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. 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 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, 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, 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.
Compiled and Edited by Robert C. Ransom
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