Fluvial Processes and Systems

6.5 Groundwater

Groundwater is stored in the open spaces within rocks and within unconsolidated sediments. Rocks and sediments near the surface are under less pressure than those at significant depth and therefore tend to have more open space. For this reason, and because it is expensive to drill deep wells, most of the groundwater that is accessed by individual users is within the first 100 m 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 limit to how deep we can go.

Porosity is the percentage of open space within unconsolidated sediment or rock. Primary porosity is represented by the spaces between grains in a sediment or sedimentary rock. Secondary porosity is porosity that has developed after the rock has formed. It can include fracture porosity, space within fractures in any rock. Some volcanic rock has a particular type of porosity related to vesicles, and some limestone has increased porosity related to cavities within fossils.

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Groundwater Flow” by the United States Geologic Survey is licensed under Public Domain.

Porosity is expressed as a percentage calculated from the volume of open space in a rock compared with the total volume of rock. 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 percent, than coarser materials (e.g., gravel). Primary porosity tends to be higher in well-sorted sediments compared to poorly sorted sediments, where there is a range of smaller particles to fill the spaces made by the larger particles. Glacial till, which has a wide range of grain sizes and is typically formed under compression beneath glacial ice, has relatively low porosity.

Terminus of the Fox Glacier, New Zealand” by Paul Morris is licensed under the Creative Commons Attribution-ShareAlike 2.0 Generic license.

Consolidation and cementation during the process of lithification of unconsolidated sediments into sedimentary rocks reduce primary porosity. Sedimentary rocks generally have porosities in the range of 10 percent to 30 percent, some of which may be secondary (fracture) porosity. The grain size, sorting, compaction, and degree of cementation of the rocks all primary influence porosity. For example, poorly sorted and well-cemented sandstone and well-compressed mudstone can have incredibly low porosity. Igneous or metamorphic rocks have the lowest primary porosity because they commonly form at depth and have interlocking crystals. Most of their porosity comes in the form of secondary porosity in fractures. Of the consolidated rocks, well-fractured volcanic rocks and limestone with cavernous openings produced by dissolution have the highest potential porosity, while intrusive igneous and metamorphic rocks, which formed under enormous pressure, have the lowest. Porosity is a measure of how much water can be stored in geological materials. Most rocks have some porosity and therefore contain groundwater. Groundwater is found under the ground and everywhere on the planet. Considering that sedimentary rocks and unconsolidated sediments cover about 75 percent of the continental crust with an average thickness of a few hundred meters and that they are likely to have around 20 percent porosity on average, it is easy to see that a considerable volume of water can be stored in the ground.

Porosity is a description of how much space there could be to hold water under the ground, and permeability describes how those pores are shaped and interconnected. This determines how easy it is for water to flow from one pore to the next. Larger pores mean there is less friction between flowing water and the sides of the pores. Smaller pores mean more friction along pore walls, but also more twists and turns for the water to have to flow-through. A permeable material has a sizable number of well-connected pores spaces, while an impermeable material has fewer, smaller pores that are poorly connected. Permeability is the most crucial variable in groundwater. Permeability describes how easily water can flow through the rock or unconsolidated sediment and how easy it will be to extract it for our purposes. The characteristic of permeability of a geological material is quantified by geoscientists and engineers using some different units, but the most common is the hydraulic conductivity. The symbol used for hydraulic conductivity is K. Although hydraulic conductivity can be expressed in a range of different units, in this book, we will always use m/s. (Earle, 2015)

Unconsolidated materials are generally more permeable than the corresponding rocks (compare sand with sandstone, for example), and the coarser materials are much 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 degree of sorting and the amount of cement present. Fractured igneous and metamorphic rocks, especially fractured volcanic rocks, can be highly permeable, as can limestone dissolve along fractures and bedding planes to create solutional openings. Both sand and clay deposits (and sandstone and mudstone) are quite porous (30 percent to 50 percent for sand and 40 percent to 70 percent for silt and clay), but while sand can be quite permeable, clay and mudstone are not.

We have now seen a wide range of porosity in geological materials and an even more extensive range of permeability. Groundwater exists everywhere there is porosity. However, whether groundwater can flow in significant quantities depends on the permeability. An aquifer is a body of rock or unconsolidated sediment with sufficient permeability to allow water to flow through it. Unconsolidated materials like gravel, sand, and even silt make relatively good aquifers, as do rocks like sandstone. Other rocks can be suitable aquifers if they are well fractured. An aquitard is a body that does not transmit a significant amount of water, such as clay, a till, or a poorly fractured igneous or metamorphic rock. These are relative terms, not absolute, and are usually defined based on someone’s desire to pump groundwater; an aquifer to someone who does not need much water may be an aquitard to someone else who does. An aquifer that is exposed at the ground surface is called an unconfined aquifer. An aquifer where there is a lower permeability material between the aquifer and the ground surface is known as a confined aquifer, and the aquitard separating the ground surface, and the aquifer is known as the confining layer.

