Interpretive Glossary of Water-related Terms and Expressions - P

packer: An expanding plug or other device used during the completion of a well when it is desired to seal off certain portions of the hole. Packers or plugs are used during cementing operations, acid treatments, and to isolate and seal off aquifers.

parts per million: ppm. Refers to the concentration of ions of solute in solution. Often measured in parts per million, ppm (wt./wt.), but usually measured in units of mg/liter (wt./vol.). A general indicator to the quality of the water. Parts per million differs from mg/liter when the specific gravity of the solution differs from the specific gravity of pure water at standard conditions of temperature and pressure. See also total dissolved solids.

Pascal’s Law: Pascal's Law states that pressure applied to any part of a free but confined liquid (e.g. water) exerts the same pressure to every other part. In a confined aquifer, either an increase in pressure or a decrease in pressure can be transmitted to all other parts of the permeable aquifer system without regard to differences in elevation, if they are hydraulically connected, subject to permeability, until any difference between water pressures reaches equilibrium.

pass-through aquifer: An aquifer carrying renewable water, where water that is being produced or has been produced is constantly being replenished by natural means. All alluvial aquifers containing renewable waters are passthrough aquifers. The aquifer contains water in a natural, permeable, usually alluvial pathway as the water migrates from one geographical location at one water pressure to another at a different pressure, or to the location of withdrawal. The migrating water stream uses the aquifer as a conduit or pipeline to pass from one location to another. See alluvial and stream, compare reservoir and recovery factor.

perched water table: A water table of a relatively small groundwater body lying above the general water table, and separated from the underlying aquifer by an aquiclude. GWAC.

percolate, percolation: Of liquids. (1) To percolate. To penetrate, under the influence of gravity, a natural or manmade porous and permeable medium. Usually in reference to the near surface soil of the ground.

(2) The liquid that percolates.

perennial flow: Year round flow.

perforating, perforations: To put holes or slots through the casing so that fluid can flow from one side of the casing to the other.

permeability: A generic term when used by itself. The ease with which water or other fluids can move through soil or rock. Influenced by many natural factors of rock and fluid. Measured in units of darcies or millidarcies. Also see resistance to flow.

Permeability is an intrinsic property of the framework of the gravel, sand, or rock constituting the aquifer and is related to the geometry of the voids and surfaces of solid materials. The most obvious influence on permeability is the interconnected void space throughout the aquifer. This is the effective pore space, or effective porosity. The permeability provided by the pores is further influenced by configuration, shape, physical dimensions of the pores and pore throats, tortuosity, continuity, pore isolation, orientation, irregularity, solid-surface roughness, angularity, sphericity, and anisotropy. In addition, the presence of allogenic minerals and authigenic mineral growths such as quartz or calcite or dolomite or clays, all, contribute to the pathway geometry within the pores. And, the pathways through the rock can be configured further by secondary porosity in all its shapes and forms, such as: dissolution porosity, replacement porosity, fissures, fractures, micro- cracks, and vugs in their various orientations within the rock framework. Finer materials with greater surface areas usually are transported the greatest distance from the source. As a generalization only, and pertaining mostly to aquifers, the permeability decreases with the distance from the source of the sedimentary materials.

(1) absolute permeability. The ability of a rock to conduct a given fluid through its interstices when the fluid is at 100% saturation.

(2) effective permeability. The ability of a rock to conduct a given fluid through its interstices in the presence of a second fluid immiscible with the first. Because of the mutual interference between the two fluids, the effective permeability to water, for example, in the presence of a second fluid, is less than the absolute permeability to water.

(3) relative permeability. The ratio of the effective permeability to a given fluid at partial saturation to the absolute permeability to the same fluid at 100% saturation. A measure of efficiency. The ratio varies between 0.0 and 1.0, where 1.0 is for a given fluid at 100% saturation. In the presence of two immiscible fluids, and because of the mutual interference between the two fluids, the sum of their respective relative permeabilities can never reach 1.0. A ratio, no units.

petrophysical log, petrophysical well log: A well log record, analog or digital. There are many different kinds of petrophysical well logs, each displaying a specific scientific measurement in the realm of chemistry or physics taken over the formations penetrated by the drill bit, from the ground surface to the deepest depth drilled. A petrophysical well log provides a permanent record of the environment of the borehole. See well log for more detailed information.

pH:      A term representing the hydrogen ion concentration in water solutions.

