Example of grain size of a conglomerate within the Trujillo Formation, Palo Duro Canyon State Park, Canyon, Texas. Photo by Raymond Straub
for storage coefficients, values for unconfined aquifers range from 0.01 to 0.30 and for confined aquifers range from 10-5 to 10-3 (Sterrett 2007). Groundwater movement occurs from areas of higher head to areas of lower head. The difference between these areas is considered the hydraulic gradient and is the difference in hydraulic head between two points divided by their distance (Sterrett 2007). Groundwater movement is in the direction of decreasing total head (Heath 1983). Willis D. Weight, Ph.D., PE, helps clarify hydraulic gradient by stating: The slope of the hydraulic gradient is proportional to the hydraulic conductivity. The lower the hydraulic conductivity, the greater the slope of the hydraulic gradient.
The zone in an aquifer that represents the thickness of saturated material between the water table and the base of the aquifer in an unconfined aquifer and the thickness of the area between confining layers in a confined aquifer is known as the saturated thickness (Weight 2008). In considering saturated thickness, it is important to consider changes in effective porosity within the saturated material in the aquifer. As was defined earlier, hydraulic conductivity is the rate at which a geologic material can transmit a liquid under a hydraulic gradient. It can also be expressed as the rate at which a geologic material can transmit a liquid through a unit cube under a hydraulic gradient. This unit is normally expressed as a cubic meter (1 m3) or as a cubic foot (1 ft3). Therefore, it is expressed in cubic dimensions and it is considered three-dimensional flow (Kasenow 2006). Twitter @WaterWellJournl
In order to express the ability of an aquifer to transmit groundwater, the entire saturated thickness must be considered. Transmissivity is the rate at which a geologic material transmits groundwater through a unit prism. The top and bottom of the prism represents the upper and lower extent of the geologic material. Since transmissivity considers the full extent of the upper and lower boundaries of the aquifer, the third dimension of flow is removed and flow only occurs in two dimensions (Kasenow 2006). Transmissivity can be expressed from Darcy’s law: T=
QL
WΔh
= Kb
where: T = Transmissivity b = Aquifer thickness W = Unit width of aquifer (Kasenow 2006)
A water budget is a quantitative approach to account for all the inputs and outputs of a hydrologic system (Weight 2008). The key factors in a water budget are recharge or input, discharge or output, and storage. C.V. Theis wrote in 1957: Under natural conditions, therefore, previous to development by wells, aquifers are in a state of approximate dynamic equilibrium. Discharge by wells is thus a new discharge superimposed upon a previously stable system, and it must be balanced by an increase in the recharge of the aquifer, or by a decrease in the old natural discharge, or by loss of storage in the aquifer, or by a combination of these.
The overarching requirement of any hydrological system is a persistent recharge. In an interview with Roger W. Lee, Ph.D., an environmental scientist with Sims Associates LLC, he stated: Based on my 40 years in hydrogeology and geochemistry, and acknowledging the importance of gravity (water flows downhill), I believe the most important concept to understanding groundwater hydrology is groundwater recharge. There are many factors affecting groundwater recharge ranging from recharge from surface-water bodies, variations in recharge based on seasonal precipitation and infiltration, pre-existing conditions prior to recharge.
The ability of a groundwater hydrologic system to discharge water is a function of the system’s recharge, storage, and hydraulic conductivity. As water resources become more stressed and we increase our use of groundwater resources, it will become even more necessary for groundwater professionals to help the public understand the basics of groundwater hydrology. As groundwater professionals, we can help further the dialogue within our areas of expertise to our communities, politicians, and lawmakers so that they can make informed decisions on groundwater-related issues as they relate to public policies. WWJ
References
Heath, Ralph C. 1983. Basic Ground-Water Hydrology. U.S. Geological Survey Water-Supply Paper 2220, Reston, Virginia: U.S. Geological Survey. Sterrett, Robert. 2007. Groundwater and Wells, 3rd Edition. New Brighton, Minnesota: Johnson Screens, a Weatherford Company. Kasenow, Michael. 2006. Aquifer Test Data: Analysis and Evalution. Highland Ranch, Colorado: Water Resources Publications LLC. Lee, Roger W. Personal interview by the author. August 25, 2013. Theis, C.V. 1957. Ground Water Notes No. 34—The Source of Water Derived from Wells. Washington D.C.: U.S. Department of the Interior, Geological Survey, Water Resources Division, Ground Water Branch. Van Deventer, Gil. Personal interview by the author. September 3, 2013. Weight, Willis D. 2008. Hydrogeology Field Manual, Second Edition. New York, New York: McGraw-Hill Companies Inc.
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