11.6: Groundwater

Groundwater is an important source of freshwater. It can be found in all places under the ground but is limited by extractable quantity and quality.

Permeability and Porosity

Most rocks are not entirely solid and contain a certain amount of open space between grains or crystals, known as pores. Porosity is a measure of the open space in rocks –expressed as the percentage of open space that makes up the total volume of the rock or sediment material. Porosity can occur as primary porosity, which represents the original pore spaces in the rock (e.g. space between sand grains, vesicles in volcanic rocks), or secondary porosity which occurs after the rock forms (e.g. fractures, dissolved portions of rock). Lithification of unconsolidated sediments will reduce porosity because it compacts grains and adds cement.

Permeability is a measure of the interconnectedness of pores in a rock or sediment. The connections between pores allows for that material to transmit water. A combination of a place to put water (porosity) and the ability to move water (permeability) makes a good aquifer—a rock unit or sediment that contains extractable groundwater. Well-sorted sediments have higher porosity because there are not smaller sediment particles filling in the spaces between the larger particles. Shales made of clays generally have very high porosity, but the pores are poorly connected, thereby causing low permeability.

While permeability is an important measure of a porous material’s ability to transmit water, hydraulic conductivity is more commonly used by geologists to measure how easily a fluid is transmitted. Hydraulic conductivity measures both the permeability of the porous material and the properties of the water, or whatever fluid is being transmitted like oil or gas. Because hydraulic conductivity also measures the properties of the fluid, such as viscosity, it is used by both petroleum geologists and hydrologists to describe both the production capability of oil reservoirs and of aquifers. High hydraulic conductivity indicates that fluid transmits rapidly through an aquifer.

Aquifers and Confining Layers

Aquifers are rock layers with sufficient porosity and permeability to allow water to be both contained and move within them. For rock or sediment to be considered an aquifer, its pores must be at least partially filled with water and it must be permeable enough to transmit water. Drinking water aquifers must also contain potable, or drinkable water. Aquifers can vary dramatically in scale, from spanning several formations covering large regions to being a local formation in a limited area. Aquifers adequate for water supply are permeable, porous, and potable.

Groundwater Flow

From the Surface into the Ground

When surface water infiltrates or seeps into the ground, it usually enters the unsaturated zone called the vadose zone or zone of aeration. The vadose zone is the volume of geologic material between the land surface and the zone of saturation where the pore spaces are not completely filled with water [34]. Plant roots inhabit the upper vadose zone and fluid pressure in the pores is less than atmospheric pressure. Below the vadose zone is the capillary fringe. The capillary fringe is the usually thin zone below the vadose zone where the pores are completely filled with water (saturation), but the fluid pressure is less than atmospheric pressure. The pores in the capillary fringe are filled because of capillary action, which occurs because of a combination of adhesion and cohesion. Below the capillary fringe is the saturated zone (a.k.a. phreatic zone), where the pores are completely saturated and the fluid in the pores is at or above atmospheric pressure [34]. The interface between the capillary fringe and the saturated zone marks the location of the water table.

Figure \(\PageIndex{1}\): Zone of saturation.

Water Table

Wells are conduits that extend into the ground with openings to the aquifers, to extract from, measure, and sometimes add water to the aquifer. Wells are generally the way that geologists and hydrologists measure the depth to groundwater from the land surface as well as withdraw water from aquifers.

Water is found throughout the pore spaces in sediments and bedrock. The water table is the area below which the pores are fully saturated with water. The simplest case of a water table is when the aquifer is unconfined, meaning it does not have a confining layer above it. A confining layer is a low-permeability layer above and/or below an aquifer that restricts the water from moving in and out of the aquifer. Confining layers include aquicludes, which are so impermeable that no water travels through them, and aquitards, which significantly decrease the speed at which water travels through them. Confining layers can pressurize aquifers by trapping water that is recharged at a higher elevation underneath the confining layer, allowing for a potentiometric surface higher than the top of the aquifer, and sometimes higher than the land surface. The potentiometric surface represents the height that water would rise in a well penetrating the pressurized aquifer system. Breaches in the pressurized aquifer system, like faults or wells, can cause springs or flowing wells, also known as artesian wells.

Figure \(\PageIndex{2}\): An aquifer cross-section. This diagram shows two aquifers with one aquitard (confining layer) between them, surrounded by the bedrock aquiclude, which is in contact with a gaining stream (typical in humid regions).

The water table will generally mirror surface topography, though more subdued, because hydrostatic pressure is equal to atmospheric pressure along the surface of the water table. If the water table intersects the ground surface, the result will be water at the surface in the form of a gaining stream, spring, lake, or wetland. The water table intersects the channel for gaining streams which then gains water from the water table. The channels for losing streams lie below the water table, thus losing streams lose water to the water table. Losing streams may be seasonal during a dry season or ephemeral in dry climates where they may normally be dry and carry water only after rainstorms. Ephemeral streams pose a serious danger of flash flooding in dry climates.

Using wells, geologists measure the water table’s height and the potentiometric surface. Graphs of the depth to groundwater over time, are known as hydrographs and show changes in the water table over time. Well–water level is controlled by many factors and can change very frequently, even every minute, seasonally, and over longer periods of time.

