radioactivity: A property of unstable isotopes which undergo spontaneous atomic readjustment with the liberation of particles and/or energy (for example, alpha or beta particles, neutrons, and gamma rays). Alpha and beta emission change the chemical nature of the element involved. The loss of energy will result in the decay or transformation of the unstable isotope into a stable isotope; or transmutation into an isotope of another element, sometimes giving rise to emission of neutrons. Also see isotopes. SPWLA

radius of invasion: The radial depth within a formation that has been penetrated by drilling mud filtrate.

radon, radon gas: A chemically inert, naturally occurring gas produced by the natural radioactive decay of uranium. The heaviest of all known gases. Radon is an invisible, odorless gas at standard conditions of temperature and pressure and is toxic to humans. Radon gas from soil, weathered rock, and ground water volatilizes and decays into radioactive particles. These particles can be transported by dust and rising air and brought to the earth’s surface in water wells and springs. If consumed or inhaled, the particles become trapped in lungs or other body tissue. Radioactive energy released from these particles damages cell tissue, increasing the risk of cancer. CSU.

raw water: Natural, untreated water.

rebound: Relative to compaction and subsidence.  It is any degree of return of the compacted clay shales and aquifers to their former compacted state as existed before subsidence took place. Complete rebound, as might be brought about by any means, is not likely to occur. During the compaction process, sediments, particularly clay shales, undergo permanent changes that cannot be reversed. See compaction (2). Some seasonal rebound might be observed, but this would have to relate to non-permanent compaction events that occur within aquifer systems. Some seasonal subsidence and rebound due to elasticity of earthen materials has been observed by Jorn Hoffman, et al, Water Resources Research, V.37, no.6, 2001. Some seasonal rebound (in the range of 4") also has been observed by InSAR measurements in the metropolitan Los Angeles, California area. This is unusual, but can happen when the mineral structure of the aquifer has sufficient elasticity and the difference in height is large between the elevation of the water table in the recharge area and the depth of the aquifer. See elastic rebound in bulk modulus (3) and (4). Limited hydraulic rebound by increasing pore pressure in aquifers bynatural or artificial means, such as injection of water, might be possible, but this can happen only in confined aquifers where there can be no pressure relief.

This behavior of producing aquifers and associated clay shales is predicated on the occurrence of compaction resulting from excessive water production and measurable subsidence occurring at the ground surface.

recharge: (1) Natural recharge. To add to or replenish water in an aquifer at its source by natural means. A renewal process. Part of the hydrologic cycle.

(2) Unnatural recharge. Augmentation. To add to or replenish any downstream supply by manmade means. A means of returning water to an aquifer after it has been put to beneficial use. A conservation measure, not a renewal process, and not a part of the hydrologic cycle.

(3) Unnatural recharge. To add to or replenish the energy to drive water to the well bore by manmade means.

recharge area: See aquifer recharge area and outcrop.

reclaim: To collect, filter and/or chemically treat water for downstream reuse of the same water. A conservation measure to prevent waste. Reclaimed water must not to be confused with renewable water because no new water is added to the original water supply to improve sustainability. Reclaimed water is not new water, it is the same water that entered the stream or system before and has been put to beneficial use at least once. But, after its primary use, the water has been reclaimed through manmade processes for further downstream recycling or beneficial uses. There is no increase in supply. The material balance of the resource remains unchanged. See effluent (1) and compare with renewable water.

recovery: There are several recovery methods. Some are natural and some are man made. For aquifers, the number is very limited. In oil-and-gas recovery, the number of methods greatly exceeds the number available for aquifers.

(1) Primary recovery is the recovery of formation fluids by natural sources of energy. In unconfined aquifers the water table is the potentiometric surface. The water table immediately beneath the overlying aerated zone can and often does vary in depth with seasonal fluctuations. The pressure of the water between the water table and the depth where water is produced is a hydrostatic head that causes water to rise in the well bore to the depth of the water table. In unconfined aquifers, the natural source of energy is gravity. Gravity always is present. But, for gravity drainage to occur, something must allow formation water to move. In unconfined aquifers, it is air from a growing aerated zone that replaces movable water. In confined aquifers, it is more water from whatever source or means. These will be discussed below. See potentiometric surface and drainage.

