Susan. Lecture 3 Flashcards
What does transmissivity mean?
A measure of how easily a layer transmits water
What is the difference between anisotropic and isotropic aquifers?
Isotropic - K at any point is independent of direction at any one point, Kx=Ky=Kz
Anisotropic - K at any one point varies with the direction of measurement
What is the difference between homogenous and heterogenous aquifers?
Homogenous - K has the same value wherever it is measured
Heterogenous - K varies with position
Describe and explain two limits in the assumptions made by Darcy
Assumption 1 - irregular complex is represented as a single homogenous medium.
Ina complex network of interconnected pore spaces, fluids flow at different velocities and in directions
Assumption 2 - only holds when groundwater velocities are small enough that flow is laminar not turbulent.
Darcy’s Law only appears to work when the Reynold’s number (Re, dimensionless) is 100. This a result of large fractures or pores
How does water move in an unsaturated medium?
As water content decreases, water is more attracted to medium than water. To flow water must flow through a network of passageways and K decreases. K declines nearly to zero as the material approaches zero water content.
What effect does water content have in an unsaturated medium?
- K is dependent on water content
- Porewater pressure is dependent on water content
- smaller pore size materials trend towards lower pore water pressures at a given water content
What happens to pore water in an unsaturated zone?
It in generally unconfined. Attraction between water molecules creates surface tension and causes capillarity. A capillary fringe develops in rocks and the thickness depends on the pores. E.g. coarse gravel: a few mm; clay: many metres
How do confined aquifers behave?
They remain fully saturated. The weight of the over-lying material is supported by the solid matrix of the aquifer and the pressure exerted by water in the pores.
What effect does removing water have on a confined aquifer?
Lowers the water pressure, granular aquifer matrix is compressed slightly
What effect does lowering water pressure have in confined aquifers?
Water can decompress and expand
What three impacts occur during the removal of water from boreholes?
- Withdrawing water causes a reduction in pressure around its intake, creates a head difference between water in the borehole and that in the aquifer
- Water flows from the aquifer to the borehole to replace that abstracted and in turn is drawn up by the pump.
- Water therefore flows from further out from the aquifer towards the borehole and so the effect continues
How does a cone of depression form?
Pumping out of water results in the lowering of the potentiometric surface, sloping smoothly to the borehole from all around the aquifer
How do we maintain a constant discharge rate when extracting water from an aquifer?
The smaller the radius of the borehole the greater the K value for a given discharge.
To maintain Q the speed of flow must increase.
An increase in kinetic energy is compensated for by a decrease in potential energy and therefore in static head.
The potentiometric surface is therefore further lowered.
What causes a cone of depression to form in unconfined aquifers?
The static aquifer thickness is reduced as the water table lowers. The hydraulic gradient increases to compensate but this depresses the cone further.
To reduce cost, drawdown should be kept to a minimum. How can this be done?
Slow pumping reduces kinetic energy through turbulent flow in the well.
Careful drilling as careless drilling can reduce permeability and can cause head loss
Test the behavior of the well.
