Water Hydraulics - Wastewater Operators - page 71

Discharge

Static water level

Ground surface Pump

Cone of depression Drawdown

Pump water level

Zone of influence

figure 2.14 Hydraulic characteristics of a well.

Static water level —The water level in a well when no water is being
taken from the groundwater source (i.e., the water level when the
pump is off; see Figure 2.14). Static water level is normally mea-
sured as the distance from the ground surface to the water sur-
face. This is an important parameter because it is used to measure
changes in the water table.

Note: The shape and size of the cone of depression are dependent on
the relationship between the pumping rate and the rate at which water
can move toward the well. If the rate is high, the cone will be shallow
and its growth will stabilize. If the rate is low, the cone will be sharp and
continue to grow in size.

zone (or radius) of inluence —The distance between the pump shaft
and the outermost area affected by drawdown (see Figure 2.14). The
distance depends on the porosity of the soil and other factors. This
parameter becomes important in well fields with many pumps. If
wells are set too close together, the zones of influence will overlap,
increasing the drawdown in all wells. Obviously, pumps should be
spaced apart to prevent this from happening.

Two important parameters not shown in Figure 2.14 are well yield and
specific capacity:

Well yield is the rate of water withdrawal that a well can supply over a
long period. Alternatively, it is the maximum pumping rate that can
be achieved without increasing the drawdown. The yield of small
wells is usually measured in gallons per minute (liters per minute)
or gallons per hour (liters per hour). For large wells, it may be mea-
sured in cubic feet per second (cubic meters per second).

Specific capacity is the pumping rate per foot of drawdown (gpm/ft), or

Specific Capacity = Well Yield ÷ Drawdown (2.19)