Susan. Lecture 6 Flashcards
Describe the two ways of flows in fissured rocks
- Continuum: treat the network of fractures as the equivalent of a porous medium
- Discontinuum: discrete flows in individual fractures
What are the three scales to the continuum approach?
Microscopic, macroscopic (homogenous) and megascopic (heterogenous)
What is the ‘Representative elementary volume (REV)’?
As scale increases, larger numbers of irregularities are encompassed and the averaging effect reduces variance.
Above the scale the variance of the aquifer is constant and the macroscopic approach is valid.
When can the cubic law be used?
During the discontinuum approach - At the level of single fractures with laminar flow in a single smooth walled planar fracture of aperture and length normal to flow.
Where do fractures tend to occur?
Closest to the surface where vertical loading stresses are lowest and unloading stresses are greatest.
What happens to k with depth?
It decreases as fractures tend to close at depth due to vertical and lateral stresses imposed by overburden loads.
However brittle nature of rock causes brittle failure so some flow will persist.
Using granite as an example describe unloading fractures and fratures in 2-more-D
- Unloading fractures - These common surface parallel fractures are caused by unloading and stress release provide up to 100m thick of aquifer space with extreme vertical heterogeneity.
Unloading fractures do not develop below this. This limit would form the base of the aquifer. - Fractures in 2- or more- D
Granites vertically jointed in 2 or 3 vertical planes in addition to horizontal planes.
Behaves like cubic system.
Will hold a large volume of water.
Why is permeability more important than porosity in karst aquifers?
Although high porosity may not translate well into permeability as pores are too small, e.g. chalk.
What creates karst aquifers?
Fracturing or solution opening along bedding planes and vertical fractures. For solution opening groundwater must be under-saturated in calcite.
What properties do karstic aquifers possess?
Generally low capacity aquifers, although the limited capacity of the system means high recharge
Passages can be large and residence time of water can be much lower than other aquifers (hours, days). This means groundwater pollution can travel fast with little filtration. High risk of nitrate pollution.
Drawdown can be a major problem in fractured systems.
Where do aquifers form in unconformities?
- Weathered and fractured zone below the unconformity immediately below the surface of the buried landscape
- Permeable zones in the coarse grained sediments laid down on top of this surface when accretion recommended.
Aquifers are not always single features and can act as two layer systems where one aquifer may share with another. Describe the rose granite case in Brittany.
Two systems co-operate. Different characteristics, complimentary allows regional water demand to be met.
The lower system is a fractured system developed along line fracture zones. High production rates but limited storage capacities.
The second layer compensates with layers of weathered sands and gravels offering lots of storage capacity. Recharge and discharge far lower than the fracture system.
Summarise how the two layers complement each other in Brittany, France
Fractures: rapid recharge-discharge, limited storage
Weathered rock: major storage reserves at a lower rate of discharge
In a two layer system a borehole should be drilled in the rapid reaction aquifer. Why?
Any major drawdown can be catered for
Recharge from rain is rapid
In rain free periods, the over lying longer term aquifer will provide a steady input.
Briefly describe the three principle aquifers in the UK
- Chalk: 55%, Insignificant matrix permeability, fractures important, considerable aerial variation in transmissivity associated with topography
- Jurassic Limestones: 25%
- Permo-Triassic sandstones: 20%, Behaviour complex and difficult to predict due to lithology (i.e. grain size variation), degree of cementation and fracturing. Unconfined storage co-effs larger than chalk as sandstone can drain
