Groundwater Resources
Importance of Groundwater -
1) Major economic resource - Houghton's drinking water comes from a well drilled into a delta deposit of the Pilgrim River. Where does your hometown's water come from?.
2) Water quality - waste disposal and pollution can threaten groundwater quality
3) Subsidence and Sinkholes - damage
Where is Earth's water distributed?
Oceans = 97% - salt water
Glaciers = 2.2% (largest reservoir of fresh water)
Shallow groundwater = 0.3% (largest reservoir of usable fresh water) - depth of 0.8 km
Lakes = 0.009%, Rivers = 0.0001%
Atmosphere (clouds) = 0.001%
Nature of Groundwater
Groundwater is located in the pore spaces (voids) between mineral grains in rocks and sediments.
The water table separates the zones of saturation and aeration.
Directly above the water table is the capillary fringe where water is drawn upward by capillary action.
Rivers and lakes represent locations where the water table intersects the ground surface.
Most groundwater (GW) originates from rainwater which infiltrates the ground and flows downward to the saturated zone. Other sources of GW include surface waters (lakes, rivers, etc.), artificial recharge (surface water pumped into the ground), irrigation, and waste water treatment systems.
Moving Groundwater
GW is in constant motion due to its ability to move between interconnected pore spaces in rocks.
Porosity is the percentage of pore spaces in a rock.
Permeability is measure of the speed that water can flow through rock.
Porous and permeable "reservoirs" of water are called aquifers (examples - sandstone, sand, and gravel). Impermeable rock layers are called aquitards or aquicludes (examples - shale and clay).
In humid regions, the water table level mimics the topography (high under hills, low under valleys), and this effect causes groundwater to flow.
This topographic effect occurs because GW moves slowly through rock and soil and tends to build up in areas of higher elevation.
Groundwater Flow
In simple systems, GW flows due to gravity.
Recharge areas - regions where water enters the GW system - areas of infiltration.
Discharge areas - regions where water leaves the GW system - springs, rivers, wells.
Effect of precipitation on the water table - Wet = high WT and a steep slope, Dry = low WT and a shallow slope (static water table, no flow).
Effect of permeability on the water table - Permeable rocks - low WT and shallow slope, Impermeable rocks - high WT and a steep slope.
An unconfined aquifer has no aquitard above it (no confining layer).
A confined aquifer is bounded above and below by aquitards.
During GW pumping, typically more water is removed than is replaced, so a cone of depression forms around the well (downward slope in the WT towards the well, vertical distance = drawdown).
Problems:
Overlapping cones of depression can cause a neighbor's well to go dry and force them to drill deeper (and often results in a lawsuit!).
Metropolitan Chicago has a huge cone of depression due to extensive GW pumping (the water table has gone from 250 m above sea level to 30 m below sea level). Thus, the city has switched to using Lake Michigan water.
This cone can also change the direction of GW flow. Pollution can be redirected into pumping wells.
A low permeability rock will have a very steep cone of depression (recharge rate of removed water is too slow to catch up to removal rate). A highly permeable rock will have a broad, shallow cone of depression.
Groundwater Supply
Groundwater supply is a function of precipitation (recharge), evaporation, surface runoff, and the nature of the earth materials involved (porosity and permeability).
Groundwater availability is determined by the balance of supply and demand for GW.
Information needed in order to manage groundwater supply effectively:
Total amount
Sustained Yield
Some GW always remains in the aquifer stuck to mineral grains - clays vs. sand and gravel
Groundwater Mining - When GW removed from an aquifer exceeds the amount produced (i.e., demand > supply).
Case Study - Ogallala/High Plains Aquifer and groundwater mining
The Ogallala Sandstone (aka High Plains Aquifer) is a very large confined aquifer that underlies much of the High Plains (NE, KS, OK, TX)and is used for irrigation. The aquifer was filled when the glaciers from the last Ice Age melted, but today there is little recharge. Demand greatly outweighs supply.
