3 – Aquifers

Question 1

Aquifer types: (openness)

Answer 1

Unconfined Aquifer:
• An aquifer with a free water surface as the upper boundary (the water table or phreatic surface) at which pressure is atmospheric.

Confined Aquifer:
• An aquifer confined between two aquitards or aquicludes, such that pressures in the aquifer are everywhere greater than atmospheric.

Question 2

Valley Aqufiers:

Answer 2

• Generally found in temperate climates where the soil is porous and permeable.
• Rainfall infiltrates and saturates the rock up to a level called the phreatic surface or water table.
• Groundwater drains to topographic lows and exits as springs or streams.

Question 3

Valley Aquifers in Arid areas

Answer 3

• In arid areas rainfall is much lower than potential evapotranspiration and surface recharge is almost zero.
• However, valleys may carry water from mountainous areas or from flash floods, which bring large quantities of water for a short time.
• This water usually infiltrates through the river bed into the aquifer and constitutes the only recharge mechanism.
• Therefore, the water table is higher beneath the valleys than elsewhere

Question 4

Heterogeneity:

Answer 4

Most geological formations are not uniform in composition and the properties of that porous medium will vary both in space and direction.

Consider hydraulic conductivity, K

Homogenous: Independent of position in a geological formation (K, y,x,z = c constant)

Heterogenous: Dependent on position (K, x,y,z =/= c constant

Question 5

Anisotropy:

Answer 5

Isotropic: Independent of direction K=/=f(0)

Anisotropic: Dependent of direction K=/=(0)

Transversely anisotropic: A layered formation where components can be considered as having only horizontal and vertical components: Kx = Ky =/= Kz

Question 6

Assumptions made in applying Darcy’s law to confined aquifers

Answer 6

• Homogenous aquifer.
• Steady state flow conditions (no variation with time).
• One dimensional flow (although there is no vertical flow, we have also ignored any flow perpendicular to the page).
• Boundary heads are constant.

Question 7

Assumptions made in applying Darcy’s law to unconfined aquifers

Answer 7

• Homogenous aquifer.
• Steady state flow conditions (no variation with time).
• One-dimensional horizontal flow. (Note that here there is vertical flow but we have ignored it. We have assumed that only horizontal flow is important, the Dupuit Assumption).
• Boundary heads are constant.
• Recharge is uniform and constant.