Groundwater level is a term that is used in a relatively loose way, normally referring to the level, either below ground or above ordnance datum, at which soil or rock is saturated. This is also referred to as the water table and represents the top of the saturated zone. Above the water table lies the unsaturated zone.
A more formal term is rest water level (RWL) which implies the water level in a borehole that has not been recently pumped, nor affected by nearby pumping. A pumped water level (PWL) refers to a water level measured while the borehole is pumped. Strictly speaking the water table refers only to unconfined aquifers; aquifers in which water is free to drain vertically from the ground surface to the aquifer. In an unconfined aquifer the groundwater may be in hydraulic continuity with surface water, in which case the surface water can be considered to be an outcrop of groundwater. However levels in an aquifer may be lower than levels in a river, in which case water may be lost from the river to the aquifer, or higher, in which case the river will gain water from the aquifer. Often rivers will gain during summer and lose water in winter.
Confined aquifers occur when a layer of impermeable rock or soil overlies an aquifer that is fully saturated, allowing the water within the aquifer to be pressurised. The groundwater level in a confined aquifer is more properly described as a potentiometric surface, and represents the theoretical level to which water would rise if the confining layer were not present. If there are multiple aquifers present in a vertical sequence there may be several distinct water levels, and where there is a significant vertical flow component present in an aquifer the potentiometric surface may vary with depth. A borehole drilled specifically to measure water level is an observation borehole. The level of water in an observation borehole will be a composite of all aquifers that are penetrated by the borehole. If it is drilled to measure water in a particular horizon and other horizons are cased off, it is called a piezometer. Piezometric surface is often used synonymously for potentiometric surface.
The term artesian is used to describe areas where the piezometric head in a confined aquifer is above the ground surface. In these areas a borehole that penetrates the confined aquifer will flow naturally. Semi-artesian is occasionally used to describe cases where a borehole penetrates a confined aquifer, but the water isn't under enough pressure to overflow at the surface, but artesian may also used in this case.
Perched water tables occur when there are lenses of impermeable material in a rock that is otherwise permeable. These allow small localised water tables to develop over the lenses, which can overlay a regional water table in the surrounding rock mass.
When drilling a borehole water strike may be recorded. This is the level at which water is first encountered. Comparisons of water strike to rest water level can be used to identify perched water table (strike > level) or confined conditions (strike < level).
Contact Andrew McKenzie for further information.