriorated or near failure, or because the hydraulic fracturing pressure has broken down the cement sheath. Isolation of the aquifers then fails. Although it is possible for gas to dissolve in ground water under natural conditions, there must be a means of communication between the source and the aquifer, and the pressure within the aquifer must be increased to cause the gas to dissolve in the ground water.
In water wells drilled through beds containing coals, sulfur contaminants and biogenic methane are common. Such constituents found in association with coals can find their way into domestic water supplies if the aquifers or the beds containing coals are not effectively isolated, or if isolation is broken down. Any communication between the aquifer of interest and coals can result in contamination of the water supply by sulfur or biogenic gas. Communication from one bed to another can be caused by faulty completion of the well or when the completion degrades or breaks down under increased pressure in the annular space, or if any form of natural fracture system has created a communicable path between aquifers and contamination source.
The source of contamination and the path the contaminant takes can be determined from appropriate well logs. See well log (2) in particular. The nature of the flammable gas, i.e. biogenic or thermogenic, can be determined by laboratory methods, and thus the source can be identified. Remedial work involving well logging, gun perforating, and squeeze cementing might be necessary. See more under flaming water. Also see methane, biogenic methane, and thermogenic methane.
compressibility: Of water. See under bulk modulus.
conditional water right: The legal reservation of a senior priority water right which becomes an absolute right when, with reasonable diligence, the water user develops the right and water is actually put to beneficial use. (CRS 37-92-103).
conductor pipe: A short length of large-diameter casing used to keep the well bore open, and to provide a means to direct the drilling mud, returning from the drill hole, into the mud pit from which the mud can be recirculated. SPWLA. Compare surface casing.
conduit: (1) A means through which water can flow from one location to another. Open ended, unending, or continuous porous and permeable aquifers serve as a means for water to flow from its source of charge and renewal to the location of withdrawal. Also see pass-through aquifer.
(3) Natural communicable breaks such as fissures and fractures in consolidated or brittle rock.
cone, cone of depression: In aquifers, the sometimes undesirable, inverted, cone-like distortion in the airwater interface where air from an overlying aerated zone, or air from the well bore or casing annulus, is drawn downward within the near environment of the well bore. Sometimes results from a form of replacement of water in gravity drainage, often appears in wells with high production rates where drawdown is large. See coning for expanded explanation.
confined aquifer: See aquifer (2).
conglomerate: A course-grained sediment mixture composed of rounded waterworn fragments of rock, cobbles, pebbles, and grains generally bound together by finer-sized cementing minerals. See consolidation and cementation.
coning: In a producing water well. There are two forms of cone that can occur in the near environment of producing water wells. One is a naturally occurring cone of depression (see 1 below) in an unconfined aquifer when air from the aerated zone replaces produced water. The other is an unnatural occurrence (see 2 below) caused by improper production and completion practices.
(1) Cone of depression. A depression in the plane of the water table. It is based on the premise that the production of water from an aquifer cannot take place unless the void left by produced water is reoccupied. Occurs particularly in unconfined aquifers where water is produced by gravity drainage. Air from the aerated zone replaces water that has drained from the near environment of the well bore. The radial effects of the drainage and re-occupation begin at the borehole at the contact of the water table and the aerated zone, and grow from top to bottom with time. The effects are greatest near the borehole and diminish as radial distance increases from the borehole, producing the appearance of a relatively flat cone-like depressed zone that has undergone the process of being drained of free water. The result is: The hydraulic head within the aquifer decreases near the well bore where the depression is the greatest and increases with radial distance from the borehole.
The size of the cone of depression is a function of the absolute permeability to water in the producing part of an aquifer, height or distance from the pump depth to the aerated zone, and the rate of water production, as well as time. As the cone of depression grows and becomes large relative to aquifer thickness, the unaffected producing area immediately below the cone decreases in size and the hydraulic head at that point in the aquifer decreases correspondingly, causing the production rate to decrease. If the drawdown is increased to compensate for the natural decrease in production, then the second form of cone (under (2) below) might occur.
(2) Coning as a form of aquifer damage and/or well damage. Production of water produces pressure gradients in all directions from the depth of pump access. For a cone to form in a confined aquifer requires a source of air and communication with the air. When the gradient to the air-water interface, wherever it might be, becomes smaller than the gradient required to drive water to the well bore, air will be drawn downward toward the depth of access to the pump, thus producing a cone. If the air reaches the pump, water production will cease. In practice, this form of coning is caused by the excessive water production rate at the pump relative to the producing thickness of the aquifer and its absolute permeability. This causes excessive drawdown that can cause the gradient driving water to the borehole to exceed the gradient to air. Thus, coning begins. As the excessive drawdown further depresses the cone, the gradient to air further decreases, thus accelerating the growth of the cone until air reaches the pump, at which time water production will become sporadic and cease.
