Question

The water requirement from an aquifer is 3 million gallons per day . The aquifer has a transmissivity of 6000 ft2/day and storage coefficient of 8*10^-4 . the available drawdown is 50 ft . It is proposed to install wells of 8 in diameter . Design the well field for well efficiency of 90%.

Answer 1

Given:

T=6000 ft²/day

S=8x10^-4

Available drawdown=50 ft

Dia of well =8in/12=0.67 ft

Step 1:

Calculate r₀ from jacob equation

r₀²=2.25x6000x(1/8x10^-4)x365

r₀=0.78x10⁵ ft

Step 2:

Calculate the value of S

S=0.5(50)/log[0.78x10⁵/0.67]

S=4.93

Step 3:

Q=2.7TS

Q=2.7X6000x4.93

Q=79866 ft³/day

Q=5.97*10⁵ galon/day

Step 4:

Number of wells=3x10⁶/(5.97x10⁵)=5.02

Step 5:

S_a=0.5(50)=25 ft

E=90%

St=Sa/Ex100

St=25x100/90

St=27.77

S_w=27.77-25

Si=1.388

Spacing=(0.78x10⁵)10^-1.388/4.93

Sapacing=40,790 ft