Susan. Lecture 4 Flashcards
What is a perched aquifer?
A limited unconfined aquifer with an underlying confining layer that lies above and is separated from the regional water table by an unsaturated zone
Name two boundaries for aquifer recharge and discharge
- Permeable hydrological boundaries
2. Natural discharge systems
When do spring lines form?
When the valley wall cuts the boundary of an aquifer and aquiclude in a horizontal line
What happens to underground river systems in karst areas?
On entering limetsone terrain rivers vanish down sink holes often vertically until they contact the water table below and are forced downhill via a series of flooded solution caverns.
What happens when a fresh water system meets a marine system? (e.g. Long Island, NY)
Long Island: Strong hydraulic connection between the unconfined groundwater system and the surrounding bodies of saline surface water. As development spread east salt water intrusion was seen in freshwater aquifers resulting in basements flooded and subsidence occurred.
What measures were taken in Long Island, NY when they realised that the salt water had intruded the fresh water aquifer?
Pumping stopped and to reduce salt water intrusion they treated waste water and storm water and fed them back into the aquifers via recharge basins
Explain the challenges of dealing with aquifers near marine environments in Brighton
In Brighton a pumping system in the chalk has been developed to limit the effects of salt water intrusion.
In winter strong flows of groundwater to the sea so water extraction takes place near the coast to intercept the flow.
In summer extraction takes place inland to reduce the danger of saline intrusion by coastal wells.
What does the Ghyben-Herzberg relationship measure?
Estimates the depth to the fresh-salt water interface for aquifers at marine margins and within small islands.
What are the assumptions of the Ghyben-Herzberg relationship?
The interface is sharp with no mixing
Hydrostatic principles apply and there is no resistance to vertical flow in the salt or freshwater reservoirs.
At the shoreline the freshwater head = sea level elevation
What are the properties of aquifers in unconcolidated fluvial deposits (such as braided rivers and meandering rivers)?
Large variability in grain sizes, great lateral and vertical heterogeneity in aquifer properties with core sample hydraulic conductivities spanning several orders of magnitude
Bedded nature of sediments gives strong anistropy
What properties do modern day aquifers in braided rivers and meandering rivers have?
Braided river: extremely coarse grained with coarse grained sands and gravels and only minor clays in abandoned channels
Meandering systems generally finer grained deposits with coarser materials in channel deposits and muds and clays in clay interbeds. Clay horizons or clay layers in floodplain sediments
What properties do aeolian aquifers have?
More homogenous than fluvial material and can have good permeability.
What properties do shale aquifers have?
Low permeability, act as aquitards in sedimentary sequence.
Low K value
Porosity abundance is such that they store a lot of water; adjacent confined aquifers can draw much of their discharge from leakage and storage in confined layers.
How do glaciogene deposits form?
Deposits laid down by active and decaying ice
What are the aquifer characteristics of glaciogene deposits?
Aquitard (if sandy)
Aquicludes (if high clay content)
Depositional structure makes aquifers in these areas perched with layered and trend heterogeneities and strong isotropies common.
Water reserves are small, difficult to model and highly vulnerable to urban pollution
