Aquifer transmissivity

Eq. [3-10b] assumes that the aquifer is horizontally isotropic (i.e., K is the same in both x and y directions). Solutions to the preceding differential equations under appropriate boundary and initial conditions describe the time-varying hydraulic head, i, in two dimensions. Several solutions are given by Carslaw and Jaeger (1959). More complicated boundary and initial conditions are treated by Hantush (1964), Reed (1980), and Wang and Anderson (1982). Note that in Eq. [3-1 Ob] the quantity (Kb) could be replaced by T, the aquifer transmissivity. 

A municipal well pumping 1200 m3/day is located 100 m from a major river. Aquifer transmissivity is 800 m2/day, aquifer thickness is 30 m, and the background hydraulic gradient (i.e., in the absence of the well) is 0.002 toward the river. 

Ferris, JJ. 1952. Cyclic fluctuations of water level as a basis for determining aquifer transmissibility. p. 1-17. In Contribution number 1. Ground Water Hydraul. Sect., Ground Water Branch, (U.S. Geol. Surv., Washington, DC. ... 

Superposition in phreatic aquifers is more problematic than superposition in confined aquifers, because changes in saturated thickness affect aquifer transmissivity and thereby create a nonlinear relationship between head gradient and total flow. However, for imconfined aquifers in which drawdovm is a sufficiently small fraction of aquifer thickness, the technique of superposition can be a useful approximation. The reader is referred to Strack (1989) for further details on superposition in imconfined aquifers. 

From a pumping test the permeability is evaluated as Transmissivity (m2/s). This parameter express the added permeability s meter by meter. By dividing the transmissivity with the thickness of the aquifer an average permeability is given. 

The soil is approx 10m and consists of clay till landfill on top. The bedrock consists of different limestone sections (Figure 89). The main aquifer is related to porous layers 30-60 m below surface. The transmissivity is approx. 2 x 10-3 m2/s. 

For application in fractured rock aquifers, it is assumed the aquifer behavior approximates that of a porous medium. Standard methodologies and their applicable assumptions are used to obtain values of transmissivity and storage, from which anisotropy is calculated. 

A limited number of aquifer pumping tests have been conducted within the perched zone. Two separate tests were conducted on a recovery well located in the southwestern portion of the refinery. Low transmissivity values of 100 and 150 gpd/ft were calculated. 

Both confined and unconfined conditions are simulated, where the hydraulic conductivity is 5 m/day and the initial saturated thickness is approximately 15.4 m for the unconfined aquifer. The transmissivity for confined conditions is 77 m2/day. The design problem is to contain the two plumes depicted on the figure, with the condition that remedial wells be located on site but not located in the building. The six candidate wells shown are used as potential pumping wells and the minimum total pumping from these wells must be determined such that the two plumes are captured. 

As stated in section 2.10, the velocity by which groundwater flows is commonly calculated from the water table gradient and the coefficient of permeability (k, or the related parameter of transmissivity). The k value is determined by a pumping test. During such a test a studied well is intensively pumped and the water table is monitored in it as well as in available adjacent observation wells. The change in water table level as a function of the pumping rate serves to compute the aquifer permeability. 

Phreatic aquifers are often regarded as recharge zones feeding adjacent confined systems. A continuous through-flow is commonly envisaged, controlled by (and deduced from) water level gradients and transmissivities. However, in certain cases a discontinuity is observed between the phreatic and confined parts of a system, reflected in abrupt changes in the chemical... 

Flow velocities of water in an aquifer, calculated by gradients and transmissivities, provide the maximum possible values. These are subject to limitations imposed by stagnation conditions. In extreme cases, confined systems may be rich in fossil karstic conduits, but with no through-flow due to complete confinement and/or burial beneath the level of the terminal base of drainage. 

Figure 1. Location of Devils Hole and salient features in its vicinity. Dashed-dotted line marks approximate boundary of the Ash Meadows groundwater system. Dashed line marks approximate boundary of highly transmissive aquifer. They also coincide with approximate position of the Spotted Range-Mine Mountain structural zone. Arrows indicate the inferred direction of groundwater flow. Shaded areas are approximate recharge areas. Adapted from Winograd et al. (1992).

In Eqs. [3-8a] and [3-8b], the quantity (K b) appears. This quantity measures the ability of an aquifer to deliver water to a well and is called transmissivity—abbreviated T, with units [L2/T],... 

First, calculate the hydraulic conductivity from the transmissivity and aquifer thickness ... 

FIGURE 3-16 Plot of drawdown as a function of time for a well in an aquifer having a finite extent. Note that drawdown increases more rapidly at longer times than it does in the case of the infinite aquifer (Fig. 3-14). Comparison of actual well pumping data with theoretical curves allows estimation of aquifer properties, including storativity, transmissivity, extent, and connection to sources of recharge (Driscoll, 1986). 

Aquifer characteristics include a 10-m saturated depth, a transmissivity of 2 X 10 3 m2/sec, a regional gradient of 0.0005, and a porosity of 0.25. It is proposed to remove, treat, and reinject the water to clean up the remaining trichloroethene. Water will be removed at well A, treated, and reinjected at well B, 200 m away downgradient. The spill extent is poorly known, but the consultant has assumed that if the steady-state pumping rate is sufficient to cause any contaminant midway between A and B to enter well A, all contaminant eventually will be captured. 

A new municipal well is installed in an alluvial aquifer having transmissivity of 0.1 m2/sec, foc of 0.5%, porosity of 0.25, and thickness of 20 m. The well is put into operation at a pumping rate of 0.5 mgd (million gallons per day). Neglect regional flow. 

A model was built using the best estimates of the variables including volumetries, stratigraphy, leak window area, hydrocarbon and aquifer properties. By producing hydrocarbons, the pressure in one block was depleted at the measured rate while the depletion profiles of the non-producing block were monitored. The least known parameter, the transmissibility of the faults, was varied until a history match was achieved between the modelled and observed depletion profiles. Comparison for different faults showed that faults with small displacements had to be modelled with higher transmissibilities than faults with large displacements. 

---