Groundwater Resources and the Water Table
Sources of groundwater
-Aquifers -Aquitards
Aquifers
-bodies of rock, sediment, or soil that are sufficiently porous and permeable to hold water -typically coarse-grained clastic sediments (gravels and sands) a) porous carbonate rocks are a distant second -unconfined and confined aquifers
sources of outflow (discharge)
-gaining streams: high water tables can lead to feeding of streams by aquifers a) essentially, the stream channel cuts into the water table, so water flows from the aquifer into the stream -springs and wetlands: where the water table intersects the ground, water-saturated environments occur -human withdrawal of groundwater from wells
sources of inflow (recharge)
-infiltration: precipitation migrates downwards through overlying unsaturated material directly -losing streams: in arid regions, water in streams is diverted through banks and beds into aquifers
balanced groundwater budget
-inflow equals outflow -depth of water doesn't change
what is a groundwater budget?
-inflow or recharge: water is added to a groundwater system -outflow or discharge: water is removed from a groundwater system
Aquitards
-low-permeability materials that serve as barriers to groundwater flow -typically fine-grained sedimentary rocks (mudrocks) and unfractured igneous or metamorphic rocks
human impacts on groundwater
-subsidence: as groundwater is withdrawn excessively, sediment will become compacted and the ground surface can sink -cones of depression: when groundwater removal exceeds recharge at a well, the water table is lowered into a local funnel-shaped depression AND development of cones of depression can lead to wells drying up as water table is lowered
the name "water table" is a bit misleading...
-this is a *very dynamic* surface -it changes in response to water sources, underlying geology, surface topography, rate of water withdrawal, and many other factors
imbalanced groundwater budget
1. Inflow exceeds outflow a) water table rises (gets shallower) 2. Outflow exceeds inflow a) water table depresses (gets deeper)
confined aquifers
a) enclosed above and below by materials of low permeability b) must be refilled with water at *recharge zones* where the aquifer approaches the surface
unconfined (open) aquifers
a) water percolates downward through overlying permeable materials to refill groundwater
the water table can be locally elevated (brought closer to the surface) or depressed (brought deeper underground) in areas where water is removed faster than it is replaced, and vice-versa
hence, position of the water table is linked to a *groundwater budget*
the water table is the surface that...
must be penetrated in order to withdraw water from a well
the water table separates what
the *saturated zone* where pore spaces are filled with water, from the *unsaturated zone* above where pore spaces are filled with air
contact between an aquifer and overlying material that does not contain groundwater is the ________ _________
water table
