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Assuring a Long-Term Ground Water Supply

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Assuring a Long-Term Ground Water Supply March 1995 AG 289 Assuring a Long Term Groundwater Supply: Issues, Goals and Tools Richard C. Peralta, USU Cooperative Extension Service and Department of Biological and Irrigation Engineering, Utah State University, Logan, UT 84322-4105 801-797-2786, FAX 801-797-1248 Introduction Groundwater is a hidden, but important resource. Generally, wells extract groundwater that is con- We can practicably define groundwater as water be- tained in the pore spaces or interstices between par- neath the ground surface that can be extracted by ticles of the aquifer material. The more intercon- wells. Other water in the ground that is not consid- nected pore spaces in the aquifer, the more water can ered to be available for man’s direct use is commonly be stored and removed. By knowing the shape, di- called “subsurface water.” Subsurface water includes mensions and effective porosity of an aquifer, one can moisture within the root zone. estimate how much water that layer can hold. But Groundwater is contained in geologic strata that does not tell us how much groundwater can be termed aquifers. Aquifers can be composed of a wide removed by wells year after year. How much can be range of materials, including sand, gravel, limestone, extracted depends on how much is initially in the and fractured granite. The more permeable the aqui- aquifer, how much new water enters (recharges) and fer to water (the greater its hydraulic conductivity), how much water leaves (discharges) in other ways. the more easily groundwater flows through it. Groundwater is part of the dynamic hydrologic Groundwater generally moves through aquifers rela- system. Most groundwater is continually in motion. tively slowly, except in fractures or solution channels. Groundwater flows from locations of recharge to lo- Solution channels form where water dissolves the ma- cations of discharge (from locations of high water terials around a fracture, gradually increasing the size surface to locations of lower water surface)1. In Utah, of the underground channel. Although underground water commonly enters aquifers in or near the moun- streams can result, they are the exception, rather than tains. It then flows through the aquifer to eventually the rule. The chance that a well will intersect an un- emerge downhill: as discharge from springs and natu- derground stream is slight. rally flowing wells; as flow to streams or lakes; as 1This is a simplification. Groundwater moves from locations of higher potentiometric head to locations of lower potentiometric head. The potentiometric surface describes the potential energy of the water. The potentiometric surface can also be called the water table in an unconfined aquifer, or the piezometric surface in a confined aquifer. 2 Groundwater velocity = (hydraulic conductivity of the aquifer)(slope of the potentiometric surface)/(effective porosity of the aquifer) 3Steady-state conditions exist when water levels no longer change significantly with time. 1 Figure 1. (a) Distinct cones of depression resulting from pumping at two separate wells at different times (b) Composite cone of depression resulting from pumping at both wells simultaneously (from Heath, 1963) (a) Well A Well B Static potentiometric surface Cone of depression if well A were Cone of depression if well B were pumping and well B were idle pumping and well A were idle Confined aquifer (b) Well A Well B Potentiometric surface Divide Cone of depression with both wells A and B pumping Confined aquifer evapotranspiration, or as discharge from pumped ing primarily on aquifer permeability, and the size of wells. How long a period water exists in an aquifer the depression, recovery can take from a few hours to depends on the distance between the recharge and dis- several days. Recovery can take much longer for charge locations, and the speed with which it moves.2 large composite cones of depression—years to de- Water entering an aquifer near a streambank might cades. discharge to the stream within a few days. Groundwa- ter might also take centuries to move greater dis- tances. Consequences of Groundwater Extraction Extracting groundwater through wells causes If recharge to the aquifer (or any part of it) ex- groundwater levels to drop. A depression in the ceeds discharge, groundwater levels (the potentiomet- groundwater surface will form around a pumping ric surface) will rise. This commonly happens near well (a cone of depression). The cone can continue to rivers at times of high river flow. As groundwater lev- expand until steady-state conditions are attained.3 The els rise within an aquifer, the volume of groundwater effect of pumping from multiple wells is roughly ad- in storage in the aquifer increases. If