Lecture 10 // Groundwater

Question 1

groundwater

Answer 1

Water that exists below the Earth’s surface within the pore spaces of sediments & rocks, and within fractured rock.

95% of total supply of fresh water in the world lies within pores and fractures within the crust!

Question 2

water table

Answer 2

The boundary beneath the
ground surface that separates the region below where pore spaces (and/or fractures) are saturated with water, and the unsaturated zone above.

Question 3

water table: level

Answer 3

The water table rises and falls throughout the year in response to recharge (how much water is infiltrating the ground).

Question 4

water table: topography

Answer 4

In humid regions, the water table mimics the topography of the land. It is higher beneath hills and drops in elevation towards the valleys.

Question 5

wetlands

Answer 5

The water table penetrates the surface in areas of wetlands and at streams.

Question 6

porosity

Answer 6

The percentage of pore space (that does or could potentially hold water or other fluids).

Question 7

permeability

Answer 7

The ability to transmit fluids. Permeability depends on the size of pore spaces, the sorting of grains, and the connectivity of pore spaces.

Question 8

• less well cemented

- well cemented

Answer 8

A less well-cemented sandstone has higher porosity & permeability than a well-cemented sandstone.

Question 9

• poorly sorted

- well-sorted

Answer 9

A well-sorted sandstone has higher porosity & permeability than a poorly sorted sandstone.

Smaller grains fill in the spaces between larger grains resulting in low porosity.

Question 10

(un)fractured shales

Answer 10

Mudstone and shale have high porosity but low permeability due to the small size of pores and the lack of connectivity. Permeability is increased if the rock is fractured.

Question 11

aquifer

Answer 11

A region of groundwater that can be accessed for human use.

An aquifer consists of porous and permeable rock or sediment, or permeable fractured rock.

Question 12

aquiclude (a.k.a. aquitard)

Answer 12

Low permeability layers that do not allow for easy uptake of water for human use.

Aquicludes are typically layers of mudstone/shale or highly cemented sandstone.

Question 13

unconfined aquifer

Answer 13

Found directly below the water table. The source of water is from infiltration directly above.

Question 14

confined aquifer

Answer 14

Found directly below an aquiclude. These aquifers are sourced from water NOT directly above. Water comes from a distant recharge area.

Note: Artesian aquifers are
under high pressure from a steep hydraulic gradient. They do not require pumping in order to flow.

Question 15

the source of water in

Metro Vancouver

Answer 15

1) North of the Fraser River: reservoirs (surface water)

2) South of the Fraser River and further out in the valley: unconfined and confined aquifers (groundwater)

Question 16

a perched water table (a perched aquifer)

Answer 16

It exists where water infiltration from above meets an aquiclude that lies above the main water table. The groundwater cannot pass through the aquiclude quickly enough (due to low permeability) and accumulates to form a small aquifer.

Question 17

groundwater:

flows

Answer 17

The groundwater flows laterally above the aquiclude and may exit as a spring if the perched water table meets a valley wall.

Question 18

groundwater issues

Answer 18

Groundwater is a resource, and it isn’t immune to problems from over usage and other issues. We have already seen one potential issue arise with groundwater usage:
1)Groundwater contamination of aquifers
Other issues with groundwater usage includes problems arising from extracting too much groundwater:
(2) Dry wells
(3) Saltwater intrusion
(4) Ground subsidence

Question 19

contamination

Answer 19

Unconfined aquifers are
especially vulnerable to
contamination sources at the surface.
- Sewage waste
- Landfills
- Pesticides
- Underground storage tanks

Question 20

dry wells

Answer 20

Pumping water faster than it is naturally replenished results in a drawdown – a cone of depression forms around the well. Wells may go dry if the cone extends deep.

Question 21

saltwater intrusion

Answer 21

Freshwater is less dense than saltwater – it sits above saltwater in coastal zones. If overpumping occurs then a cone of ascension forms in addition to a cone of depression. If the drawdown and updrawn regions meet, then wells go salty

Question 22

Hornby Island

Answer 22

The Gulf Islands in the Strait of Georgia are especially prone to saltwater intrusion. Precipitation is the only source of groundwater recharge and thus during the dry summer months the water table is naturally drawn down.

Question 23

Subsidence

Answer 23

Continued pumping of groundwater gradually lowers the region’s water table. Without water in pore spaces, the ground settles.