Consider an upper confined aquifer overlying a lower aquifer, separated by a low
ID: 287660 • Letter: C
Question
Consider an upper confined aquifer overlying a lower aquifer, separated by a low-permeability confining layer. The flow is primarily horizontal in the aquifer with water exchange via leakage. In the cross-section, each aquifer is 10 m by 100 m. The aquitard thickness between two aquifers is 20m. The planar area is 1000 m by 100 m.
Other than the information in the diagram, you are given the following:
(1) The hydraulic conductivities of both the lower aquifer is 2.2x10^-6m/s, and the hydraulic conductivity of the upper aquifer is 1.4x10^-6 m/s
(2) The vertical conductivity of the low-permeability confining layer (dark gray) is 1.1x10-9 m/s;
(3) The head in the lower aquifer average about 0.83 m higher than those in the upper aquifer;
(4) The hydraulic gradient in the near vicinity of the inflow areas of both aquifers is 0.1.
a. Calculate the horizontal inflow Q (m3/s) into each of the two aquifers.
b. Calculate the vertical leakage rate across the confining layer. What is the direction of leakage?
c. Calculate the horizontal outflow Q (m3/s) out of each of the two aquifers.
d. Calculate the horizontal hydraulic gradients in the vicinity of the outflow areas of both aquifers. (for part d. If you know the outflow rate from each aquifer, and you already know the hydraulic conductivity and the flow area, you can figure out the hydraulic gradient!)
100 m 1000 m 10 m 20 m 10 mExplanation / Answer
a) According to Darcys's law we know that-
Amount of water flowing through a porous media , Q = KA (H/L)
where K = hydraulic conductivity, A= Area of cross section, H/L= hydraulic head
Therefore amount of water through the lower aquifer = QL= 2.2x10^-6x10x100 (0.83/100) = 2.65x 10-1 m3/s
amount of water from the upper aquifer= QU= 1.4x10-6x10x100 (0.83/100) = 1.6 x 10-1 m3/s
b) vertical leakage rate of the confining medium is = QVL= 1.1x10-9x20x100 (0.83/100) = 26.5 x 10-5 m3/s
c) an aquifer generally has the ability to yield as well as retain water in natural conditions.
we are asked to calculate the amount of water that is horizontally outflowed. So it will be the same as the input. Therefore the amount of water from the upper aquifer would be 1.6x10-1m3/s and the outflow from the lower aquifer would be 2.65x10-1 m3/s.
d) Since the outflow rate is the same for both the cases as the inflow rate. Therefore the hydraulic gradient would also be the same that is (0.86/100) for the aquifers= .86x10-2.