Question

An aquifer test with a constant pumping rate of 200 gpm was conducted in a confined aquifer with a saturated thickness of 12 m. Head measurements in an observation well located 76.2 m (250 ft) from the pumping well yields the following plot. Use the Cooper-Jacob (1946) straight-line method to compute transmissivity [m/d], hydraulic conductivity [m/d and m/s], storativity and specific storage m ] of the aquifer 0 0.2 0.4 0.6 0.8 1.2 0.1 10 100 1000 Time (min)

Answer 1

Ans) Given,

Pumping rate (Q) = 200 gpm

Thickness of aquifer (D)= 12 m

Distance between observation well and pumping well (r) = 250 ft

Step 1) Find t_o from the plot

From the above pot we can see that at time = 0.30 min , drawdown = 0

therefore, t_o = 0.30 min

Step 2) Find $\Delta$ Z for values t₁ = 10 min and t₂ = 100 min

Z(drawdown) at t = 10 min , S = 0.70 m

At t = 100 min , S = 1.10 m

$\Delta$Z=1.10 - 0.70

$\Delta$Z = 0.40 m

or 1.31 ft

Step 3) Compute Transmissivity (T),

T = 264 Q / $\Delta$ Z

  T = 264 x 200 / 1.31

  T = 40305.34 gpd / ft

or 500.5 m²/day

Step 4) Compute storativity (S)

S = Tt₀ / 4790r²

S = 40305.34 x 0.30 /(4790 x 250²)

S = 4.04 x 10^-5

Step 5) Compute Hydraulic conductivity (K)

T = K x D

500.5 = K x 12

K = 41.708 m/day

or K = 4.83 x 10^-4 m/s

Step 6) Compute Specific storage (S_s)

S = S_s x D

4.04 x 10^-5 = Ss x 12

Ss = 3.36 x 10^-6 m^-1