This term is a measure of either acidity or alkalinity of water solutions. A neutral water solution has a pH of 7. A pH lower than 7 is acidic, and that above 7 is basic or alkaline. A pH lower than 7 can be corrosive to copper pipes in homes and businesses. Also see Langelier Index.

piezometer: A borehole or standpipe in the earth extended below the depth of interest for the observation and measurement of the hydrostatic water level. The standpipe has an opening at or near the bottom to allow water to rise to the level that balances the pressure in the aquifer. The static water level is the potentiometric surface whether the standpipe is in an unconfined or confined aquifer. Piezometric pressure is potentiometric pressure. See potentiometric surface.

phreatic, phreatic zone: Pertains to the water-saturated zone underlying the water table. Overlain by the aerated zone.

plastic shale: Shale that contains a large amount of water and lacks rigidity. Plastic shale supporting a heavy geostatic load can find relief by squeezing into a drilled hole. The pressure exerted by plastic shale must be counterbalanced by that of the drilling mud, or the drill pipe can be seized and the hole lost.

point of diversion: A specifically named location where water is removed from a body of water. CSU.

point source pollution: Pollution coming from a single identifiable source such as discharge pipes from a commercial enterprise, sewers, private and municipal waste treatment plants, feedlots, golf courses, dumps for garbage or contents from septic tanks, faulty completions or abandonments of wells drilled for oil and gas, uranium or other minerals used for exploration or production, or any other concentrations of organic or inorganic waste, or means of pollutant discharge. Also see soil vapor intrusion.

pollutants: Contaminants. Any introduced gas, liquid, solid, inorganic or organic matter, or living microorganisms that makes a resource unfit for a specific purpose.

pore pressure: Pressure exhibited by the fluid in the pores of rock. Formation pressure. In aquifers, it is water pressure. See also hydraulic pressure, normal pore pressure, and abnormal pore pressure.

porosity: Generic. The fraction or percentage of void space in a unit of volume of a rock.

porosity feet: Can be the product of the average porosity and number of feet within a specific thickness of formation from which the porosities were measured. Or can be the sum of accumulated porosities over a selected net thickness within a formation.

porous: Pertains to soil or rock with open spaces or void spaces between the particles and grains of the rock. Sometimes intergranular voids, sometimes voids in fractures, sometimes voids created by dissolved solids. Often interconnected, creating permeability and pathways for water (and other natural fluids) to migrate through the host rock.

potable water: Water safe for human consumption. Water suitable for drinking and cooking purposes.

potentiometric surface: (1) It is a real or imaginary water-level surface or web that spreads across the aquifer that corresponds to water levels measured in well bores of strategic static water wells and monitoring wells. Usually these are measurements of artesian water levels.The artesian water level reflects the artesian head, or hydrostatic head. This, in turn, is a measure of the amount of abnormal pore pressure (overpressure). The water levels in the well bores from which these measurements are taken are not necessarily the same as water levels in the aquifers, or formation. There might be no such corresponding water level in the aquifers because in confined aquifers there might be no source of air for an air-water interface to be present. The static water-level in the well bore corresponds with the hydrostatic head that balances the formation-water pressure at the drilled face of the aquifer.

The potentiometric surface is the water table in unconfined aquifers. In confined aquifers, where no water table exists, the potentiometric surface is considered to be an imaginary water table.The height of the potentiometric surface above the level that water emerges from the aquifer describes the potential for water production. If the actual static water level in the monitoring well or static water well falls below the upper boundary of the aquifer the hydrostatic head will have dissipated and little or no water production can be expected by this means of pressure. Water saturation still remains at 100%, but the remaining water must be removed by forceful drainage methods. If the elevation of the surface of the ground should fall below the elevation of the potentiometric surface, ponds, lakes, springs, and water flows will be found. The warping downward of the interface between water and air in a cone of depression is a depression of the potentiometric surface. See cone. See also abnormal pore pressure (1) and drainage.

For any specific potentiometric surface measurement, the measurement is site specific. The reference datum level is the elevation of the level that water emerges from the aquifer in a specific water well.

