6. A water table aquifer encounters the surface of the earth and forms a stream.

Question

6. A water table aquifer encounters the surface of the earth and forms a stream. The surface elevation of the stream is 75 feet. Two wells penetrate this aquifer. Well A is 180 feet south of the stream and well B is 130 feet north of the stream. The wells are open to the aquifer for their entire length. The surface elevation of well A is 140 feet. The surface elevation of well B is 135 feet. The depth to the water table in well A is 20 feet; in well B it is 15 feet. The total depth of well A is 125 feet. The total depth of well B is 120 feet. For each well find: (a) total head, (b) elevation head, (c) pressure head, (d) fluid pressure at bottom of well, (e) hydraulic gradient from each well to the stream.

Explanation / Answer

As per norms I should answer for first 4 sub division only

Given:

Solution:

b) Elevation head:

The elevation head is the height of the measuring point above the datum. In our case we have taken our datum to be mean sea level, therefore the elevation head for each well is calculated as:

For well A = Elevation of surface - well depth = 140-125 = 15 ft

For well B = Elevation of surface - well depth = 135 -120 = 15 ft

c) The pressure head is the height of the water above the depth of the well. Hence it is calculated by:

For well A = well depth - water table depth = 125 - 20 = 105 ft = 32 m

For well B = well depth - water table depth = 120 - 15 = 105 ft = 32 m

a) Total head:

Total head for well A = Elevation head + pressure head = 15+105 = 120 ft

Total head for well B = Elevation head + pressure head = 15+105 = 120 ft

d) Fluid pressure = Pressure head *density *gravity

For well A = 32 * 1000 * 9.81 = 313920 psi

For well B = 32* 1000 *9.81 = 313920 psi

A B Elevation at Surface 140 ft 135 ft Depth of  well 125 ft 120 ft Depth of water table 20 ft 15 ft

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