8.3: Groundwater

Groundwater is the water stored in the open spaces within unconsolidated sediment and the underlying bedrock. This is freshwater, though it can be affected by the sediment and rock it is stored in. Sediment and rocks near the surface are under less pressure than those at depth and therefore tend to have more open space. For this reason, and because it’s expensive to drill deep wells, most of the groundwater that is accessed by individual users is from within the first 100 meters of the surface. Some municipal, agricultural, and industrial groundwater users get their water from greater depth, but deeper groundwater tends to be of lower quality than shallow groundwater, so there is a practical limit as to how deep we can go.

Porosity is the percentage of open space within an unconsolidated sediment or a rock. It is the spaces between grains in a sediment or sedimentary rock and spaces that developed after or during rock formation due to fracture or with the formation of gas bubbles in lava rock. Porosity is expressed as a percentage calculated as the volume of open space in a rock compared with the total volume of rock. Typical porosities for several different geological materials are shown in Figure \(\PageIndex{1}\). Unconsolidated sediments tend to have higher porosity than consolidated ones because they have no cement, and most have not been strongly compressed. Finer-grained materials (e.g., silt and clay) tend to have greater porosity—some as high as 70%—than coarser materials like gravel.

Figure \(\PageIndex{1}\): Variations in Porosity of Unconsolidated Materials (in red) and Rocks (in blue)

Porosity relates to how much water can be stored in geological materials. Almost all rocks have some porosity and therefore contain groundwater. Groundwater is present under your feet and everywhere on the planet. Considering that sedimentary rocks and unconsolidated sediments cover about 75% of the continental crust with an average thickness of a few hundred meters, and that they are likely to have around 20% porosity on average, it is easy to see that a huge volume of water is stored in the ground.

While porosity is a measure of open space, permeability is related to the sizes of those spaces and how they are shaped and interconnected, and so determines how easy it is for water to flow through the material. Larger pores mean there is less friction between flowing water and the sides of the pores. Smaller pores mean more friction along pore walls, and more twists and turns for the water to have to flow-through. Permeability controls how quickly water can flow through the rock or unconsolidated sediment and how easy it will be to extract the water for our purposes. Permeability is the most important variable in controlling groundwater flow rate and the quality of an aquifer as a source of water. Permeability can be expressed in a range of different units; in this book we will use meters per second, which is also termed the hydraulic conductivity (K, m/s).

As shown on Figure \(\PageIndex{2}\): there is a wide range of permeabilities in geological materials from hydraulic conductivities of 10⁻¹² meters per second (0.000000000001 m/s) to approaching 1 m/s. Unconsolidated materials are generally more permeable than the corresponding rocks (compare sand with sandstone, for example), and the coarser materials are generally more permeable than the finer ones. The least permeable rocks are unfractured intrusive igneous and metamorphic rocks, followed by unfractured mudstone, sandstone, and limestone. The permeability of sandstone can vary widely depending on the rock’s degree of sorting and cementation. Volcanic rocks can be highly permeable—especially if they are fractured—as can limestone that has been dissolved along fractures and bedding planes to create solutional openings.

Figure \(\PageIndex{2}\): Variations in Permeability of Unconsolidated Materials (in red) and Rocks (in blue)

Surface water and groundwater are both active parts of the hydrologic cycle that moves water from where it has been deposited on the land surface as rain or snow, back to the ocean. The water within a watershed is all linked.

There is ongoing exchange of water between surface water (streams, lakes etc.) and the groundwater. Figure \(\PageIndex{3}\) shows how groundwater is stored below the surface. As water infiltrates into the ground, it fills the pore spaces. The line marked with "Water table" separates the area called the saturated zone where the pore spaces are filled with water (below the line) and the area called the unsaturated zone where the pore spaces are not filled with water (above the line). The water table goes up as more water infiltrates into this area, saturating more of the ground, and moves down when water is lost in various ways addressed later in this section.

Notice in Figure \(\PageIndex{3}\) (top), the level of the stream is higher than the water table off to the sides, so water is moving into the surrounding materials from the stream, and by water percolating downward from surface.

