![]() a) Illustration of the definition of the storage coefficient, which is the volume of water released from, or added to, storage per unit change in head normal to the earth’s surface per unit area. Storativity of an unconfined aquifer (dimensionless)Īverage thickness before and after a water level change (L)įigure 49 – Schematic of components of storativity ( S) of an unconfined aquifer. ![]() The storativity ( S) of an unconfined aquifer is composed of two components as shown in Equation 44. Specific yield reflects the volume of water that drains by gravity when the water table is lowered, or fills with water when the water table is raised (Figure 49). The storativity for an unconfined aquifer is dominated by the gravity drainage term, specific yield ( S y). Storativity is dimensionless and is expressed as a decimal. It is defined as the volume of water removed or stored per unit change in head normal to the earth’s surface over a unit area. Storativity describes the capacity of an aquifer to store or release water. The older term, storage coefficient, is also used to describe the same aquifer storage property. The storage capacity of an aquifer is referred to as the storativity, S. These data are then analyzed using analytical equations and/or numerical computer simulations to estimate transmissivity values (e.g., Lohman, 1972). Most commonly, T values are determined from carefully designed aquifer tests were wells are pumped while water levels are measured in nearby observation wells. Transmissivity values are often estimated by combining K values from laboratory tests or textbook tables with field measurements of aquifer thickness, b. Head of an unconfined aquifer decreases in the downgradient flow direction, so saturated thickness decreases ( b 1 > b 2) and so does transmissivity ( T 1 > T 2). b) In an unconfined aquifer, transmissivity needs to be defined at a specified location because saturated thickness depends on the hydraulic head (e.g., based on the elevation of the water table). a) In a confined aquifer of uniform geologic thickness, saturated thickness, b, is constant ( b 1 = b 2) even when the hydraulic head is different. Figure 48 – Transmissivity, T, is the product of hydraulic conductivity, K, and saturated thickness, b, of the aquifer. In areas with large water table gradients, average thickness may be used to compute one representative value of T. When the water table slope is small, a single value of T is commonly used to represent the aquifer. ![]() The saturated thickness of an unconfined aquifer varies with space as the water table slopes in the direction of flow, thus, T values change with distance from a given location (Figure 48b). By definition the head of a confined aquifer is higher than the top of the aquifer, so the complete thickness of the confined aquifer is saturated, thus b is a constant when T is determined. The transmissivity of a confined aquifer of uniform thickness is a constant value for an isotropic and homogeneous set of conditions as shown in Figure 48a. Aquifers with multiple horizontal layers with different hydraulic conductivities can be represented by the sum of the T value for each layer. The higher T of the second aquifer indicates that it can transmit more water, thus if all else is equal, it would be a better target for a water supply well. If another aquifer has a hydraulic conductivity of 50 m/d and is 300 m thick, its transmissivity is T = 15,000 m 2/d. For example, if the hydraulic conductivity of a confined aquifer is 100 m/d and the thickness is 10 m, then T is 1,000 m 2/d. Transmissivity describes the overall transmission capacity of an aquifer system, not just the properties of a small volume of the aquifer. The term transmissibility is an outdated term that is occasionally used for transmissivity. Transmissivity, the capacity of an aquifer to transmit water (L 2/T) Transmissivity is defined as the product of hydraulic conductivity, K, and saturated aquifer thickness, b, as shown in Equation 43. However, the capacity of an unconfined or confined aquifer to transmit water is described as transmissivity. When describing the transmission capacity of a small representative volume of porous media the hydraulic conductivity is used. The groundwater transmission and storage properties of geologic formations including aquifers and confining units can be described by three hydrogeologic terms: transmissivity, T specific storage, S s and storativity, S. 6.4 Properties of Aquifers and Confining Units
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