Earths Water: Focus on Groundwater

Water is important for….

•  sustaining life, unique on earth that we have liquid water 

• Can exist in a solid form, liquid, and vapour 

Water covers___

• 71% of Earth’s surface (covered by oceans)

• Usable water: limited supply-2.67% is non saline and a large amount is in glaciers 

• 97% in oceans 

• Ground water is 0.62% of the usable water 

Hydrologic (water) cycle

Constant movement of water driven by sun's energy 

• Precipitation (rain or snow)

• Groundwater flow (flows from high topography to low topography)

• Surface runoff

• Evaporation (water returned to the atmosphere, liquid water converted to vapour)

• Transpiration (water returned to the atmosphere, induced by plants through capillary action and excess gos from the leaves) 

• evapotranspiration: mix of transpiration and evaporation

Groundwater

• Exists in spaces of loose sediments and rocks under the surface 

• Pores: open spaces in rocks and sediments 

• Total volume of open space limits amount of groundwater that can be held: porosity (amount of space available)

• geologic materials exhibit a wide range of porosities

Primary porosity

• pore space forms when the rock forms

  • voids in sediments

  • vesicles in basalt

  • open-reef framework

• crystalline rocks have very little primary porosity (Texture of rock is interlocking—igneous and metamorphic)

What kind of rock has high porosity?

•  sedimentary rocks: lots of pore space when sediments deposit

• clastic sedimentary rocks are derived from preexisting rocks, when deposited the pore space is in between (sometimes reduced lithification); loose sediments have lots of pore spcae between the grains

•  Well sorted sediements: pore spaces remain open  

  • And well sorted sandstone will have high porosity 

What kinds of rocks have low porosity?

• Igneous rocks have low porosity 

  • Interlocking each other, no open spaces 

  • but, some igneous rocks like basalt has vesicles-could also hold water 

• Metamorphic

  • exception: Foliation planes produced by alignment of minerals, water can get trapped between layers

• Smaller grains filling up pore spaces=low sorted so low porosity 

  • Poorly sorted sandstone will have low porosity 

Marble is….

quite crystalline: limestone (protolith) gets re crystallized during metamorphism so it is organized (interlocking)

• Limestone has porosity 

  • Chemical sedimentary rock/biochemical 

Calcite is a ____

very soluble mineral, water can dissolve it

• can have porosity created in limestone through dissolution

Secondary Porosity

• develops later

  • fracturing

  • faulting

  • dissolution of minerals to create open space (limestone)

• Enhance porosity of the rock 

Groundwater can be _____

primary or secondary porosity

Peremeability

• The ease/measure of water flow due to interconnectedness of the pores 

• Less permeable=flow of water is blocked (shale–lots of porosity but pores aren't connected) (clay-prevents water from moving)

• High permeable=water can flow (sand)

In terms of groundwater perspective:

want a rock that is both porous and permeable

• porosity enables a rock to hold large amounts of water

• permeability ensures the ability to extract water effectively

Shale is an example of what?

poor permeability

• has lots of porosity, but the pores aren’t connected so water doesn’t flow through

• this is the reason why fracking is needed to extract oil from shale because oil is held within the pores and can’t flow out easily

What is an example of a poorly permeable material?

clay

What is a highly permeable material?

sand

Aquifer

 an underground layer of water-bearing permeable rock or unconsolidated materials (gravel-sand) from which groundwater can be usefully extracted using a water well

• high porosity and permeability 

Aquitard

is a zone within the earth that restricts the flow of groundwater from one aquifer to another. Aquitards are layers of either clay or non-porous rock

• high porosity but little permeability 

Saturated Zone

where all pore spaces is saturated with water

Unsaturated Zone

Shallow levels, transitions where there is some water and open pore spaces

Water Table

• The boundary between the saturated and unsaturated zones

  • Beneath the water table, all spaces filled with water 

  • Above-not completely saturated 

    • Depth is variable

      • in humid settings, the water table is closer to the surface

      • in arid settings, it may be tens to hundreds of meters down

• Any surface water body (pond or river) is an intersection of the water table with the earth's surface 

Is the Water table a flat line?

No

• it is a topography that enables the flow of groundwater

• p refers to pressure that is the measure of the thickness of that water layer, pressure exerted by water (slide 11)

Groundwater flows _____

slowly under the influence of gravity

Unsaturated Zone water flow

• flow is straight downward, driven by gravity

• Any precipitation that falls in the unsaturated zones will recharge the water through gravity (infiltration of water)

Saturated zone water flow

• water flow is more complicated—governed by gravity and pressure

• doesn’t flow in straight lines

  • Gravity and pressure creates the curved path 

  • Curved path=when the water table is low and goes against gravity (recharge bodies of water)

  • Pressure difference from water (river and saturated area-one is thicker) 

Tapping groundwater

• springs are locations of natural groundwater discharge, where water table intersects the ground surface

• Slide 13: top right layer shows a permeable and non-permeable layer, when water is infiltrating and reaches a non-permeable surface, it will create a spring

What happens when there is no natural seepage of groundwater?

A well needs to be drilled into the saturated zone

• water is recovered by lifting or pumping

• as we use the water, it will be replenished as water flows from the aquifer into the well

What is Drawdown?

If water is pumped from a well faster than the aquifer can recharge, the water table is lowered

Groundwater depletion

•  lowering of water table (dragdown of water table) 

• Wells that are originally below the water table, are now above and dry (where recharge does not occur well)

• severe water table decline can alter surface water flow

  • by capturing flow, wells may dewater streams and lakes

  • especially problematic in arid and semi-arid regions

Groundwater depletion in costal regions

• intrusion of saltwater

• Freshwater floats above saltwater (less dense-salt water is dense cause of salt)

• pump out freshwater at a faster rate=intrusion of salt water so well is no longer usable (excess groundwater removal)

Groundwater quality

• often good quality because sediments and soil are good filters against substances 

  • clay minerals can absorb certain dissolved ions

• In contact with rocks and minerals–can be dissolved into the water (unwanted substances)

  • Issue: could lead to hard water (ions), contains basalt or salt ions of calcium and magnesium, won't lather well with soap (limestone regions)

Groundwater contamination

• many sources

  • Can be point sources of contamination: septic tanks, gas and oil reservoirs (leakage will contaminate)

• pollution is often not recognized until damage occurs

• cleanup is slow, expensive, and limited

  • Really expensive to reimmediate the contaminated water (can spread to aquifer quite fast) 

Water well density in Alberta

• Intense in southern part–more arid places and rely on groundwater fro farming and household purposes 

• Increase form 20,0000-230,000 

• We have an extreme drought triggered by longtime decline by climate change and el nino year (severe drought conditions)