<P> A confined aquifer is an aquifer that is overlain by a relatively impermeable layer of rock or substrate such as an aquiclude or aquitard . If a confined aquifer follows a downward grade from its recharge zone, groundwater can become pressurized as it flows . This can create artesian wells that flow freely without the need of a pump and rise to a higher elevation than the static water table at the above, unconfined, aquifer . </P> <P> The characteristics of aquifers vary with the geology and structure of the substrate and topography in which they occur . In general, the more productive aquifers occur in sedimentary geologic formations . By comparison, weathered and fractured crystalline rocks yield smaller quantities of groundwater in many environments . Unconsolidated to poorly cemented alluvial materials that have accumulated as valley - filling sediments in major river valleys and geologically subsiding structural basins are included among the most productive sources of groundwater . </P> <P> The high specific heat capacity of water and the insulating effect of soil and rock can mitigate the effects of climate and maintain groundwater at a relatively steady temperature . In some places where groundwater temperatures are maintained by this effect at about 10 ° C (50 ° F), groundwater can be used for controlling the temperature inside structures at the surface . For example, during hot weather relatively cool groundwater can be pumped through radiators in a home and then returned to the ground in another well . During cold seasons, because it is relatively warm, the water can be used in the same way as a source of heat for heat pumps that is much more efficient than using air . </P> <P> The volume of groundwater in an aquifer can be estimated by measuring water levels in local wells and by examining geologic records from well - drilling to determine the extent, depth and thickness of water - bearing sediments and rocks . Before an investment is made in production wells, test wells may be drilled to measure the depths at which water is encountered and collect samples of soils, rock and water for laboratory analyses . Pumping tests can be performed in test wells to determine flow characteristics of the aquifer . </P>

Water that falls into a terrestrial habitat becomes groundwater