Water Glossary: R

radionuclide	A material with an unstable atomic nucleus that spontaneously decays or disintegrates, producing radiation.*
radium (Ra)	A naturally occurring radioactive element (in the form of radium-226 or radium-228) created in the decay of the uranium and thorium series. Radium can be removed from water by cation exchange softening.*
radium-226 + radium-228 (Ra-226 + Ra-228)	The sum of the naturally occurring radioactive isotopes of radium. The regulation for radium by the US Environmental Protection Agency is for the sum of the [two] isotopes.*
radon	A colorless, radioactive, inert gaseous element formed by the radioactive decay of radium; exposure to high levels can cause cancer
recharge	Replenishing an aquifer with stormwater or imported water
reclaimed Water	Wastewater that has been cleaned so that it can be reused for most purposes except drinking.
reclamation	Historically, a wide-ranging federal program to irrigate arid lands throughout the West. More recently, a euphemism for treating sewage water so it can be reused for nonpotable purposes. See recycled.
recycled	Wastewater cleaned for re-use, usually for nonpotable purposes such as irrigating landscape and refilling aquifers.
regulation	A governmental order having the force of law
reservoirs	A pond or lake where water is collected and stored until it is needed.
residuals	Any gaseous, liquid, or solid by-product of a treatment process that ultimately must be disposed of. For example, in a fixed-bed filter for removing particles from water, both the filter backwash water and the solids in the backwash water are residuals.*
reverses osmosis	A process where water is cleaned by forcing it through an ultra-fine semi-permeable membrane that allows only the water to pass through and retains the contaminants
rills	Small grooves, furrows, or channels in soil made by water flowing down over its surface; also another name for a stream-usually a small stream.
runoff	Liquid water that travels over the surface of the Earth, moving downward due to the law of gravity; runoff is one way in which water that falls as precipitation returns to the ocean.
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