| Groundwater is stored in subsurface void spaces below the
water table. The geologic material that stores, transmits, and yields
groundwater to wells and springs is called an aquifer. The
key word here is yields. To be an aquifer, it must store
and transmit water at rates fast enough to supply reasonable amounts to
wells and springs. Therefore, not all groundwater is stored in an
aquifer. Productive aquifers can be comprised of sand and gravel, sandstone, limestone and dolomite, or basalt flows; whereas fractured igneous or metamorphic rocks are only marginal aquifers. |
|
|
|
Three main types of aquifers exist:
|
| An unconfined aquifer has no confining layers
between the zone of saturation and the land surface. If a fully screened well is drilled into an unconfined aquifer, the water level will rise to the water table, which is the top of the zone of saturation. (Most private wells are not fully screened.) Unconfined aquifers exist in most parts of Portage County. |
|
|
|
A confined aquifer is overlain by a confining
layer or aquitard, which is geologic material with little or no permeability/hydraulic
conductivity. This layer does not allow water to pass through
or the rate of movement is extremely slow. Often clays and silts,
or rocks such as shale comprise confining layers. When a well is drilled into a confined aquifer, the water level in this well may rise higher than the top of the aquifer due to pressure. This well is called an artesian well. The level to which the water rises is called the potentiometric surface (similar to the water table in unconfined aquifers). If the potentiometric surface is above the land surface, then a flowing artesian well or spring may occur. Water flows without the use of a pump. |
| A perched aquifer is a saturated zone within the zone of
aeration that overlies a
confining layer. A perched aquifer sits above the main water
table.
A well may be drilled into a perched aquifer, but it usually only yields enough water for a household. |
|
