### formation:

Aquifer
* Many aquifers are also potentially at risk.
* Most aquifers contain aqueous concentration
  - arsenic concentration
  - high concentration
* Most aquifers have layers
  - water tables
* Some aquifers are large enough to supply water to thousands of households and business
  - can hold water for centuries without losing water to evaporation or seepage
* Some aquifers result in compaction
  - land subsidence
  - yield much more water than others, as shown on the map below
* act as reservoir s for groundwater
  - like giant filters, made up of sand, gravel and rock
* are a major source of drinking water
  - able to transport water and supply water to wells, rivers, springs and marshes
  - also more protected from pollutants than are surface sources
  - at an all-time low
* are geologic formations that are capable of storing and tranmitting ground water
  - where ground water can be withdrawn in usable quantities
  - geological formations that contain water
  - important groundwater structures
  - layers that allow water to flow
  - natural underground formations holding water that can be pumped for public consumption
  - rock or sediment layers that contain water
  - rocks with high permeability
  - susceptible to contamination around withdrawal wells
* are underground formations containing groundwater
  - reservoirs
  - unrenewable water sources
  - used by people as a freshwater source
  - very sensitive to abuse, however, such as pollution and over-abstraction
  - water reservoirs that are conditioned to hold the gas
  - water-bearing geologic formations into which wells are dug
* can also receive water from surface waters like lakes and rivers.
* can be hundreds of miles long and wide or narrow, shallow veins running through rock
  - only a few feet below the surface or more than a thousand feet deep
  - contain cracked and fractured rocks and other geological variations
  - cover a few miles or spread out over hundreds of thousands of square miles
  - occur at various depths
  - take thousands of years to recharge
* come in all sizes.
  - elevated levels of arsenic in some areas of Michigan
  - saturated permeable material which yields water to wells and springs
* exist at various depths depending on the local geology.
* form in the loose soil lying directly above a layer of clay or less permeable rock.
* generally hold sufficient water to be used as a water supply.
* have high porosity and high permeability
  - porosity and permeability
* located near the surface are more vulnerable to potential contamination.
* occur in two types of geologic formations.
* provide water through springs and wells.
* recharge very slowly.
* represent major fresh water resources throughout the country.
* store fresh water beneath over half of all land surface in Texas.
* supply the water for wells and springs.
* transmit ground water from areas of recharge to areas of discharge.
* typically are of great volume and under tremendous pressure
  - consist of gravel, sand, or fractured rock such as limestone and sandstone
* typically consist of gravel, sand, sandstone, or fractured rock such as limestone
  - or fractured rock, like limestone
* underlie and cross watershed boundaries.
* vary greatly in size.
+ Aquifer, Exploitation: Engineering :: Ecology
  * Aquifers can occur at various depths. Those closer to the surface are not only more likely to be exploited for water supply and irrigation, but are also more likely to be topped up by the local rainfall. Many desert areas have limestone hills or mountains within them or close to them which can be exploited as groundwater resources. If more water is taken out than get in again one speeks of over exploitation. In these cases the aquifer contains brackish water.