In the 1940's before GW pumping occurred, the average saturated zone was > 60 meters thick. By 1980 the average had decreased to ~3 meters thick. In some areas there was a 30-60 meter decline in saturated zone thickness.
Problems:
Use of inefficient center pivot sprinklers
Little incentive to conserve water
Government prices support the farming of water-hungry crops
A farmer can lose water-use rights if a minimum amount of GW is NOT extracted each year
In the 1980's, the situation improved due to better water management (meters on water wells to prevent overuse, reuse of waste water), heavy rains, and new technologies (new irrigation nozzles that < evaporation loss by up to 98 % over previous methods).
Projections are that 25% of the water in Ogallala will be used by 2020
Extending Water Supply
Water diversion projects - aqueducts
Reservoirs - for urban water supplies
Desalinization - expensive and energy intensive
Water Quality - general principles
Structure of water - unique physical properties that are related to its molecular structure.
H2O is a dipolar molecule
Water forms hydrogen bonds with other water molecules.
The unique physical properties of water include:
1.High melting and boiling point
2.High heat capacity
3.High capacity to dissolve salts
All natural water contains dissolved chemicals. The measure of the total amount of dissolved material in water is given as the Total Dissolved Solids (parts per million) = TDS
Standards for drinking water = < 500 ppm TDS
freshwater = < 1000 ppm TDS.
Representative TDS values and general chemical compositions for natural waters are:
Rain water = ~ 4 ppm (extremely low TDS)
River water = ~ 120 ppm (dilute freshwater, major TDS = HCO3-, SO42-, Ca2+, SiO2)
Ocean water = ~ 34,000 ppm (saline or saltwater, major TDS = Na+ and Cl- )
Groundwater is extremely variable - near surface GW is similar to river water, deep GW (100's of meters) is even more saline than seawater!
Why are the TDS so different for different locations?
Rainwater is dilute because it foms by the process of evaporation
Rivers and GW derive their dissolved chemicals from complete or partial dissolution of rocks by water
TDS in GW is a function of its residence time
Long residence times = long time for rock dissolution reactions and high TDS accumulation.
Typical residence times for natural waters are:
1.Atmosphere = 10 days
2.Ocean water = 37,000 years
3.Lakes, rivers, and shallow GW = variable, but closer to atmosphere times (not 1000's of years)
4.Deep GW = closer to or greater than ocean water
Seawater is more saline that rivers (that drain into oceans)
evaporation of incoming river water (distilled) that contributes additional salts,
is relatively stagnant (long residence time).
Seawater differs chemically because chemical reactions occur in the ocean to remove each dissolved species
- Na and Cl are removed very slowly,
all other species are quickly removed by sedimentation or organisms
Groundwater Pollution
Pollutants and sources that contaminate surface waters can also pollute groundwater.
Examples: leaky landfills, industrial waste lagoons, agricultural activities, and underground storage tanks.
Polluted groundwater can be a more serious problem. Why?
Contaminant plume -
Most GW contamination occurs in shallow unconfined aquifers located near a contaminant source.
Common sources:
1) Leaking underground storage tanks (gasoline stations) before 1980, tanks were made of metal and tended to corrode and leak pollutants into the soil and GW. Now, leak detectors are required, old storage tanks are being dug up and replaced, and the soil has to be cleaned up or destroyed(incinerated). In Denver, 80 liters of organic solvents contaminated 4.5 trillion liters of GW, affecting an area 5 km in length!
2) Septic tanks - bacterial and chemical pollution can be released when:
1.there are heavy rains and clay-rich soils cause the water to float the raw sewage to the surface
2.extremely permeable or fractured bedrock do not effectively filter the waste as it passes through
Groundwater treatment -
1.Reduce or stop input and then wait for nature to remove or destroy pollutants
Problem with this approach?
2.Extract GW - pump out the contaminated water and try to treat it, remove the soil and treat or destroy it.
Problem?
3.In-situ - add chemicals to immobilize heavy metals; add oxygen or nutrients along with microorganisms to stimulate them to munch
on the pollutants
Links about groundwater
Water Erosion Prediction Project--Purdue