Groundwater

Question 1

Define vadose zone

Answer 1

Unsaturated zone between the land surface and the phreatic zone

Question 2

Define phreatic zone

Answer 2

Saturated zone, below water table

Question 3

Examples of groundwater inflows

Answer 3

Infiltration through the vadose zone,
injection at wells,
groundwater flow from areas outside the region of interest

Question 4

Examples of groundwater outflows

Answer 4

-Evaporation/ transpiration where the water table is shallow

-Water withdrawal from pumping wells

-Natural discharge at springs/lakes/wetlands

Question 5

Define aquifer

Answer 5

a water bearing stratum that will release water easily. It consists of permeable rock or unconsolidated materials capable of holding and releasing groundwater.

Question 6

Define aquiclude

Answer 6

a layer that does not transmit water effectively, and therefore acts as a barrier to groundwater flow

Question 7

Unconfined aquifer

Answer 7

aquifer formation extends to the land surface, also known as water table aquifers, because the water table marks the top of the groundwater system

Question 8

Confined aquifer

Answer 8

isolated from overlying formations and the land surface by one or more confining impermeable layers

Question 9

Perched aquifer

Answer 9

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)

Question 10

Define aquitard

Answer 10

An aquiclude is more of a theoretical concept as most layers will let a small amount through. Instead, an aquitard just slows the flow

Question 11

Characteristics of a confined aquifer

Answer 11

-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

Question 12

Compare intergranular and fissure flow

Answer 12

Intergranular has more storage potential but slower flow

Question 13

What does Darcy’s Law state

Answer 13

• The flow between two points is directly proportional to the pressure difference between the points, the distance and the connectivity of the flow (permeability)

Question 14

Define porosity

Answer 14

Volume of void space as a proportion of the total volume

Question 15

Controls of porosity

Answer 15

o Sorting
o Cementation
o Overburden stress (effecting compaction)
o Grain shape

Question 16

Hydrogeological Boundaries

Answer 16

-Lake (discharge/recharge)
-Fault (barrier)
-Fold (barrier)
-Watercourse (discharge)
-Fault (recharge)
-Facies (barrier)

Question 17

Important aquifer properties

Answer 17

Porosity (specific yield)
Permeability (hydraulic conductivity)

Question 18

Define specific yield

Answer 18

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

Question 19

Define storage coefficient

Answer 19

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

Question 20

Define transmissivity

Answer 20

• 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

Question 21

Groundwater Budget

Answer 21

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