Groundwater & Aquifers
Aquifers, water table, porosity & contamination
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Reading: Groundwater & Aquifers
Groundwater & Aquifers
About 2.5% of all Earth's water is fresh water, and most groundwater is not sitting in open underground caverns, it fills the tiny spaces between particles of rock, sand, and gravel, like water in a sponge. Groundwater is the largest reservoir of liquid fresh water on the planet, far more than all rivers and lakes combined. Billions of people depend on it for drinking water and irrigation.
Key idea: groundwater fills pore spaces in rock and sediment below the water table. How much it can hold depends on porosity (space). How easily water flows through depends on permeability (connected space).
How Groundwater Works
When precipitation infiltrates the soil, it percolates downward through the unsaturated zone (where pore spaces contain both air and water) until it reaches the saturated zone (where all pore spaces are filled with water).
The water table is the boundary between the unsaturated and saturated zones. It is not flat, it follows the general shape of the surface, rising under hills and dipping near valleys, streams, and wells. The water table fluctuates: it rises after wet periods (recharge) and drops during dry periods or heavy pumping.
Groundwater moves slowly through rock and sediment under the influence of gravity, flowing from areas of higher water table elevation to lower. Typical flow rates are centimeters to meters per day, far slower than surface water.
Porosity & Permeability
Porosity is the percentage of a rock or sediment volume that consists of open space (pores).
Permeability is the degree to which pore spaces are connected and allow water to flow through. Clay has high porosity but very low permeability because its tiny pores restrict flow. Gravel has high permeability because its large, well-connected pores allow water to pass easily.
Key distinction: porosity determines how much water a material can hold; permeability determines how easily water flows through it. A good aquifer needs both.
Aquifer Types
An aquifer is a body of rock or sediment that stores and transmits usable quantities of groundwater. Not all underground rock is an aquifer, it must have sufficient porosity and permeability.
Contamination & Protection
Contamination sources include leaking underground fuel tanks, agricultural chemicals, septic systems, landfill leachate, industrial waste, road salt, and improperly disposed household chemicals.
Once a contaminant enters groundwater, it forms a plume that moves in the direction of groundwater flow. Because groundwater moves slowly, contamination can persist for decades or longer and is extremely expensive to remediate.
Protection measures include wellhead protection zones, water quality monitoring, and proper chemical disposal. Natural filtration through soil and rock removes many (but not all) contaminants, sand and gravel filter particles and some bacteria effectively, but dissolved chemicals and certain pathogens may pass through.