Water Treatment Processes - Mathematics and Statistics for the Environment - page 604

Addition of
Coagulant

Water
Supply

Mixing
Tank

Flocculation
Basin

Settling
Tank

Sand
Filter

To Storage and
Distribution

Screening

Sludge
Processing

Disinfection

FIGURE 23.1 Conventional water treatment model.

The primary sources of freshwater include the following:

• Captured and stored rainfall in cisterns and water jars
• Groundwater from springs, artesian wells, and drilled or dug wells
• Surface water from lakes, rivers, and streams
• Desalinized seawater or brackish groundwater
• Reclaimed wastewater

23.2.1 W ater s ourCe C alCulations
Water source calculations covered in this section apply to wells and pond or lake storage capacity.
Specific well calculations discussed include well drawdown, well yield, specific yield, well casing
disinfection, and deep-well turbine pump capacity.

23.2.1.1 Well Drawdown
Drawdown is the drop in the level of water in a well when water is being pumped (Figure 23.2).
Drawdown is usually measured in feet or meters. One of the most important reasons for measuring
drawdown is to make sure that the source water is adequate and not being depleted. The data col-
lected to calculate drawdown can indicate if the water supply is slowly declining. Early detection
can give the system time to explore alternative sources, establish conservation measures, or obtain
any special funding that may be needed to get a new water source. Well drawdown is the difference
between the pumping water level and the static water level:

Drawdown (ft) = Pumping water level (ft) - Static water level (ft) (23.1)

Discharge

Static water level

Ground surface Pump

Drawdown

Cone of depression

Pump water level

Zone of influence

FIGURE 23.2 Hydraulic characteristics of a well.