Groundwater Movement
The water table (or the potentiometric surface of a confined aquifer) is not a flat surface: rather, there are high areas and low areas just like the hills and valleys found on land. Just as surface water tends to flow downhill, groundwater tends to move downgradient from water-table areas (or potentiometric regions) of higher elevation to water-table areas (or potentiometric regions) of lower elevation.
Normally, but not exclusively, the higher water-table areas of uncon-fined aquifers coincide with higher elevation at the land surface, and the lower water-table areas coincide with low areas. As a result, groundwater in unconfined aquifers tends to flow towards, and discharge to, streams, lakes, and wetlands, because these waterbodies often occur in low points of the watershed. Even groundwater from confined aquifers tends to discharge to larger area-wide rivers.
Two other common discharge areas for groundwater are springs and wells. A spring is an area where groundwater has access to the land surface. In some cases, precipitation infiltrating downward from the ground surface encounters a relatively impermeable rock or sediment layer as it moves down toward the underlying aquifer. The groundwater, which cannot pass through the low-permeability layer, moves along the top of the layer until the layer is exposed at the ground surface and the water can emerge as a spring. In this typical "gravity spring," the most common form of spring, gravity is the driving force for water movement. Such springs commonly occur at the side of a hill, or at an outcrop such as a bluff or canyon wall (see the small figure below). In other cases, fractures allow groundwater to move from the aquifer to the surface. Groundwater from a spring can issue onto the land surface, or directly into a stream, lake, or ocean.