A confined aquifer (porosity n = 0.33) with an initial thickness of 14.7 m and a
ID: 803709 • Letter: A
Question
A confined aquifer (porosity n = 0.33) with an initial thickness of 14.7 m and an area of 30.0 km2 experienced land subsidence of 0.058 m when the pressure head was uniformly lowered by 18.1 m.
a) Calculate the vertical matrix compressibility, specific storage, and storativity of the aquifer. (8 marks)
b) What geological material is the aquifer made of? Explain your reasoning. (5 marks)
c) Assuming that pumping caused the pressure drop, calculate the total volume of water pumped in m3 (i.e., released from storage). (8 marks)
d) Suppose the same volume of water calculated in (c) above was pumped from an unconfined aquifer having the same matrix compressibility and a specific yield of 0.15. Would the decline in head in the unconfined aquifer be the same, greater than, or less than the 18.1 m decline measured in the confined aquifer? Explain your answer. (4 marks)
Explanation / Answer
specific storage = Ss = Dg ( a + nB)
where D = density of water
g= acceleration due to gravity
a= compressibility of an aquifer
n = porosity
B= compressibility of water = 4.4 * 10-10 msec2/ kg
= for a = (db / b) / dp
b = thickness
dp = pressure change
= 14.7 / 18.1
= 0.81 m
Ss = 1000 * 9.8 [ 0.81 + (0.33) (4.4 *10-10]
= 9800 [ 0.81 + 1.452 * 10-10]
= 0.8*10-6
so the rock material is rock and sound
storativity S= Ss* b
0.8*10-6 * 14.7
= 11.76 * 10-6
vertical matrix or pore compressibility
= P = g dw where P = compressibility
g = acceleration due to gravity = 9.8kN/m3
dw = land subsidence ie change in water table
= 9.81 * 0.058
= 0.56m2/N
b)sound rock from the specific storage value.
c) total volume of water drained = A * d * n
A = area D = thickness
n = porosity
= 9*105 * 14.7 * 0.33
43.659*105m 3
this is the amount of water drained through the aquifer.
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