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3.18 Evaluation of Aquifer Behavior
At least 6˝
Rock outcrop
Slope
is approached with the water table or the piezometric surface
at a lower level.
The effect of drawdown on aquifer recharge is diffi-
sealed off
up from bottom
cult to evaluate. Additional recharge may be induced into an
At least 4˝ larger
diameter than casing
aquifer through the former discharge areas by reversing the
hydraulic gradient. In leaky aquifers, inflow may be induced
by the reduction of heads, the contributions being roughly
proportional to drawdown. Additional recharge may also be
Shattered rock
to be sealed off
induced in the recharge areas if drawdown causes a dewater-
ing in areas where recharge was limited because the aquifer
was full. This is referredtoas thecaptureof rejectedrecharge.
Water-bearing stratum Drill hole Cement grout forced Top soil to be available for development and a new equilibrium condition
to be sealed off Well casing Liner
Sound impervious rock 3.18.2 Safe Yield of an Aquifer
Not less than 20´ Cement grout forced The yield of an aquifer depends on the following:
into sound rock
up from bottom
1. The characteristics of the aquifer
Drill hole acteristics of its boundaries
Water-bearing formation to be tapped 2. The dimensions of the aquifer and the hydraulic char-
3. The vertical position of each aquifer and the hydraulic
Figure 3.16 Drilled well and its sanitary protection (After Iowa characteristics of the overlying and underlying beds
State Department of Health). Conversion factor: 4. The effect of proposed withdrawals on the recharge
′
′′
1 = 1ft = 0.3048 m; 1 = 1in. = 25.4 mm.
and discharge characteristics of the aquifer
Thus it is evident that the safe yield of an aquifer is not
corroding or encrusting), and (c) any interference from neigh- necessarily a fixed quantity and it is not strictly a characteris-
boring wells. If the performance of the well declines, ren- tic of the groundwater aquifer. It is a variable quantity depen-
ovation measures should be undertaken that may include dent on natural hydrogeologic conditions and on recharge and
mechanical cleaning, surging, and chemical treatment. discharge regimes. Safe yield has been defined in a variety of
ways, each definition placing emphasis on a particular aspect
of groundwater resource development. These include, within
3.18 EVALUATION OF AQUIFER BEHAVIOR
economic limits, (a) development to the extent that with-
Planning for the optimum utilization of the groundwater drawals balance recharge and (b) development to the extent
resource in an aquifer system requires the evaluation of the that change in the quality of groundwater allows.
merits of alternative strategies of development. The steps
involved in predicting the consequences of various plans are 3.18.3 Water Budget (Hydrologic Budget)
(a) quantitative assessment of the hydraulic and hydrologic
characteristics of the aquifer system and (b) elaboration of The continuity of the water flow through a system can be
the cause-and-effect relationships between pumping, replen- measured by a water budget, which is an indication of water
ishment, and water levels. resources sustainability. Equation (3.43) can be rearranged
to be a mass balance equation in which the change of water
storage (ΔS) over a time period within the water system
3.18.1 Hydrologic Equation equals the inflows or recharges (I) to the system minus the
outflows or discharges (O) from the system. The water budget
The basic groundwater balance equation is an expression of
equation for a watershed can be derived as follows:
material balance:
ΔS = (G + R) − (E + E + G out + Q out ) = I − O
t
in
Inflow (or recharge) = outflow (or discharge)
(3.44)
± change in storage
Similarly the water budget equations for a lake can also be
I = O ±ΔS (3.43) derived using Eq. (3.43):
This equation must be applied to a specific area for a ΔS = (G + R + Q ) − (Q out + G out + E + E ) (3.45)
t
in
in
specific period of time. When drawdowns imposed by with-
G = G − G out =ΔS − R + E + E − Q + Q out
in
in
t
drawals reduce the hydraulic gradient in the discharge areas,
the rate of natural discharge is reduced. These flows become (3.46)