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106 Chapter 3 Water Sources: Groundwater
operations and the installation of other elements of the system. Periodic disinfection of
the well during the drilling is a good practice and should be encouraged. In the case of an
artificially gravel-packed well, all gravel-pack material should be sterilized before being
placed in the well. Solution strengths of 50 to 200 mg/L chlorine are commonly used for
sterilizing wells. The effectiveness of disinfection should be checked after the comple-
tion of the work. Disinfection of the system is also necessary after repairs of any part of
the system.

3.17.10 Maintenance

Good maintenance extends the life of a well. The maintenance of the yield of a well
depends on (a) the well construction, (b) the quality of water pumped (water may be cor-
roding or encrusting), and (c) any interference from neighboring wells. If the performance
of the well declines, renovation measures should be undertaken that may include mechan-
ical cleaning, surging, and chemical treatment.

3.18 EVALUATION OF AQUIFER BEHAVIOR

Planning for the optimum utilization of the groundwater resource in an aquifer system
requires the evaluation of the merits of alternative strategies of development. The steps
involved in predicting the consequences of various plans are (a) quantitative assessment
of the hydraulic and hydrologic characteristics of the aquifer system, and (b) elabora-
tion of the cause-and-effect relationships between pumping, replenishment, and water
levels.

3.18.1 Hydrologic Equation
The basic groundwater balance equation is an expression of material balance:
Inflow (or recharge) outflow (or discharge)
change in storage
I O
S
This equation must be applied to a specific area for a specific period of time. When draw-
downs imposed by withdrawals reduce the hydraulic gradient in the discharge areas, the
rate of natural discharge is reduced. These flows become available for development and a
new equilibrium condition is approached with the water table or the piezometric surface at
a lower level.
The effect of drawdown on aquifer recharge is difficult to evaluate. Additional
recharge may be induced into an aquifer through the former discharge areas by revers-
ing 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 induced in the recharge areas if drawdown causes a dewatering in areas
where recharge was limited because the aquifer was full. This is referred to as the cap-
ture of rejected recharge.

3.18.2 Safe Yield of an Aquifer
The yield of an aquifer depends on the following:
1. The characteristics of the aquifer
2. The dimensions of the aquifer and the hydraulic characteristics of its boundaries