Water Treatment Processes - Mathematics and Statistics for the Environment - page 605

■ EXAMPLE 23.1
Problem: The static water level for a well is 70 ft. If the pumping water level is 90 ft, what is the
drawdown?

Solution:

Drawdown = Pumping water level (ft) - Static water level (ft) = 90 ft - 70 ft = 20 ft

■ EXAMPLE 23.2
Problem: The static water level of a well is 122 ft. The pumping water level is determined using the
sounding line. The air pressure applied to the sounding line is 4.0 psi, and the length of the sounding
line is 180 ft. What is the drawdown?

Solution: First calculate the water depth in the sounding line and the pumping water level:

1. Water depth in sounding line = 4.0 psi × 2.31 ft/psi = 9.2 ft

2. Pumping water level = 180 ft - 9.2 ft = 170.8 ft

Then calculate drawdown as usual:

Drawdown = Pumping water level (ft) - Static water level (ft = 170.8 ft - 122 ft = 48.8 ft

23.2.1.2 Well Yield
Well yield is the volume of water per unit of time that is produced from the well pumping. Usually,
well yield is measured in terms of gallons per minute (gpm) or gallons per hour (gph). Sometimes,
large flows are measured in cubic feet per second (cfs). Well yield is determined as follows:

Well yield (gpm) = Gallons produced/Duration of test (min) (23.2)

■ EXAMPLE 23.3
Problem: When the drawdown level of a well was stabilized, it was determined that the well pro-
duced 400 gal during a 5-min test. What was the well yield?

Solution:

Well yield = Gallons produced/Duration of test (min) = 400 gal/5 min = 80 gpm

■ EXAMPLE 23.4
Problem: During a 5-min test for well yield, a total of 780 gal was removed from the well. What
was the well yield in gpm? In gph?

Solution:

Well yield = Gallons produced/Duration of test (min) = 780 gal/5 min = 156 gpm

Then convert gpm flow to gph flow:

156 gpm × 60 min/hr = 9360 gph

23.2.1.3 Specific Yield
Specific yield is the discharge capacity of the well per foot of drawdown. The specific yield may
range from 1 gpm/ft drawdown to more than 100 gpm/ft drawdown for a properly developed well.
Specific yield is calculated as follows:

Specific yield (gpm/ft) = Well yield (gpm)/Drawdown (ft) (23.3)