What is ground water and why should we care about it?
Most important fresh water source
Where is ground water found
Held in pore space of sediments
Found in fractures and joints of rocks
Easy to pollute but hard to clean up
How is water held in pores of rocks and soils?
Hygroscopic water
Thin film of water molecules on soil particles
Created by polar nature of water and polar nature of clays
This water bonds to soil and doesnt move through soil easily
Capillary water
Water that is held in pore space due to surface tension of water
Adheres to soil particles and moves by pressure changes
Gravitational water
Associated with large particles such as sand and gravel
Moves down due to gravity
Responsible for recharging ground water system
Parts of the ground water system
Zone of aeriation
Capillary zone
Zone of saturation
Water table
Perched water table
Spring
Ground water motion
Follows Surface topography
water flows down hill
Ground water is always in motion
How fast does groundwater move?
Two primary factors
Porosity
The amount of pore spaces in a rock or sediment
Determines how much water a rock or sediment can hold
Well sorted sediment with low cementation--> good porosity
Fractures and joints--> good porosity
Permeability
how well connected the pore spaces are in a rock or sediment
determines how water flows through a system
round, well sorted clasts with a random packing
poorly cemented or heavily jointed
best permeability--> sand and gravel
worst permeability--> clay an silt
Water movement
Primary controlΰ permeability
low permeability = slow movement of water
Secondary controlΰ porosity
small pores = slow movement of water
Aquifer
A medium that transmits water freely
What type of material would make a good aquifer?
Good aquifers
Sand, gravel or other permeable and porous material
Highly fracture rocks
Aquitard
impermeable layer of material
What type of material would make a good aquitard?
Good aquitards
Clay
Lines aquifers and acts as a pipe
Recharging?
How do water tables recharge?
Infiltration: Slow process--> dependent on permeability and porosity of the sediments/rocks
Influent streams: streams that feed the water table
How long does it take for ground water to recharge?
How long does it take to remove water from system?
What is a well?
Man made
created to harvest ground water
uses some type of mechanical pump
Natural well (artesian)
water rises above the aquifer through hydraulic pressure
1 aquifer and 2 aquitards
reaches surface by fracture or pipe
no pump is required
Other ground water exits
What happens when a water table intersects the surface of the earth?
Springs
Geysers
Effluent streams
Social issues surrounding water
Water use in USA (1995)
surface water 321 billion gal/day
ground water 77 billion gal/day
220 billion gal/day returned to streams after use (55%)
What sectors of society use water
power generation: 39%
Irrigation: 39%
Public supply: 13%
Industry and mining 7%
Commerical 1%
Livestock 1%
How are water supplies (streams and groundwater) affected by the population
overuse
pollution
climate
Overuse (Numbers from the United Nations)
1950 ~ 2.6 billion people
2000 ~ 6 billion people
2050 projected to be ~9 billion people
Current over use/mismanagement problems?
Diversion water from rivers for crop irrigation
Low water flow to downstream populations
Rio Grande--> Mexico and Texas
Disrupt sediment balance
Grand Canyon and Colorado River
Building levees to stop flooding
rich sediments not provided to flood plane
Divert to irrigate fields
increases salinity of farmland
Competition between wells
overpumping
draw down and cone of depression
dry wells
subsidence
salt water incursion
Salt water incursion
Mining of water tables
short term non-renewable
must apply safe yield concepts
establish hydrologic equilibrium for each basin
how much water is available
recharge rate vs withdraw
long term monitoring
limit wells
drawdown < recharge
Pollution?
Ground water contamination
dumps
pesticides and other chemicals
gas stations
septic tanks
farm waste (lagoons)
industrial
pipes and tanks leaking
dumping of toxic material
Why is it so difficult to clean up ground water systems?
How can ground water contamination be prevented or mitigated?
Use sewers not septic systems
line lagoons
limit pesticide/chemical use
rapid response to spills
monitor wells and tanks
Natural purification?
Sediment removes bacteria
physically
oxidation
assimilation
long resident time required
small pore spaces