The expression Δp / Δd in Darcy’s equation is the pressure gradient or hydraulic pressure gradient where the initial energy is related to Δp measured between the elevation of the water table at the recharge area (outcrop) and the elevation of the perforations or the level where water emerges from the aquifer. At distance Δd, the distance from the recharge area to the well site, the potentiometric surface has declined and the energy remaining for production, Δp, has declined accordingly. At the producing well site, Δp is the hydrostatic head that determines the excess water pressure in the aquifer. Here, the potentiometric surface has fallen to a lower elevation and is the level inside a static well bore that balances the formation-water pressure. The energy remaining for production, Δp, is measured between the elevation of the new potentiometric surface at the well site and the elevation of the level where water emerges from the aquifer. See the basic explanation of Darcy’s equation at Darcy’s equation.

For a detailed discussion of Darcy’s equation relative to the potentiometric surface go to hydraulic conductivity.

The potentiometric surface was first defined by O. E. Meinzer in 1923. Meinzer is considered to be the father of hydrogeology.

(2) Conversion of confined aquifers to unconfined aquifers. The potentiometric surface at or near the elevated recharge area of a confined aquifer will drop as long as water consumption exceeds recharge. Precipitation and other ground-surface water is the source of renewal of water throughout the recharge area. Below the aerated zone in the recharge area lies the water table created over geologic time. As the demand grows for ground water produced from the confined aquifer, a pressure decline takes place and the water table falls correspondingly. The drop in the water table might reach the depth where it enters the confined portion of the aquifer. It is possible then for air at atmospheric pressure to enter the former confined portion of the aquifer and cause it to become unconfined. Thus, conversion begins and production will resume.

(3) An immediate result of the conversion of confined aquifers is the loss of former actively producing water wells in and near the recharge area. These wells will have ceased production due to the drop in the elevation of the water table.

Another consequence of this conversion that is not first recognized is: As the water table falls, so does the potentiometric surface throughout all of the aquifer fed from this recharge area. The consequence of this continuing downward adjustment of the water table is that the potential for water production at all dependent wells will decrease right along with the falling water table at the recharge area. In Darcy’s equation, the fall in the potentiometric surface results from a reduction of the initial Δp . Every well in the aquifer ultimately will experience this reduction in formation pressure as the potentiometric surface adjusts to the falling water table at the recharge area. Some wells at remote locations from the recharge area will cease production earlier than others depending on the absolute permeability to water, shown as symbol k, in Darcy’s equation, and their elevations at the depth level of water production. These wells often will experience a decrease in the value of k as distance from the source of the sedimentary materials increases. Near the source of the sedimentary materials, larger particles and grains are the first to fall out of blowing wind and moving water. Finer particles and grains are carried farther from the source by wind or water. Smaller grains and particles exhibit larger surface areas resulting in lower permeabilities in their deposits and larger amounts of irreducible water. As distance increases, this will compound the decrease in the hydrostatic head at the producing well site.

Also, relative to depleting confined aquifers, see the discussion on flooding by the injection of compressed air under drainage (2) and recovery (2).

precipitation: (1) Meteoric water. Water in any form derived naturally from the atmosphere. Rain, snow.

(2) From water solutions. In a saturated or nearly saturated solution containing a uniform mixture of a dissolved substance (a solute), if conditions are changed slightly the solution can become over saturated and the solute will drop out of solution by precipitation or crystallization until the solution is no longer over saturated. See authigenic.

(3) From sea water. The process whereby materials, substances, secretions, siliceous skeletons of diatoms, or other organic or inorganic forms of matter settle out of water by virtue of their weight.

preservation: The protection from destruction, depletion, or pollution of any natural resource for future use. The act of setting aside or saving a natural resource. This involves limiting the use, waste, exploitation, and pollution of a resource. Preservation is supported by any conservation means including the reuse through reclamation or other treatment of a used resource. Also see conservation and environmental concerns.

pressure: The force per unit area applied to a surface or cross section. The hydraulic pressure that drives water to the well bore.

pressure drop: The loss of pressure, caused by the resistance to flow, measured between one location and another. Also, the difference between one potentiometric surface to another at lower pressure.

