The top of an aquifer may be high enough in some places to meet the surface of the ground. This often happens on a slope. The water flows out of the ground and creates a spring. A spring may be just a tiny trickle, or it may be a big gush of water.
Water flowing out of the ground at a spring may flow downhill and enter a stream. If the water from a spring can’t flow downhill, it may spread out to form a pond or lake instead. In the desert, the only reliable water may be from springs (Figure 1). A spring may allow wildlife to inhabit an uninhabitable area.
Sometimes an aquifer is confined. A confined aquifer is trapped between two impermeable rock layers. Pressure from the rock layer on top forces the water out where the aquifer reaches the ground surface. Water that flows up to the surface naturally is an artesian spring. If people drill a well into a confined aquifer, the water may flow to the surface without assistance. This is an artesian well (Figure 2).
Mineral Springs and Hot Springs
Some springs have water that contains minerals. Groundwater dissolves minerals out of the rock as it seeps through the pores. The water in some springs is hot because it is heated by hot magma. Many hot springs are also mineral springs. That’s because hot water can dissolve more minerals than cold water.
Springs in Yellowstone National Park are hot and contain dissolved minerals. Morning Glory Pool (Figure 3) has a bright green color from dissolved minerals. Along the edge are thick orange mats of bacteria. The bacteria use the minerals in the hot water to make food.