Aquifer =
Saturated, permeable geological unit that stores groundwater and allows it to flow under normal conditions
Aquiclude =
Saturated geological unit incapable of transmitting significant quantities e.g. could have unconnected voids
Aquitard =
Less permeable bed in a stratigraphic sequence
- may be significant in regional groundwater balance
- insignificant for local production well
Where does groundwater flow?
B/w grains (minute spaces)
Small cracks/fractures in limestones and chalk
How much of the water available to the biosphere is groundwater?
97.5%
Total porosity =
nt
vol voids/total rock vol
Effective porosity =
ne
vol voids accepting water/total rock vol
Drainable porosity =
nd
vol voids drained by gravity/total rock vol
Kinematic porosity =
nk
vol flowing water/total rock voil
Porosities and their relationship to each other (reasons)
nt>ne>nd>nk
- some voids too small to accommodate water molecules
- some water held against gravity by surface tension
- some pores bypassed/not connected to flowing network
Darcy’s Law
Q = -KA x /\h/L
Darcy flux
q = Q/A
Hydraulic head eqn
hydraulic head = pressure head + elevation head
Pressure head eqn
pressure head = P/(density x gravity)
Fluid potential
Flow from high to low potential regardless of direction in space; for flow through porous media = mechanical energy (“work done” per unit mass of fluid
Converting fluid potential to hydraulic head, steps:
Fluid potential = an energy
- Potential energy (raise elevation)
w1 = mgz - Kinetic energy (accelerate)
w2 = mu^2/2 - pressure energy (raised fluid P)
w3 = mP/density
Take unit m=1
Divide by g
= h
= z + w
Work = (equations)
mass x gravity x height
mass x acceleration
pressure/density x mass
(= (force/unit area)/(vol/unit area) x mass)
Observation well
Open borehole
Water level = average h over length of open section
Unconfined aquifers water level = level of prevailing water table –> integrated h
Piezometer
Device for measure h AT A POINT (or averaged over relatively small depth)
–> More precise point measurement > observation well
Water level in open piezometer well x = water table in unconfined conditions
Hydraulic conductivity =
Ease of flow of fluid
K
What does Poiseuilles formula calculate?
The flow through an individual hole
k =
Intrinsic permeability
= nD^2/32 (messy bit of Darcy/Poiseuilles formula)
Method for using Darcy’s/Poiseuilles formula
- Compare the two equations
- Get AK = the messy bit
- Since A=BH, K = messy bit
- Recall porosity n = vol voids/total vol
n = vol of cylinder (void)/vol of cub (total vol) - Relate K = and n =
- Separate fluid component and geometric component
- k = messy bit
so K = fluid component
Hydraulic head =
Elevation head + pressure head
= elevation at surface - depth to water
