Groundwater Flashcards
how is earths water distributed?
Oceans: 97.2% Glaciers: 2.15% Groundwater: 0.62% Rivers and lakes: 0.02% Soil moisture: 0.005% Atmosphere: 0.001%
What is groundwater?
Water below Earth’s surface, in pore spaces
Resides mostly in a zone of continuously saturated sediment or rock defined by being below the ‘water table’
The shape of the water table usually mimics the …
surface topography
what is the residence time of underground water?
280 years
why is underground water important?
residence time
sea level
chemical weathering
why did sea levels fall by 5mm in 2010-2011?
increase in water mass on continents
groundwater recharged by increased precipitation
why is groundwater not a renewable resource?
Long residence (& recharge) times
Depleted aquifers cannot usually be replenished within the human life span
Supply & demand management
how do groundwater systems recharge?
by precipitation, infiltration & percolation
where do groundwater systems store water?
in deep aquifers
where do groundwater systems discharge water?
to surface (springs & river baseflow)
water will flow from…
higher pressure (hydraulic head) to lower pressure
What controls groundwater flow?
1. Substrate –porosity & permeability 2. Pressure differences –due to weight of water at higher levels 3. Elevation differences –potential energy (gravity)
How does groundwater flow?
–from areas of high pressure →low pressure
–water moves down where water table is high due to ↑pressure
–then upward beneath areas with low pressure
what is hydraulic gradient?
slope of water table
where does water move when water table elevation is high (high pressure)?
down
How fast does groundwater flow?
Rates: ~ 1 m/day –1 m/yr
what does flow speed depend on?
(1) Slope of water table (hydraulic gradient)
(2) Permeability (hydraulic conductivity
what is the equation for flow speed?
Flow speed (m/s) = hydraulic gradient x hydraulic conductivity (m/s)
what is porosity?
% volume of pore space in rock/sediment/soil (how much water the substrate can hold)
Space between mineral grains, fractures etc
Moisture-holding capacity
E.g., 30-50% for most soils
what is permeability?
(how easily can water flow in the substrate)
Pore shape and “interconnectedness”
- ability of substrate to transmit water
how does groundwater weather rock?
Groundwater dissolves carbonates by dissolution weathering
Groundwater flow transports dissolved ions
Precipitation from groundwater to form stalactites etc
- water loses dissolved CO2 and evaporates, leaving calcite behind
what is an aquifer?
permeable region that transmits groundwater freely (such as sands, gravels, fractured rock)
what is an aquitard?
impermeable layer that hinders or prevents water movement (such as clay)
where does natural groundwater discharge occur?
where water table intersects surface: rivers, lakes, springs..