In practice, this form of coning is caused by the excessive water production rate at the pump relative to the producing thickness of the aquifer and its absolute permeability to water. This causes excessive drawdown that can cause the gradient to air to be smaller than the gradient driving water to the borehole. Thus, coning begins. As the excessive drawdown further depresses the cone, the gradient to air further decreases. The air-water interface between the cone and the water is a depression in the potentiometric surface. Water production will continue as long as the potentiometric surface is above the pump, and as long as there is pressure to drive the water out of the rock. The cone might reach equilibrium with the production rate and stabilize, or the cone might continue to grow until air reaches the pump, at which time water production will become sporadic and cease.
To reduce the likelihood of producing a cone, the production rate must be decreased. Once coning has occurred it might be difficult to reverse the damage to the aquifer because air has been introduced where it did not exist before, and the effective permeability to water within the volume of the cone might be permanently decreased. See permeability.
The thickness of the aquifer of interest, and the number of beds and laminae and their thicknesses opened to production, will influence the size and shape of the cone in both cases (1) and (2), and ultimately will influence the performance of the water well. Whether or not a cone will form depends on a source for air, the quality of completion and/or amount of drawdown.
(3) Conversion of confined aquifers to unconfined aquifers. For the conversion of a confined aquifer to an unconfined aquifer requires a source of air. No water can be produced unless the volume vacated by produced water is re-occupied by something, whether it be more water from a remote distance, or air. For air to re-occupy the vacated space there must be a source, and a communicable path between the aquifer and the source of air. If there is no air, or there is no communicable path to a source of air, there will be no conversion.
Under natural conditions, the usual confined aquifers do not have access to air except at distant outcrops, or if the completion of the well were faulty, or if air dissolved in water comes out of solution as pressure declines.
Air creeping into a confined aquifer as a result of the water table falling in outcrops will be discussed under potentiometric surface (2).
The possibility for air to communicate with a confined aquifer because of poor quality well completion is conditional. However, a means by which the conversion could take place would be for cones of depression from a number of producing wells to overlap or merge with one another. This would allow a layer of air to overlie the water and, therefore, allow air to take the place formerly occupied by water as water is produced. If the re-occupation were to take place, drainage of water in this manner would be by gravity drainage.
Coning universally is thought of as a result of overproduction due to a flow rate that is too high for the thickness of the aquifer, or as a result of faulty completion practices. All aquifers lying above the selected producing aquifer must be completely isolated from the producing aquifer, and from each other. If not, crossflow can occur between aquifers, and the water produced at the surface might include water from one or more of the other aquifers. To prevent this, the annular space between the production casing and the face of all aquifers must be sealed, usually with cement, to ensure isolation and prevent communication. If the cementing is performed properly there will be no communicable path to air via the annular space. If the completion is faulty, not only might there be communication with air in the aerated zone, but there will be communication with other aquifers as well.
If the annular space between the production casing and the face of all aquifers has been properly completed and sealed, then the only other communication with air is through the annular space between tubing and production casing. This space usually is sealed by bolted fittings and connections at the ground surface. If leakage should occur, the water level in this annular space during the pumping operation should be prevented from falling to the level of the pump.
The air-water interface between the cone and the water is a depression in the potentiometric surface. Water production will continue as long as the potentiometric surface is above the pump, and as long as there is pressure to drive the water out of the rock. The cone might reach equilibrium with the production rate and stabilize, or the cone might continue to grow until air reaches the pump, at which time the pump will try to produce air.
(4) If the conversion from confined to unconfined aquifers were to take place there would be consequences. The benefit of the conversion where cones of depression merge is that there might be the possibility for a layer of air to grow in the upper part of the saturated aquifer. If the leakage of air were to continue, and in sufficient quantity, the layer of air would allow water locked in the aquifer to drain by gravity drainage.
However, depending on its depositional history the aquifer might have been formed by material transported by wind or water and deposited as sand dunes or sand bars. Air, because of its lighter density, would seek the higher elevations in the aquifer undergoing conversion. The consequence of this is that any wells drilled into the accumulation of air might incur production problems. New wells drilled into the higher elevations where air has become trapped will have a shorter beneficial life, and old wells will deplete early as air replaces water. See depositional environment and sedimentary.
Also, relative to depleting confined aquifers, see the discussion on flooding by the injection of compressed air under drainage (2).
connate water: The adjective connate is from the Latin meaning born together, originated together, or deposited along with the sediment. Connate waters are waters entrapped within the pores or spaces between the grains or particles of rock constituents (sedimentary or extrusive igneous) at the time of their deposition. Connate water is derived from sea water, meteoric water, or ground surface water. It is not to be used interchangeably with interstitial water. All connate water is interstitial water, but not all interstitial water is connate water. Compare interstitial water.
conservancy district: A special taxing district, created by a vote of the district’s electors, that has authority to plan, develop, and operate water supply and/or potable water projects. CSU.
