Aquitards are often
saturated, but due to low
permeability do not yield
water freely to
wells. However, aquitards may
transmit appreciable water
quantities to and from
adjacent aquifers1, and depending on thickness may constitute a
significant groundwater
storage zone.
The usage of the term
aquitard is also relative. What is considered an aquitard in one area of the world could be considered an
aquifer in other parts. For example, in
The Netherlands a
stratum of
clay is almost always considered an aquitard, because it is an
order of magnitude less
permeable than the (often) adjacent
strata of
sand. In areas with predominantly hard, rocky
subsurface strata, a
layer or
formation of clay would probably be considered an
aquifer, because here it is an
order of
magnitude more permeable than the surrounding
rock.
Sources:
College material - node your homework
http://www.geocities.com/Eureka/Gold/1577/hg_eng.html
1 This is because the flow between strata passes through the total area of contact between the strata, thereby balancing out the low conductivity per surface area.