Page 76 - Water and Wastewater Engineering Design Principles and Practice

GENERAL WATER SUPPLY DESIGN CONSIDERATIONS 2-19

The depth to the bottom of the aquiclude ( Figure 2-5 ) sets the limit of drawdown of the
piezometric surface. If the piezometric surface drops below the bottom of the aquiclude, ground
settlement will begin to occur and, in addition to structural failure of the well, structural damage
will occur to buildings and roadways. In populated areas of the United States, regulatory agen-
cies gather hydrogeologic data reported by well drillers and others that may be used to estimate
the depth to the aquiclude. In less densely populated areas, exploration and evaluation by a pro-
fessional hydrogeologist is required.
The existing piezometric surface sets the upper bound of the range of drawdown. That is,
the difference between the existing piezometric surface and the bottom of the aquiclude ( s max
in Figure 2-5 ) is the maximum allowable drawdown for a safe yield. As noted above, in popu-
lated areas, regulatory agencies will have a database that includes this information. Otherwise,
a hydrogeologic exploration will be required.

Drawdown Estimation. The derivation of equations relating well discharge to water-level
drawdown and the hydraulic properties of the aquifer is based on the following assumptions
(Bouwer, 1978):
1. The well is pumped at a constant rate.
2. Flow toward the well is radial and uniform.

3. Initially the piezometric surface is horizontal.
4. The well fully penetrates the aquifer and is screened for the entire length.
5. The aquifer is homogeneous, isotropic, horizontal, and of infinite horizontal extent.
6. Water is released from the aquifer in immediate response to a drop in the piezometric
surface.

Although the steady state will seldom occur in practice, it may be approached after pro-
longed pumping when the piezometric surface declines at a very slow rate. The Thiem equation

FIGURE 2-5
Geometry and symbols for a pumped well in a confined aquifer. (Source: H. Bouwer, 1978.)