conservation: Providing protection for any part of our environment by placing limits on the use, waste, exploitation, and pollution of a natural resource; and by the reuse through reclamation or other treatment of a used resource for the purpose of stemming the exhaustion of a natural resource. Also see preservation and environmental concerns.
conservation district: A geographical area designated by the State Legislature for water management purposes with a board appointed by county commissioners. CSU.
conservation storage: The storage of water in a reservoir or other containment for later release for beneficial purposes.
consolidation: Pertains to the rigidity of sedimentary rock. The precipitation and growth of water borne minerals on and around the grains, particles, or fragments of sediment serve as binding material that increases adhesiveness and rigidity. Typical binding materials are authigenic cementing crystals of quartz and carbonates. Also see cementation.
consumer confidence report: An annual water quality report prepared for consumers by their supplier. GWAC.
contamination: The undesirable introduction of pollutants or other extraneous matter into a system free of impurities. Can occur in water wells from faulty completion practices. Also see soil vapor intrusion.
conversion of confined aquifers to unconfined aquifers: This is not a technical term, but it is a sometime behavior that is said to occur relative to depleting confined aquifers.
Confined aquifers have no water table. Unconfined aquifers have a water table underlying air at atmospheric pressure in the aerated zone. For the conversion of confined aquifers to unconfined aquifers to take place there must be a source of air and a communicable path to the air, and air must replace produced water. The drainage of water from the aquifer in this manner is by gravity drainage as long as air is continually replenished, and as long as mobile water is available. See irreducible water.
Some hydrogeologists look upon the conversion of confined aquifers to unconfined aquifers as a potential, beneficial occurrence. If it happens it is possible for water formerly locked in the aquifer to be released by gravity drainage, but there are consequences.
At present, there are three speculative proposals how air might enter confined aquifers for conversion to take place: (1) Air creeping in from distant outcrops. (2) Merging cones of depression. (3) Dissolved air coming out of solution as pressure declines.
There are conditions where this conversion can and cannot occur. In general, for air to exist in the producing well system, there must be some form of communicable path to air. Confined aquifers, if they contain renewable water, are recharged at distant outcrops. They have no other communication to air unless completion practice for the well is faulty. See a discussion on how this conversion could take place by faulty completion practice under coning (3), and a discussion on the falling water table at outcrops and consequent fall in potentiometric surface of confined aquifers under potentiometric surface (2).
conveyance loss: Water that is lost in transit from a pipe, canal, conduit, or ditch by leakage or evaporation. Generally, the water is not available for further use; however, leakage from an irrigation ditch, for example, may percolate to a groundwater source and be available for further use. GWAC.
corrosion: (1) The chemical, electrochemical, or erosional destruction of metal or its surfaces by its natural or unnatural environment.
(2) A process of erosion whereby rocks and soil are removed or worn away by natural chemical processes, especially by the solvent action of running water, but also by the reactions, such as hydrolysis, hydration, carbonation, and oxidation. NSSH.
corrosivity index: One of the methods for assessing the scale dissolving (corrosive) or scale forming potential of water. A positive number indicates a tendency to deposit calcium carbonate. If the result is negative, it is an indication that the water will dissolve calcium carbonate and enhance corrosion. Also see Langelier Index. CSU.
crossflow: (1) The undesirable condition of fluid-flow out of one or more permeable strata into one or more thief strata. In aquifers, it can result in the depletion of one or more strata and the loss of integrity and/or contamination of others when water flows out of one into another. A condition that often occurs in water wells when faulty completion practices fail to isolate permeable strata. Also see channeling.
(2) The undesirable condition in a producing water well when water from a restricted or unauthorized aquifer flows behind casing to the level of the authorized aquifer and is produced, consumed or sold along with the water from the approved aquifer. A thief condition that often occurs in water wells when faulty completion practices are accepted and fail to isolate unapproved permeable strata and, therefore, fail to prevent production from unapproved aquifers. This condition can result in the unauthorized depletion of an unapproved aquifer.
(3) In oil or gas wells the production casing that passes through the surface casing must be completed properly. If not completed properly, oil or gas and sometimes salt water from producing wells, or shut-in wells, or capped uneconomical wells, or plugged and abandoned wells, under pressure from the formation, can fill the hole or enter channels in the annular space between the production casing and the face of the drilled hole, and thus allow communication and ultimate contamination of any or all unprotected aquifers. See completion.
critical year: Usually considered a year in which the annual precipitation was considerably less than average and runoff in most of the streams was low. The critical year is used to test the dependability of water rights under "worst case" conditions.
cryptosporidium and giardia: Found in Colorado’s rivers and streams, cryptosporidium and giardia are microscopic organisms that, when ingested, can result in diarrhea, fever and other gastrointestinal symptoms. GWAC.
cubic foot per second: cfs. A unit of water measurement equal to the flow of one cubic foot of water each second. Equivalent to 448.8 gallons per minute or 1,984 acre feet per day.
Compiled and Edited by Robert C. Ransom
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