In confined aquifers a means occurs when there is positive vertical separation between the level of the potentiometric surface at the well site and the level in the aquifer where water emerges. The vertical separation between these two levels produces a hydrostatic head giving rise to overpressure. This overpressure causes ground water to rise in the well bore to the level of the potentiometric surface. In aquifers this overpressure is called artesian head (this can be measured in nearby monitoring wells). When the potentiometric surface falls to the level of water emergence, there is insufficient pressure to push water into the well bore above the level of emergence. The natural hydrostatic energy will have dissipated and water production will decrease markedly and perhaps cease for all practical purposes.

There are a number of natural means to maintain overpressure, otherwise called artesian pressure, in the aquifer. Water pressure in the formation can be the result of natural movement of formation water into the environment of the producing well. This movement of water to replace water being produced is called water drive and can be the result of decreased water pressure at the well site due to water withdrawal, or it can be related to events occurring at remote locations at higher elevations, particularly at the recharge area. Also, formation pressure can be sustained by dissolved gas coming out of solution, and gas-cap expansion. In aquifers, primary recovery consists of several occurrences, all of which decrease with distance from the source. In aquifers, the natural events acting simultaneously and cumulative, by whatever magnitude are: (a) Recharge at the recharge area. (b) Drainage by gravity from the recharge area to the producing wells where the difference in depth between the potentiometric surface and depth of water emergence from the aquifer constitutes the energy for water production. (c) Water expelled from the aquifer and surrounding clay shales by compaction resulting from the decline in aquifer water pressure due to pumping. (d) Water expansion as a result of the decline in aquifer water pressure. (e) Dissolved gases coming out of solution resulting from the decline in aquifer water pressure. All natural occurrences identified here contribute to overpressure, and decrease the rate of decline of the decreasing pore pressure, often referred to as artesian pressure. Because of these several factors, the rate of recharge at the source is masked by these other factors.

The occurrences listed in the paragraph immediately above are natural drive mechanisms that create overpressures or maintain existing pressures until these actions dissipate. These occurrences constitute means for primary recovery. When the primary recovery has been exhausted, secondary recovery efforts are employed. In aquifers, secondary efforts are extremely limited.

Normal pore pressure that remains in formations after overpressure has been dissipated contributes little to the recovery process because the drainage mechanism or mechanism to drive fluid into the well bore has dissipated or does not exist due to the relatively great distances from the recharge area. Under this condition, there is very little fluid of any kind to replace potentially producible fluid. Observe the effect caused by distance Δd in the rewritten Darcy equation:

q = (kA/μΔd ) × Δp

When primary recovery means are exhausted secondary recovery methods are employed. See recovery, drainage, potentiometric surface, normal pore pressure, and abnormal pore pressure.

(2) Secondary recovery is man made. When no more formation fluid will flow by natural drainage methods, then fluid can be injected into a nearby well bore to increase the pressure within the formation to force the desired fluid to enter the producing well bore. In oil and gas wells, the fluid injected usually is water to displace oil or gas and increase or maintain pressure. This is called a water flood. Other fluids that may be injected might be natural gases. See flood.

In aquifers, it is not an option to inject surface water to increase the pressure to drive ground water out of the aquifer. There might be no other fluid that is as cost-effective as air. It is possible to inject air by compressors on the ground surface. The appropriate locations for compressors and appropriate wells for the air injection process must be selected with great care after studying the geology of the aquifer. If not, the injected air might accumulate in the aquifer at water-producing wells causing them to cease production.

(3) Tertiary recovery usually is the costly method used to recover as much formation fluid as possible that remains in the rock after secondary recovery operations have been exhausted. Tertiary recovery is not an option in aquifers. These recovery methods might be the injection of chemicals in solution, surface active agents, miscible solvents, or air. In the case of air injection in oil and gas wells, the oil will ignite by spontaneous combustion, thus heating the rock and producing combustion gases at the same time. In an oil reservoir the increased temperatures might reduce the viscosity of oil, and at the same time the gases produced might increase the pressure to drive the oil to the well bore of the oil or gas well. See drainage.

recovery factor: The percentage or fraction of the total volume of the resource that can be recovered by applicable recovery means. The recovery factor is a function of and is dependent on the means of drainage that takes place or actually can be implemented under in situ conditions. Applies to reservoirs and other forms of resource deposits where the capacity or quantity is a finite, determinable volume. Meaningless when applied to a resource that is renewable, such as water in a pass-through aquifer, or where boundaries and volume are indeterminate. See drainage, renewable, reservoir, recovery, and irreducible water.

