Water as a Resource Flashcards
NASA’s motto in the hunt for extraterrestrial life: “follow the water”
- Perfect solvent
- Water flows -> provides a way to transfer substances between cells and environment
Where is the water?
Total water on Earth = 100%
- Oceans and salt lakes: 97.41%
- Ice and snow: 1.984%
- Groundwater: 0.592%
- Lakes and rivers: 0.0071%
- Soils, wetlands, and biota: 0.0059%
- Atmospheric water: 0.001%
Lots of water BUT only a tiny fraction by volume of the world’s water supply is available for human use
Storage of Freshwater (2.5% of total)
- Ice caps and glaciers: 68.6% (not readily available)
- Groundwater: 30%
- Rivers and lakes: 1.4%
Freshwater is not distributed equally on the surface, resulting in:
deserts, rainforests, and glaciers…
Lakes hold less than 0.4% of the world’s freshwater
- 80% of the water stored in lakes worldwide is held in <40 large lakes.
- They are only important as local sources of water
Rivers hold about 1% of the world’s freshwater
Rivers supply abundant water to cities:
- Hydroelectric power generation
- Household use
- Industrial use
- Agriculture
- Etc.
In Canada, most of our water use is for:
power generation and industrial purposes
Of the 38 billion m3 of water withdrawn in 2013, ~90% was returned to the
rivers and lakes of origin, 3.6billion m3 were consumed (consumed water cannot be used again immediately)
Power generation uses ~27bil m3 of water;
2% is consumed
Agriculture uses ~2bil m3 of water; 80-90% is consumed
Worldwide, agriculture extracts >6.4 trillion litres of water per day (82% for irrigation)
- This is suspected to increase to 56.4 trillion litres of water per day by 2050, BEYOND dependable flow of the world’s rivers
- Water availability is expected to limit pop growth in the 21st century
In many areas, consumption exceeds local surface water supplies, so water must be imported or mined.
- It is estimated that 3000x more water is stored as groundwater in the upper 800m of the continental crust than water found in all the world’s rivers and streams
- About 30% of the population of Canada depends on groundwater
Hydrogeologists
Groundwater geologists that specialize in distribution, circulation, and contamination of groundwater
Misconception is that groundwater exists in vast, underground rivers and lakes
This is generally not true, except in karsted carbonate rocks
Where is groundwater stored and how does it move?
Most groundwater is stored in pore spaces and fractures in rocks
Outcrops
Areas of exposed rock that provide a glimpse of subsurface conditions that we can’t see otherwise. Fractures and pore spaces can be microscopic to macroscopic
Porosity
Volume of empty space/total volume of rock (expressed as percent or fraction of 1)
- Shales can have higher porosity than sandstones, but the pore size is very small
- Primary porosity in limestones and other crystalline rocks may be very low, but secondary porosity (e.g vugs or fractures) may be high
Permeability
A measure of the ability of a rock or sediment to transmit fluid. Related to the size and interconnectedness of pores or fractures
The permeability of a medium is related to the porosity, but also to
the shapes of the pores in the medium and their level of connectedness.
High porosity, from high to low permeability: gravel, sand, silt, clay
High permeability allows for lots of drainage, whereas low permeability
minimizes drainage
Water that doesn’t run off enters porous rock through:
Infiltration.
- Fractured and porous rocks can store water
If these rocks are permeable, they can also transmit water.
- Infiltrating water tends to percolate downwards (gravity)
- The pores fill with water until it is saturated
- The top of the saturated zone is called the water table
- Above the water table in the unsaturated zone, pores may contain a mix of water and air
The shape of the water table mimics topography, but is subdued
The water table can move up and down with precipitation/drought withdrawal
In some situations, groundwater contributes to the base flow of a stream
In others, streams contribute to the groundwater
- If an area is dry, the water table may be far below the stream (disconnected losing stream)
Types of Streams
Gaining stream: base of stream is in the water table and gains base flow of water Losing stream (connected): base of stream touches the water table but loses base flow Losing stream (disconnected): base of stream does not touch the water table, losing base flow to ground and infiltrates down to water water
Wetlands often occur where
The water table intersects the land surface
Springs generally form where
The water table intersects a slope
Aquifer
Saturated rock that contains and transmits “significant” quantities of water
- Depends on usage
Unconfined aquifer
Recharged by water seeping directly from the ground above
- The level of the water can change over time
Confined aquifer
Fully saturated and bound by ~impermeable layers above and below the aquifer
How does water get into confined aquifers?
Recharge from other places
What are some groundwater issues we are facing?
Contamination:
Acid mine drainage:
- acidic solution gets into surface water: harms terrestrial, vegetation, and aquatic systems, and weathering increases.
- Acidic solution gets into groundwater: difficult to recharge or fix once contaminated