If a person were to go out into their garden or a forest or a park and start digging, they would find that the soil is moist (unless you are in a desert), but it is not saturated with water. This means that some of the pore space in the soil is occupied by water, and some of the pore space is occupied by air (unless you are in a swamp). This is known as the unsaturated zone. If a person were to dig down far enough, they would get to the point where all of the pore spaces are 100 percent filled with water (saturated), and the bottom of your hole would fill up with water. The level of water in the hole represents the water table, which is the surface of the saturated zone.

Water falling on the ground surface as precipitation (rain, snow, hail, fog, etc.) may flow off a hill slope directly to a stream in the form of runoff, or it may infiltrate the ground, where it is stored in the unsaturated zone. The water in the unsaturated zone may be used by plants (transpiration), evaporate from the soil (evaporation), or continue past the root zone and flow down to the water table, where it recharges the groundwater.

In areas with topographic relief, the water table generally follows the land surface, but tends to come closer to surface in valleys, and intersects the surface where there are streams or lakes. The water table can be determined from the depth of water in a well that is not being pumped. Although, that only applies if the well is within an unconfined aquifer. In this case, most of the hillside forms the recharge area, where water from precipitation flows downward through the unsaturated zone to reach the water table. The area at the stream or lake to which the groundwater is flowing is a discharge area.

What makes water flow from the recharge areas to the discharge areas? Recall that water is flowing in pores where there is friction, which means it takes work to move the water. There is also some friction between water molecules themselves, which is determined by the viscosity-ty. Water has a low viscosity, but friction is still a factor. All flowing fluids are always losing energy to friction with their surroundings. Water will flow from areas with high energy to those with low energy. Recharge areas are at higher elevations, where the water has high gravitational energy. It was energy from the sun that evaporated the water into the atmosphere and lifted it to the recharge area. The water loses this gravitational energy as it flows from the recharge area to the discharge area.

The situation gets a lot more complicated in the case of confined aquifers, but they are es-essential sources of water, so we need to understand how they work. There is always a water table, which applies even if the geological materials at the surface have incredibly low permeability. Where there is a confined aquifer, meaning one that is separated from the surface by a confining layer, this aquifer will have its own “water table,” which is called a potentiometric surface, as it is a measure of the total potential energy of the water. However, if we drill a well through both the unconfined aquifer and the confining layer and into the confined aquifer, the water will rise above the top of the confined aquifer to its potentiometric surface. This is known as an artesian well because the water rises above the top of the aquifer. In some situations, the potentiometric surface may be above the ground level. The water in a well drilled into the confined aquifer in this situation would rise above ground level, and flow out if it is not capped. This is known as a flowing artesian well.

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Artesian Well” by Andrew Dunn is licensed under the Creative Commons Attribution-ShareAlike 2.0 Generic license.

It is critical to understand that groundwater does not flow in underground streams nor form underground lakes. Except karst areas, with caves in limestone, groundwater flows very slowly through granular sediments or solid rock with fractures in it. Flow velocities of several centimeters per day are possible in significantly permeable sediments with significant hydraulic gradients. However, in many cases, permeabilities are lower than the ones we have used as examples here, and in many areas, gradients are much lower. It is common for groundwater to flow at velocities of a few millimeters to a few centimeters per year.

As already noted, groundwater does not flow in straight lines. It flows from areas of higher hydraulic head to areas of lower hydraulic head, and this means that it can flow “uphill” in many situations. 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 112 and 110 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. Many limestone caves also extend below the water table and into the saturated zone. Here water behaves in a similar way to any other groundwater, and it flows according to the hydraulic gradient.

The Water Table

For a groundwater aquifer to hold the same amount of water, the amount of recharge must equal the amount of discharge. In wet regions, streams are fed by groundwater; the stream’s surface is the top of the water table. In dry regions, water seeps down from the stream into the aquifer, often dry much of the year. Water leaves a groundwater reservoir in streams or springs, where people take water from aquifers.