At a different time of year, when the water table is higher, water may flow from the surrounding materials into the stream, supplementing the stream flow (Figure \(\PageIndex{3}\), bottom).

Figure \(\PageIndex{3}\): Examples of the Transfer of Water From Surface Reservoirs to Groundwater (top) and from Groundwater to Surface Water (bottom)

The porosities and permeabilities of different geological materials help us to define their water storage and water supply capabilities so that we can identify which materials might be useful as a groundwater source, and which might not. An aquifer is a body of sediment or rock that has sufficiently high permeability that we can easily extract water for our use using a well. An aquitard (or confining layer) is a body that has too little permeability to be useful. This concept is illustrated on Figure \(\PageIndex{4}\) which shows an aquifer at the top (labeled “Unconfined aquifer”). The level of the water in well A is at the same height as the water table at that location.

The unconfined aquifer is underlain by a layer of relatively impermeable material, here labeled a “Confining layer” which could also be called an aquitard. This is underlain by another permeable layer which is labeled as a “Confined aquifer”. Water can enter the confined aquifer only in the area where it is exposed (upper left) and because that is at a relatively high elevation the groundwater at depth in this aquifer is under pressure. The red-dashed line defines the pressure at any point in this aquifer, and that is equivalent to the height to which water would rise in a well at that location. This is called the potentiometric surface. A well drilled at B is known as artesian because the water rises above the upper surface of the confined aquifer. The well at C is also artesian, but because the potentiometric surface is above the ground surface at that location this is a flowing artesian well. There could be more than one confined aquifer in a region. In that case, each would have its own potentiometric surface.

Figure \(\PageIndex{4}\): Aquifers and Confining Layers (Aquitards), the Water Table and a Potentiometric Surface, a Water Table Well (A), and Two Artesian Confined Aquifer Wells (B and C)

It is important to note that the distinction between an aquifer and an aquitard is subjective. As shown on Figure \(\PageIndex{5}\), a groundwater user with only a modest need for water may consider a body with low permeability to be an aquifer, while someone with a need for a lot of water might consider that same body to be an aquitard.

Figure \(\PageIndex{5}\): The upper layer is an aquifer to the water user on the left, who has only minimal water needs, but the upper layer is an aquitard (or confining layer) for the water user on the right, who has significant water needs.

Groundwater flows from areas of high hydraulic head (high water table or potentiometric surface) to areas of lower hydraulic head. This is shown on Figure \(\PageIndex{6}\), which also illustrates the concept of a groundwater divide (red-dashed lines), where the slope of the water table changes directions. Precipitation on the ground surface leads to recharge of the groundwater system. Water doesn’t flow across a groundwater divide, which is why it is also termed a no-flow boundary. Groundwater flow lines always cross hydraulic head contour lines (also known as equipotential lines) at right angles.

Figure \(\PageIndex{6}\): Possible Groundwater Flow Paths (Blue Lines) in an Unconfined Aquifer. The dashed grey lines are hydraulic head contours (equipotential lines), and the numbers 70 to 90 represent the hydraulic heads (in meters) along those lines and at the locations shown.

It is critical to understand that—except in limestone karst (cave) regions—groundwater does not flow in underground streams, nor does it form underground lakes. In almost all cases, groundwater flows very slowly through the pores in granular sediments, or through the fractures in solid rock. Flow velocities of several centimeters per day are possible in permeable sediments with significant hydraulic gradients. But in many cases, permeabilities are lower than the ones used in the examples above, and gradients are much lower. It is not uncommon for groundwater to flow at velocities of a few centimeters per year, or even just a few millimeters per year.

Problems With Wells

Cone of Depression

When a well is pumped at a rate that is faster than rate at which water can flow into it (as governed by the permeability of the aquifer) the water level in the well will drop and a cone of depression will develop around the well. This is illustrated for well A in Figure \(\PageIndex{7}\). (Wells B and C are not being pumped currently.)

Figure \(\PageIndex{7}\)

Subsidence

is the lowering of the ground surface due to the removal of groundwater. When water is removed, the weight of the overlying geologic materials causes the materials to compress, reducing the amount of pore space. This space is permanently lost resulting in a lowered surface. One excellent example occurred in the San Joaquin Valley in California and is depicted in Figure \(\PageIndex{8}\). The ground lowered 9 meters (30 feet) in 52 years.