groundwater dis- ditive. The cumulative depression resulting from charge exceeds recharge, groundwater levels will pumping all the wells approximately equals the sum drop. That means that some of the discharged water of the depressions from the individual wells (Figure has been obtained by reducing the total volume of 1). Composite cones of depression can become very groundwater stored in the aquifer. large. Extracting more groundwater than is recharged is The depressed region around a single well can re- termed groundwater mining. Groundwater mining is cover shortly after the well stops pumping. Depend- not inherently bad. Some mining is necessary to make 2 best use of an aquifer. An aquifer undeveloped by resources. Coordinated management of ground water man has natural recharge rates and natural discharge and surface water resources is commonly referred to rates. Extracting groundwater by pumping changes as conjunctive water management. If done carefully those rates. The first groundwater that is pumped (with appropriate consideration of interactions be- comes from storage. However, groundwater pumped tween the two resources), transferring of water rights later can also come from increasing recharge; and between parties can improve conjunctive water man- from reducing other discharges. agement. Transfer can be accomplished by sale or Some beneficial side effects can result from trade. groundwater mining and declining groundwater lev- Agencies attempt to assure that groundwater els. Dropping groundwater levels increases flow to- pumping will not cause significant problems, such as ward the pumping wells and can: (1) increase re- are listed above. They commonly use proven equa- charge to the aquifer from rivers, lakes, or adjacent tions or computer simulation models to predict the aquifers; (2) reduce discharge from the aquifer to sur- consequences that will result from continuing current face water bodies; (3) reduce groundwater contami- pumping rates, or the pumping rates that would result nation by causing water levels to be below the reach if everyone that wants to use groundwater is permit- of degradable leaching contaminants; (4) reduce un- ted to do so. desirable groundwater loss (discharge) due to Computer simulation models contain equations phreatophytes or evaporation from the capillary describing how groundwater levels and flows respond fringe; (5) reduce other undesirable groundwater dis- to groundwater pumping, changes in recharge rates, charges; improve crop yields in previously water- or changes in other hydrologic features, such as rivers logged areas; (6) reduce septic tank problems result- or lakes. The models also contain estimates of aquifer ing from high water table elevations; or (7) reduce parameters (such as hydraulic conductivity, effective moisture in basements. porosity) and recharge rates, and the locations at However, excessive mining can be harmful. which these occur. Computer simulation models are Problems that can result from declining groundwater not used to predict the future, until they have been ac- levels include: (1) increase in energy required to raise ceptably calibrated for the region of interest and have a specific volume of groundwater to the ground sur- been proven to acceptably simulate what happened in face; (2) reduction in well yield or total loss of well the past. functionality due to diminished aquifer saturated Agencies frequently use properly calibrated simu- thickness in the well screened interval; (3) increased lation models to predict what the consequences will migration of salty or otherwise contaminated water be of any increase in pumping rates. An agency might into previously uncontaminated portions of the aqui- use a model after receiving a request from someone fer; (4) reduction in flow from springs; (5) reduction wishing to drill a new well, or increase pumping. If of flow in rivers due to induced recharge from river the model predicts that approving the new request to aquifer or reduction in discharge from aquifer to will harm those already pumping (or other legal or river; (6) dewatering of wetlands; (7) economic hard- environmental interests), the agency may deny the ship due to previously listed problems; and, last but new request. This is a common appropriate use of a not least, (8) social conflict and litigation. simulation model. If the model predicts that continuing current groundwater pumping will cause significant regional Goals of Groundwater Management problems, it might be the agency’s responsibility to Within its legal capacity, a water management attempt to reduce groundwater extraction. Because agency usually tries to assure that water users will the aquifer is not a uniform and homogeneous sys- have a long-term reliable source of water of adequate tem, reducing pumping in one part of the region quality
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