Although groundwater levels do not rise and fall as rapidly as at the surface, over time, the water table will rise during wet periods and fall during droughts. One of the most interesting but extremely atypical types of aquifers are found in Florida. Although aquifers are very rarely underground rivers, in Florida, water has dissolved the limestone so that streams travel underground and aboveground. (Components of Groundwater | Geology, n.d.)

Groundwater Use

Groundwater is a significant water source for people. Groundwater can be a renewable resource, as long as when the water pumped from the aquifer is replenished. It is essential for anyone who intends to dig a well to know how deep the water table is beneath the surface. Because groundwater involves interaction between the Earth and the water, the study of groundwater is called hydrogeology. Some aquifers are overused; people pump out more water than is replaced. As the water is pumped out, the water table slowly falls, requiring wells to be dug deeper, which takes more money and energy. Wells may go completely dry if they are not deep enough to reach into the lowered water table.

The Ogallala Aquifer supplies about one-third of the irrigation water in the United States. The aquifer is found from 30 to 100 meters deep over about 440,000 square kilometers! The water in the aquifer is mostly from the last ice age. People widely use the Ogallala Aquifer for municipal and agricultural needs. About eight times more water is taken from the Ogallala Aquifer each year than is replenished. Much of the water is used for irrigation of crops in the Breadbasket of the central plains. Currently, there is great concern about the long-term health of this vast aquifer because it is being tapped into and used at a higher rate than being replenished by natural processes. This could have huge implications regarding food production in the country if this critical water source is depleted. At current rates of use, 70 percent of the aquifer could be gone by 2050.

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Ogallala Aquifer and Keystone Pipeline” is licensed under the Creative Commons Attribution-ShareAlike 3.0 Unported license.

Overuse and lowering of the water tables of aquifers could have other impacts as well. Lowering the water table may cause the ground surface to sink. Subsidence may occur beneath houses and other structures. When coastal aquifers are overused, salt water from the ocean may enter the aquifer, contaminating the aquifer and making it less useful for drinking and irrigation. Saltwater incursion is a problem in developed coastal regions, such as in Hawaii.

Springs and Wells

Groundwater meets the surface in a stream or a spring. A spring may be constant, or may only flow at certain times of the year. Towns in many locations depend on water from springs. Springs can also be a vital source of water in locations where surface water is scarce.

A well is created by digging or drilling to reach groundwater. When the water table is close to the surface, wells are a convenient method for extracting water. When the water table is far below the surface, specialized equipment must be used to dig a well. Most wells use motorized pumps to bring water to the surface, but some still require people to use a bucket to draw water up.

Finally, it is also essential to understand how water is cleaned, filtered, and delivered to our homes and work. Many of us do not know where our water comes from, and we take it for granted. This often leads to the wasteful water use of our lawns, showers, and other appliances.

Groundwater Quality

One good thing about groundwater as a source of water is that it is not as easily contaminated as surface water. However, there are two caveats to that: one is that groundwater can become naturally contaminated because of its very close connection to the materials of its aquifer, and the second is that once contaminated by human activities, groundwater is very difficult to clean up. (Earle, 2019)

Natural Contamination of Groundwater

Groundwater moves slowly through an aquifer, and unlike the surface water of a stream, it has many contacts with the surrounding rock or sediment. In most aquifers, the geological materials that make up the aquifer are relatively inert or are made up of minerals that dissolve very slowly into the groundwater. Over time, however, all groundwater gradually has increased material dissolved within it as it remains in contact with the aquifer. In some areas, that rock or sediment includes some minerals that could contaminate the water with elements that might make the water less than ideal for human consumption or agricultural use. Examples include copper, arsenic, mercury, fluorine, sodium, and boron. In some cases, contamination may occur because the aquifer material has unusually high levels of the element in question. In other cases, the aquifer material is just natural rock or sediment, but some particular feature of the water or the aquifer allows contaminants to build up to significant levels.

Rural residents in the densely populated country of Bangladesh (over 1,000 residents/km2, compared with 3.4/km2 in Canada) used to rely mostly on surface supplies for their drinking water, and many of these were subject to bacterial contamination. Infant mortality rates were among the highest in the world, and other illnesses such as diarrhea, dysentery, typhoid, cholera, and hepatitis were common. In the 1970s, international agencies, including UNICEF, started a program of drilling wells to access abundant groundwater supplies at depths of 20 m to 100 m. Eventually, over 8 million such wells were drilled. Infant mortality and illness rates dropped dramatically, but it was later discovered that the water from a high proportion of these wells has arsenic above safe levels.