Subsidence

Figure \(\PageIndex{8}\): Land surface subsided ~9 m from 1925 to 1977 due to compaction related to groundwater removal. Signs on the telephone pole indicate the former elevations of the land surface in 1925 and 1955.

Saltwater Intrusion

Many important aquifers are in coastal areas, but aquifers close to a coast are at risk from intrusion by salt water which is called saltwater intrusion. As illustrated on Figure \(\PageIndex{9}\), groundwater beneath the ocean is salty, while that beneath the land is fresh. Because fresh water is less dense than salt water the fresh groundwater in near-shore areas exists in a lens that is approximately 40 times as deep as the height of the water table above the ocean surface. A well drilled within this lens (A) will produce fresh water, but one that penetrates the salty groundwater (B) will produce salt water. In the scenario shown, well A is at risk of producing salt water if it is overused, as that will draw the water table down, and bring the fresh-salt interface up.

Figure \(\PageIndex{9}\): Depiction of the fresh water–salt water interface in near-shore areas

Figure \(\PageIndex{10}\): The areal extent of salt water intrusion in the Biscayne Aquifer, Florida

A significant salt water intrusion problem exists in the eastern part of Florida around Miami (Figure \(\PageIndex{10}\)). The area is prone to intrusion for several reasons. First, it has very subdued topography; much of southern Florida is only a few meters above sea level. Second, the main near-surface aquifer in the region—the Biscayne Aquifer—includes limestone units with significant porosity and permeability related to dissolution that took place when the unit was above surface. In addition to those natural factors, there are also several anthropogenic factors that contribute to salt intrusion. The region is densely populated and most of the public water supply is extracted from wells in the Biscayne Aquifer. Starting in the 19th century large areas of the nearby Everglade wetlands were drained to create land for agriculture and urban expansion. The consequent drop in the water level allowed sea water to flow inland. Furthermore, draining the wetlands was largely accomplished by building canals, and those canals have allowed sea water to extend well inland and then seep into the underlying aquifer.

The extent of salt water intrusion along Biscayne Bay is illustrated on Figure \(\PageIndex{10}\). As of 2011 about 1200 km² were affected. Salt extended further inland in some areas in 1955—particularly along canals—but steps have been taken in recent decades to restrict the inland flow of salt water in the canals and so some of those areas have recovered.

Groundwater and Drainage Basins

Groundwater is the basis for water movement in a drainage basin. It is literally the base. Any water that cannot flow out of the drainage basin while still under the ground will end up flowing into low spots on the landscape, contributing to the flow in streams and lakes. Those flows are supplemented by surface runoff.

What we need to recognize is that pumping any amount of groundwater takes away from surface water somewhere in the drainage basin (unless we later rehabilitate that water and return it to the environment). It’s OK to use groundwater, but we need to recognize that groundwater and surface water are really all the same thing, and so we need to be careful not to use so much that we negatively affect other users, including ecosystems.

Media Attributions

• Figure \(\PageIndex{1}\): Steven Earle, CC BY 4.0
• Figure \(\PageIndex{2}\): Steven Earle, CC BY 4.0
• Figure \(\PageIndex{3}\): Steven Earle, CC BY 4.0
• Figure \(\PageIndex{4}\): Steven Earle, CC BY 4.0
• Figure \(\PageIndex{5}\): Steven Earle, CC BY 4.0
• Figure \(\PageIndex{6}\): Steven Earle, CC BY 4.0
• Figure \(\PageIndex{7}\): Steven Earle, CC BY 4.0
• Figure \(\PageIndex{8}\): USGS, Public Domain
• Figure \(\PageIndex{9}\): Steven Earle, CC BY 4.0
• Figure \(\PageIndex{10}\): Steven Earle, CC BY 4.0

Reference

Nile Creek-Qualicum Bay restoration and enhancement: Reconnecting coastal cutthroat trout and salmon to their habitat. (2011).Trout Unlimited Canada. https://tucanada.org/project/nile-cr...d-enhancement/