Darcy's Law

In 1856, French engineer Henry Darcy developed a hypothesis to show how discharge through a porous medium is controlled by permeability, pressure, and cross-sectional area. To prove this relationship, Darcy experimented with tubes of packed sediment with water running through them. The results of his experiments empirically established a quantitative measure of hydraulic conductivity and discharge that is known as Darcy's law. The relationships described by Darcy’s law have close similarities to Fourier’s law in the field of heat conduction, Ohm’s law in the field of electrical networks, or Fick’s law in diffusion theory. Darcy’s Law provides a quantitative measure of hydraulic conductivity and discharge.

\[Q=K \cdot A \cdot \dfrac{Δh}{L} \nonumber\]

Figure \(\PageIndex{3}\): Pipe showing apparatus that would demonstrate Darcy’s law. Δh would be measured across L from a to b.

• Q = flow (volume/time)
• K = hydraulic conductivity (length/time)
• A = cross-sectional area of flow (area)
• Δh = change in pressure head (pressure difference)
• L = distance between pressure (h) measurements (length)
• \(\dfrac{Δh}{L}\) is commonly referred to as the hydraulic gradient

Cone of Depression

Pumping water from an aquifer lowers the water table or potentiometric surface around the well. In an unconfined aquifer, the water table is lowered as water is removed from the aquifer near the well. The amount of change from before pumping to pumping level is termed drawdown. Drawdown is the greatest nearest the well, resulting in a concentric pattern of drawdown termed the cone of depression. In a confined aquifer, pumping water reduces the pressure or potentiometric surface around the well.

When one cone of depression intersects another cone of depression or a barrier feature like an impermeable mountain block, drawdown is intensified. When a cone of depression intersects a recharge zone, the cone of depression is lessened.

Figure \(\PageIndex{4}\): Cone of depression.

Recharge

The recharge area is where surface water enters an aquifer through the process of infiltration. Recharge areas are generally topographically high locations of an aquifer. They are characterized by losing streams and sediment or permeable rock that allows infiltration into the aquifer. Recharge areas mark the beginning of groundwater flow paths. In the Basin and Range region, the recharge zones for the unconsolidated aquifers of the valley areas are along the valley margins, near the foothills of the mountains.

Recharge can be induced through the aquifer management practice of aquifer storage and recovery. Injection wells and infiltration galleries (basins) allow for humans to increase the rate of recharge into an aquifer system [35]. Injection wells pump water into an aquifer where it can be stored. Injection wells are regulated by state and federal governments to ensure that the injected water is not negatively impacting the quality or supply of the existing groundwater in the aquifer. Some aquifers are capable of storing significant quantities of water, allowing water managers to use the aquifer system like a surface reservoir. Water is stored in the aquifer during periods of low water demand and high water supply and later extracted during times of high water demand and low water supply.

Figure \(\PageIndex{5}\): Different ways an aquifer can be recharged.

Discharge

Discharge areas are where groundwater emerges at the land surface. Groundwater emerges at the land surface when the potentiometric surface or water table intersects the land surface. These areas are characterized by springs, flowing (artesian) wells, gaining streams, and playas (dry lake bed). Discharge areas mark the end of groundwater flow paths. In the Basin and Range of the western United States, discharge zones are typically in the middle of the valley basins, where playa lakes, springs, and gaining streams signify groundwater emerging at the land surface.

Groundwater Mining and Subsidence

Groundwater as a Limited Resource

Like other natural resources on our planet, the quantity of fresh and potable water is finite. The only natural source of water on land is from the sky in the form of precipitation. Because of a slow rate of travel, limited recharge areas, and intensifying extraction and demand, in many places groundwater is being extracted faster than it is being replenished. When groundwater is extracted faster than it is recharged, groundwater levels (potentiometric surfaces) decline and areas of discharge can diminish or dry up completely. Regional pumping-induced groundwater decline is known as groundwater mining or groundwater overdraft. Groundwater mining can lead to dry wells, reduced spring and streamflow, and subsidence. Groundwater mining is happening in places where more water is extracted by pumping than is being replenished by precipitation, and the water table is continually lowered. In these situations, groundwater must be viewed as a ore body and in its depletion, the possibility of producing ghost towns.

Subsidence

In many places, water actually helps hold up the skeleton of the aquifer by the water pressure exerted on the grains in an aquifer. This pressure is called pore pressure and comes from the weight of overlying water. If pore pressure decreases because of groundwater mining, the aquifer can compact, causing subsidence, or the sinking of the surface of the ground. Areas especially susceptible to this effect are aquifers made of unconsolidated sediments. Unconsolidated sediments with multiple layers of clay and other fine-grained material are at higher risk because clay can compact considerably when drained of water [36, 37].

In many cases, the amount of compaction in one area will be greater than the amount of compaction in an adjacent area. The different amounts of compaction in areas that are next to each other can cause the land to offset and develop cracks and fissures. Subsidence from groundwater mining has been documented in southwestern Utah, notably Cedar Valley, Iron County, Utah. Groundwater levels have declined more than 100 feet in certain parts of Cedar Valley, causing earth fissures and measurable amounts of land subsidence.

Figure \(\PageIndex{6}\): Example of an earth fissure in Cedar City.

The photo shows documentation of subsidence from the pumping of groundwater for irrigation in the Central Valley in California. The dates marked on the pole show land elevation in the past.

Figure \(\PageIndex{7}\): The pole shows subsidence from groundwater pumping for irrigation in the San Joaquin Valley of California.

References

34. Freeze AR, Cherry JA (1979) Groundwater, 1 edition. Prentice Hall

35. Reese RS (2014) Review of Aquifer Storage and Recovery in the Floridan Aquifer System of Southern Florida

36. Galloway D, Jones DR, Ingebritsen SE (1999) Land subsidence in the United States. U.S. Geological Survey

37. Knudsen T, Inkenbrandt P, Lund W, et al (2014) Investigation of land subsidence and earth fissures in Cedar Valley, Iron County, Utah: Utah Geological Survey