pressure gradient: (1)  Generally considered to be the variable hydraulic pressure gradient because the hydrostatic pressure gradient for water is fixed at 0.433 (psi per foot of height) times the specific gravity of the water of interest. The gradient is a measure of the rate of change in the difference in water pressure from one location, level, or depth to another. In producing aquifers, the rate of change is directly related to the hydraulic pressure required to drive water toward and into the well bore. This pressure is directly related to the drawdown and the resulting hydrostatic head for the water level in the well bore under producing conditions. In producing water from the aquifers, the gradient is influenced by both the hydrostatic head within the water-well casing and the formation pressure and the absolute permeability to water in the producing part of the aquifer. Without sufficient formation pressure and/or sufficient resulting gradient, neither resistance to flow nor back pressure will be overcome and water in an aquifer will not flow, but simply will remain as water in place. See in place, pumping process, and withdrawal process.

The highest efficiency for the hydraulic pressure gradient is found opposite those producing aquifers where drawdown is least for any specific production rate, regardless of hydraulic pressure in the aquifer. This signifies that the resistance to flow can be overcome easily by whatever formation pressure exists. Also see drainage area and resistance to flow.

(2)  Hydraulic pressure gradient. The change in pressure per unit length within an aquifer. Either horizontal or vertical. See the equation form of hydraulic pressure gradient under hydraulic conductivity as it appears in Darcy’s equation.

pressure tank: Usually a water storage tank equipped with an air bladder maintained under pressure that regulates the water pressure in the house. When the pressure at the tank decreases to a predetermined minimum pressure, the well pump is turned on. When the pressure in the tank reaches a maximum predetermined level, the pump is turned off.

primary recovery: See under recovery.

prior appropriation: Doctrine for prioritizing water rights based upon dates of appropriation (first in time, first in right). Common method for allocating water rights in the western United States. GWAC.

priority: The relative seniority of a water right as determined by its adjudication date and appropriation date. In some cases, other factors are involved in determining priority. The priority of a water right determines its ability to divert water in relation to other rights during periods of limited supply.

priority date: The date for establishment of a priority is the date of application for the water right.

production: (1) The state of a water well when producing water.

(2) The total volume of water produced from a well, accumulated from the date the well was first put on production, or the total volume of water over a specified time period.

production casing, production string: In the case of water wells, surface casing also can serve as the production casing. In the case of oil and gas wells, the production casing always will be steel for strength, and the production casing is run through all intermediate casings, including the surface casing, all the way to the bottom of the drilled hole. The production casing encloses tubing and pump and protects all downhole equipment used for production purposes in completed wells. Running production casing in the drilled hole is part of the completion process, and the casing must be properly cemented in place to isolate all permeable zones.

proration: Relative to water, a means for determining equity decreed by a government agency or court.

protection casing, protection string: (1)Sometimes called conductor pipe when it is the primary casing set in an oversized borehole for the purpose of preventing the collapse of the hole, or to protect the aerated zone and near-surface aquifers from contamination and damage. See conductor pipe. Compare surface casing and production casing.

(2) An intermediate casing string. One or more protective casing strings sometimes set between the surface casing and the production casing in order to protect portions of the formation that are sensitive to drilling mud chemicals or drilling mud pressures in order to prevent lost circulation; and, in formations exhibiting overpressure, to prevent collapse of the borehole opposite plastic shale or to prevent blowouts of oil or gas from permeable hydrocarbon-bearing beds.

pumping process: In a water well, the well pump withdraws water from the well bore, not from the formation. (Side note. The water-well pump cannot draw water to the well bore by suction. A vacuum, even if it could be created, cannot drive water to the well bore because a vacuum is neither a source of energy nor a form of force. See suction.) The pump does not draw water from the aquifer, but reduces the hydrostatic head of the water inside the well casing, thus reducing the back pressure that opposes the flow of water into the well bore. Water that emerges from the aquifer has been driven from the aquifer into the well bore, and its casing, where the pump forces the water to the surface through tubing. Assuming that water enters the well bore faster than it is produced, the height that water can be lifted and the rate that water can be produced depends on the lifting capacity of the pump. Also see withdrawal process and drainage.

pumping water level: See water level (2).


Compiled and Edited by Robert C. Ransom


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