Aquifer Recharge, Storage, and Recovery - Southwest Hydrology ...

What About the “R” in ASR?Betsy Woodhouse – Southwest Hydrology, University of ArizonaSouthern Nevada Water Authority ASR wellGetting water into the ground isfairly straightforward: water drainsdown from a basin or goes downa well. But then what happens to it? Willthe water really be there when it’s needed?Does it matter if the exact same water isthere to recover? Was water actually stored?The answers depend on the goals of theproject and the local regulatory framework.When fresh water is stored in saline orbrackish aquifers, common in the Southeast,mixing of the waters is undesirable: thegoal is to recover essentially the same waterthat was recharged. When fresh water isstored in fresh-water aquifers, as is typicalin the Southwest, recovering the same wateris less critical. The goal of such projectsmay be to reverse water level declinesin the aquifer or to store water long-termfor future drought or development.What Storage MeansFrom a physical standpoint, water mustremain in a location that is definable andaccessible for recovery in order to beconsidered “stored.” Rising water levelsin wells demonstrate that the volume ofwater stored in an aquifer is increasing.The Vidler Water Company has beenrecharging about 30,000 acre-feet peryear of Central Arizona Project waterSometimes recoveryof an equivalentmass matters morethan getting the samemolecules back.in its spreading basin facility in westernArizona since 2000; nearly a 200-foot risein water levels has been observed. TheSouthern Nevada Water Authority storeswater in a highly transmissive confinedaquifer that also is used by other entities. Itoperates on a seasonal cycle, storing waterduring the wet months when demand islow (and natural recharge also replenishesthe aquifer) and pumping it during drymonths. Water levels fluctuate 35 to 40 feetbetween the two seasons, of which 13 to18 feet are attributed to artificial recharge.Permeability tests, tracer experiments(primarily using chloride), and flowand transport models have been used tostudy the behavior of recharged water inan aquifer. In general, recharged waterusually stays in a somewhat coherentmass in the subsurface for a period oftime after some initial mixing at theentry zone. How much and how quicklythe recharged water mixes with nativegroundwater depends on parameters suchas regional groundwater flow velocityand the dispersivity, transmissivity,and heterogeneities of the aquifer.If water is stored only briefly or theaquifer is not highly transmissive, therecharged water mass will likely maintainits integrity, permitting recovery of mostof the stored water. If the water is storedlonger, its mass may eventually dissipatethroughout the aquifer, but if the aquiferis well-constrained, storage will still beevident through elevated water levels.If the aquifer is very large or highlytransmissive, however, physical storagemay be measurable only briefly if at all.Another Kind of StorageFrom a regulatory perspective, storagecan simply mean credit for recharging acertain quantity of water which providesthe storing entity a right to withdraw waterin the future. The water need not stay inany particular location, although ideallyit should stay within the groundwaterbasin. In some states or regions, a storingentity receives an equal amount of creditsfor withdrawal as was recharged. Inother cases, a “tax” may be levied. TheArizona Water Banking Authority takesa five percent “cut to the aquifer” forrecharge of Central Arizona Project waterin recognition that some amount of wateris lost in the aquifer. However, that fivepercent does not have a scientific basis.Robert Maliva of SchlumbergerWater Services points out that so-26 • May/June 2008 • Southwest Hydrology