Rainfall and snowmelt can runoff into streams or soak into the ground. The process of water soaking into the ground to become groundwater is known as groundwater recharge. The area on the surface where water soaks in is call the recharge area. Groundwater flows underground until it reaches a discharge zone, an area where the water is above the land surface. Springs are clearly visible discharge zones.
Less obvious is the groundwater seeping into wetlands or contributing to stream flows. If the water table is close to the land surface during the growing season, large amounts of groundwater might be withdrawn by plant roots and released into the air by plants.
A well that is drilled into a confined aquifer at a point where the elevation of the ground is lower than that of the water table at the recharge area is an artesian well and is free flowing, that is, it requires no pumping to get the water to the surface. In more complex geological environments, the water table may be more complicated.
If there is a low-permeability clay layer, an aquiclude, in a high-permeability sand formation, the aquiclude may lie below the water table in a shallower aquifer and above the water table in a deeper aquifer. The water table in the shallower aquifer is called a perched water table because it is above the main water table in the lower aquifer.
Although all groundwaters flow through aquifers slowly, some flow more slowly than others. In the middle of the nineteenth century, Henri Darcy, town engineer of Dijon, France, proposed an explination for the difference in flows. Here are his findings:. The dashed orange lines are equipotential, meaning lines of equal pressure. The blue lines are the predicted groundwater flow paths. The dashed lines red lines are no-flow boundaries, meaning that water cannot flow across these lines.
Groundwater flows at right angles to the equipotential lines in the same way that water flowing down a slope would flow at right angles to the contour lines.
The stream in this scenario is the location with the lowest hydraulic potential, so the groundwater that flows to the lower parts of the aquifer has to flow upward to reach this location. It is forced upward by the pressure differences, for example, the difference between the and equipotential lines.
Groundwater that flows through caves, including those in karst areas — where caves have been formed in limestone because of dissolution — behaves differently from groundwater in other situations.
Caves above the water table are air-filled conduits, and the water that flows within these conduits is not under pressure; it responds only to gravity. In other words, it flows downhill along the gradient of the cave floor Figure Many limestone caves also extend below the water table and into the saturated zone.
Skip to content Chapter 14 Groundwater. Exercise Previous: Next: Share This Book Share on Twitter. The length of ground-water-flow paths ranges from a few feet to tens, and sometimes hundreds, of miles.
A deep ground-water-flow system with long flow paths between areas of recharge and discharge may be overlain by, and in hydraulic connection with, several shallow, more local, flow systems Figure 6. Thus, the definition of a ground-water-flow system is to some extent subjective and depends in part on the scale of a study.
Figure 7. The concept of "hydraulic head" or "head" at a point in an aquifer. Consider the elevations above sea level at points A and B in an unconfined aquifer and C in a confined aquifer.
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