recycle: The act of collecting water or any other exhaustible product for treatment and reuse. A conservation measure to prevent waste, not a renewal process.

reef: A significant limestone body of rock, sometimes of reservoir quality, formed under water by the skeletal remains and secretions of organisms, particularly corals. It is resistant to sea action and usually rises or stands above surrounding sediments.

relative permeability: See permeability (3).

relief: In porous and permeable aquifers, the equilibration process where water at higher pressure at one location always will seek to reduce the excess pressure through communication with any location where pressure is lower. See pressure gradient, communication, hydraulic pressure, and conduit.

renewable: That which can be renewed. Describes the state of an original, natural, usually exhaustible resource that can be replaced, is being replaced, or is being regenerated by natural means, such as by meteoric precipitation, rivers and streams, wind, solar, or geothermal. Water is renewable through the hydrologic cycle. Not synonymous with reusable. Compare effluent (1).

renewable water: Water originating only from tributary sources such as: meteoric precipitation, rivers, lakes and streams. Alluvial aquifers deriving its water from these sources are annually recharged through the natural means of the hydrologic cycle, so that the water supply is sustainable over time. The primary criterion for renewable water is that it is renewed by natural means, i.e. it is not derived from prior beneficial use or consumptive use of any water supply. Not to be confused with reusable or reclaimed water. Compare with effluent (1).

renewal: That which renews. A process by which any resource can be restored by natural means. The rate of depletion of this resource can be lower than, the same as, or greater than the rate of renewal, depending on consumption. See also hydrologic cycle. Compare with effluent (1).

reservoir: An above ground vessel, impoundment, pond, or lake; or an underground porous and permeable rock body that can entrap water, oil, or gases in a finite, measurable volume that can be filled and emptied of its contents. The basic premise of a reservoir is that it be a closed system with dimensions that can be determined so that volumetric calculations can be performed. Sometimes natural, sometimes manmade. Also see trap and lens, compare pass-through aquifer.

resistance to flow: Relative to water in an aquifer, it is the composite of those factors within the aquifer that decrease permeability and oppose the flow of water, such as: decreased porosity, friction, capillarity (adhesion, cohesion), cementation, tortuosity, discontinuity, decrease in grain size (increase in grain-surface area), angularity of the grains, efficiency of grain packing, decrease in size of pores and pore throats, increase in mineralization, presence of fines and clays and other pore infill and obstructive materials. Can be overcome only by sufficient hydraulic pressure to maintain or force water to flow toward the well bore. See permeability, also Darcy’s equation.

Resume: A monthly publication by the water court of a summary of water rights applications filed in the water court that month. CSU.

return flow: The amount of water returning to the water supply after it has been released from the point of use and thus becomes available for reuse.

reusable: Describes a substance that has been used before and has been treated to become usable again. Not synonymous with renewable. Reclaimed water is reusable water. See also reclaim.

reuse: To use again after manmade reclamation or treatment has taken place. Water sometimes is reclaimed for reuse. A conservation measure to prevent waste. See also reclaim.

riparian: Of or pertaining to rivers, streams, lakes and their banks.

rock: A generic term used to refer to earthen formations, beds, or strata without reference to mineral composition, properties, consolidation, or fluid content. Also relative to samples brought to the surface for examination. More specific:

(1) Sedimentary. The usual aquifer is sedimentary material. Can be sediment or fragments of source material that have been transported by water and then deposited as river beds and stream beds. Or can be material transported by wind and water to be deposited as sand dunes, beaches and sand bars. Can be unconsolidated or consolidated. Usually has some permeability. Or, it can be solid matter, such as salt or gypsum, that has precipitated from water, or skeletal remains or secretions of organisms precipitated to form limestone. Here, permeability exists in fractures, dissolution porosity and cavities, and karsts.

(2) Igneous. Formed from the cooling of magma or lava. Usually impermeable except for fractures.

(3) Metamorphic. Igneous and sedimentary rocks that have undergone changes in crystallization as a result of having been buried at depth within the earth and subjected to elevated temperatures and pressures, and different chemical environments.

risk assessment: Evaluation or the potential for exposure to contaminants and the associated hazards. GWAC.

rugosity: The degree of roughness and irregularity of the borehole wall.

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