How and Where Does Groundwater Flow?
GROUNDWATER FLOWS BENEATH THE SURFACE in ways that are controlled by several key principles. The direction and rate of groundwater flow are largely controlled by the porous nature of the materials, the slope of the water table, and the geometry and nature of the subsurface rock. Some rock types allow easy groundwater flow, whereas others prevent significant movement.
What Is the Geometry of the Water Table?
The water table defines the boundary between unsaturated and saturated rock and sediment. It usually is not a horizontal surface but instead has a three-dimensional shape that mimics the shape of the overlying land surface. The shape of the water table commonly has slopes, ridges, hills, and valleys. These features control which way groundwater flows.
- In most environments, the water table has the same general shape as the overlying land surface but is more subdued. Where the land surface is high, the water table is also high. The similarity in shape between topography and the water table is less straightforward in some arid environments and in places where humans have pumped out groundwater faster than it can be replenished by precipitation.
- The water table generally slopes from higher to lower areas. It is generally deeper below the surface under mountains than under lowlands, so its slope is less steep than that of the land surface. The shape of the water table is largely independent of the geometry of rock units through which the water table passes.
- Groundwater just below the water table flows down the slope of the water table. In this example, it flows from left to right, from areas with a higher water table to areas with a lower water table. The red arrows show flow directions of water right below the water table.
- Where the water table is horizontal, for example near this lake, groundwater may flow very slowly or not at all. Deeper water may flow in directions different from near-surface water.
- The terminology used to describe features of a water table is derived from topography. A high part of the water table separating parts sloping in opposite directions is called a groundwater divide. Groundwater flows in opposite directions on either side of a groundwater divide into different drainage basins.
- Where the water table intersects the land surface, there may be lakes, wetlands, or a flowing stream. The stream in this figure occurs where the water table is at the surface. However, streams do not necessarily coincide with the water table, because some flowing streams are underlain by unsaturated materials.
What Controls the Rate of Groundwater Flow?
The rate of groundwater flow is typically measured in meters per day, but can be much slower. Rate is primarily controlled by permeability, a material’s ability to transmit fluid, which can vary greatly from one material to the next. The rate is controlled to a lesser extent by the steepness of the water table because flow is driven by gravity. Other factors being equal, water flows faster down a steep water-table slope and slower down a more gentle one. The slope of the water table is called 08.06.b1 the hydraulic gradient.
The rate of groundwater flow is strongly controlled by the permeability of the rock type. In this diagram, flow is fastest in this highly permeable rock layer. Flow is moderately fast in a somewhat less permeable layer. Flow is slowest in layers that have a very low permeability.
What Is an Aquifer?
An aquifer is a large body of permeable, saturated material through which groundwater can flow well enough to yield significant volumes of water to wells and springs. To be a good aquifer, a material must have high permeability.
- The most common type of aquifer is an unconfined aquifer where the water-bearing unit is open (not restricted by impermeable rocks) to Earth’s surface and atmosphere. Rainwater or surface water can seep unimpeded through the upper layers of rock and sediment into an unconfined aquifer.
- A confined aquifer is separated from Earth’s surface by rocks with low permeability. Here, a permeable aquifer is bounded above and below by layers of low-permeability rock.
- A low-permeability unit, such as the thin gray layer in the middle, can restrict flow. An impermeable unit blocks flow completely. Such units are the opposite of an aquifer and are referred to as an aquiclude.
How Are Wells Related to the Water Table?
A well is a hole dug or drilled deep enough to intersect the water table. If the well is within an aquifer, water will fill the open space to the level of the water table. This free-standing water can be drawn out by buckets or pumps.
This well has been drilled from the land surface downward past the water table. The aquifer is unconfined and has filled with water to the height of the water table.
In dry seasons, or during periods of high groundwater use, some wells run dry. This occurs when the water table drops and the well was not drilled deep enough into the aquifer.
A perched water table sits above the main water table and generally forms where a discontinuous layer or lens of impermeable rock blocks and collects water infiltrating into the ground. Perched water bodies can make it difficult to predict the best site to drill an adjacent well.
Artesian Wells and Water
We often hear the word artesian in the context of bottled water or certain beverages. What does this term imply? Does it mean that the water is better tasting, more natural, or more healthy? The short answer to these three questions is no, or at least not necessarily.
The term artesian means that groundwater is in a confined aquifer and is under enough water pressure that the water rises some amount within a well. The water does not have to reach the surface for the well to be called artesian, but many artesian systems have enough pressure to force the water all the way to the surface, creating a well or spring. Although it is a catchy advertising term, the term artesian is not indicative of how the water tastes, whether it is more natural than other types of groundwater, or whether it is healthy. It only means that the groundwater is confined and under pressure, and as a result rises some amount in the well.
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