Unconfined aquifers are water table aquifers that are at atmospheric pressure. Confined aquifers are overlain by an impermeable layer, such as clay or shale, that prevents the upward movement of water. The overlying confining layer allows aquifers to develop pressures that result in artisan aquifers. Most confined aquifers are actually semi-confined because the confining layer is not completely impermeable. The depth to groundwater in many shallow wells is directly influenced by water levels in nearby streams, and can fluctuate several feet in response to seasonal stream changes.
Seasonal variations can also cause water quality changes.
When drilling a well near streams or rivers during periods of high water levels, seasonal water table changes must be taken into account. The screened interval must be deep and long enough to allow for seasonal fluctuations. The well screen should be placed deep enough in the target water-bearing zone to avoid having the well go dry or avoid insufficient water yields during low water table conditions. For example, shallow wells next to agricultural lands can show drastic water table rises during irrigation periods. Well screen depth should be designed to prevent the impacts from agricultural practices such as the application of fertilizers or pesticides during irrigation periods.