Groundwater Flashcards
Define vadose zone
Unsaturated zone between the land surface and the phreatic zone
Define phreatic zone
Saturated zone, below water table
Examples of groundwater inflows
Infiltration through the vadose zone,
injection at wells,
groundwater flow from areas outside the region of interest
Examples of groundwater outflows
-Evaporation/ transpiration where the water table is shallow
-Water withdrawal from pumping wells
-Natural discharge at springs/lakes/wetlands
Define aquifer
a water bearing stratum that will release water easily. It consists of permeable rock or unconsolidated materials capable of holding and releasing groundwater.
Define aquiclude
a layer that does not transmit water effectively, and therefore acts as a barrier to groundwater flow
Unconfined aquifer
aquifer formation extends to the land surface, also known as water table aquifers, because the water table marks the top of the groundwater system
Confined aquifer
isolated from overlying formations and the land surface by one or more confining impermeable layers
Perched aquifer
Occur above discontinuous aquitards, which allow groundwater to mound above them. Perched aquifers sit above the water table, and within the vadose zone (there is an unsaturated layer below the perched aquifer)
Define aquitard
An aquiclude is more of a theoretical concept as most layers will let a small amount through. Instead, an aquitard just slows the flow
Characteristics of a confined aquifer
-Typically under considerable pressure
-Remain saturated over their entire thickness even as water is removed by pumping wells. Water extracted from the aquifer comes only from the depressurisation of the aquifer. The water pressure is decreased, but the pore spaces remain fully saturated. As pressure reduces the aquifer compresses slightly
-Usually deeper underground than unconfined
-Replenished or recharged by rain or stream water infiltrating the rock at a considerable distance from the confined aquifer
Compare intergranular and fissure flow
Intergranular has more storage potential but slower flow
What does Darcy’s Law state
- The flow between two points is directly proportional to the pressure difference between the points, the distance and the connectivity of the flow (permeability)
Define porosity
Volume of void space as a proportion of the total volume
Controls of porosity
o Sorting
o Cementation
o Overburden stress (effecting compaction)
o Grain shape
Hydrogeological Boundaries
-Lake (discharge/recharge)
-Fault (barrier)
-Fold (barrier)
-Watercourse (discharge)
-Fault (recharge)
-Facies (barrier)
Important aquifer properties
Porosity (specific yield)
Permeability (hydraulic conductivity)
Define specific yield
Proportion of water occupying void spaces that drains away under gravity
- Some water is bound or absorbed to the aquifer particles or fractures
- The specific yield is always lower than the porosity
- The attraction between water molecules and the aquifer is due to the polar nature of water and surface tension
- In larger grained or fractured rock, the specific yield is only slightly less than porosity
- In finer grained it may only be a few percent of the porosity because the surface area of the particles is much larger, and the pores are smaller allowing the aquifer to retain more water
- In unconfined aquifers, the specific yield controls the amount of water that can be extracted by pumping
- Effective Porosity= Specific Yield + Specific Retention
Define storage coefficient
ratio of the volume of water that an aquifer will deliver, to the volume of rock/soil, for a unit fall in piezometric head.
In unconfined aquifers this is the same as the specific yield
Define transmissivity
- Hydraulic Conductivity x Saturated Thickness
- Determined by pumping tests
- Measures the ability of the aquifer to transmit groundwater through its entire saturated thickness
- Implies extraction possibilities
Groundwater Budget
Inflows= Outflows +- Change in Storage
- Changes in storage are manifested as changes in the potentiometric surface, either due to a drop in the water table in unconfined aquifers or reduction in elastic storage as the aquifer is depressurised in confined aquifers.
- In a steady state the change in storage is 0- normally considered over a year or 10 years as they have seasonal variation
- Non steady state is due to heavy pumping for irrigation, industry or water supply