Most of the wells in the affected areas are drilled into recent sediments of the vast delta of the Ganges and Brahmaputra Rivers. While these sediments are not particularly enriched in arsenic, they have enough organic matter to use up any oxygen present. This leads to water with a naturally low oxidation potential (anoxic conditions); arsenic is highly soluble under these conditions, and so any arsenic present in the sediments easily gets dissolved into the groundwater. Arsenic poisoning leads to headaches, confusion, and diarrhea, and eventually to vomiting, stomach pain, and convulsions. If not treated, the outcomes are heart disease, stroke, cancer, diabetes, coma, and death. There are ways to treat arsenic-rich groundwater, but it is a challenge in Bangladesh to implement the simple and effective technology that is available. (Earle, 2019)

Anthropogenic Contamination of Groundwater

Groundwater can become contaminated by pollution at the surface (or at depth), and there are many different anthropogenic (human-caused) sources of contamination. The vulnerability of aquifers to pollution depends on several factors, including the depth to the water table, the permeability of the material between the surface and the aquifer, the aquifer’s permeability, the slope of the surface, and the amount of precipitation. Confined aquifers tend to be much less vulnerable than unconfined ones, and deeper aquifers are less vulnerable than shallow ones. Steeper slopes mean that surface water tends to run off rather than infiltrate (and this can reduce the possibility of contamination). Contamination risk is also less in dry areas than in areas with heavy rainfall. (Earle, 2019)

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Crop Circles, Kansas ” by NASA is licensed under Public Domain.

The principal sources of anthropogenic groundwater contamination include the following:

  • Chemicals and animal waste related to agriculture, and chemicals applied to golf courses and domestic gardens
  • Agriculture
  • Landfills
  • Industrial operations
  • Mines, quarries, and other rock excavations
  • Leaking fuel storage tanks (especially those at gas stations)
  • Septic systems
  • Runoff from roads (e.g., winter salting) or chemical spills of materials being transported

Landfills

In the past, domestic and commercial refuse was commonly trucked to a “dump” (typically a hole in the ground), and when the hole was filled, it was covered with soil and forgotten. In situations like this, rain and melting snow can easily pass through the soil used to cover the refuse. This water passes into the waste itself, and the resulting landfill leachate that flows from the bottom of the landfill can seriously contaminate the surrounding groundwater and surface water. In the past few decades, regulations around refuse disposal have been significantly strengthened, and significant steps have been taken to reduce the amount of landfill waste by diverting recyclable and compostable materials to other locations.

A modern engineered landfill has an impermeable liner (typically heavy plastic, although engineered clay liners or natural clay may be adequate in some cases), a plumbing system for draining leachate (the rainwater that flows through the refuse and becomes contaminated), and a network of monitoring wells both within and around the landfill. Once a part or all of a landfill site is full, it is sealed over with a plastic cover, and a system is put in place to extract landfill gas (typically a mixture of carbon dioxide and methane). That gas can be sent to a nearby location where it is burned to create heat or used to generate electricity. The leachate must be treated, and that can be done in a typical sewage treatment plant.

The monitoring wells are used to assess the water table level around the landfill and collect groundwater samples so that any leakage can be detected. Because some leakage is almost inevitable, the ideal placement for landfills is in areas where the depth to the water table is significant (tens of meters if possible) and where the aquifer material is relatively impermeable. Landfills should also be situated far from streams, lakes, or wetlands to avoid contamination of aquatic habitats.

Today, hundreds of abandoned dumps are scattered across the country; most have been left to contaminate groundwater that we might wish to use sometime in the future. In many cases, it is unlikely that we will be able to do so. (Earle, 2019)

Mines, Quarries, and Rock Excavations

Mines and other operations that involve the excavation of substantial amounts of rock (e.g., highway construction) have the potential to create severe environmental damage. The exposure of rock that has previously not been exposed to air and water can lead to the oxidation of sulfide-bearing minerals, such as pyrite, within the rock. The combination of pyrite, water, oxygen, and a particular type of bacteria (Acidithiobacillus ferrooxidans) that thrives in acidic conditions leads to the generation of acidity, in some cases, to pH less than 2. Water that acidic is hazardous by itself, but the low pH also increases the solubility of certain heavy metals. The water that is generated by this process is known as acid rock drainage (ARD). ARD can occur naturally where sulfide-bearing rocks are near the surface. The issue of ARD is a primary environmental concern at both operating mines and abandoned mines. Groundwater adjacent to the contaminated streams in the area is very likely contaminated as well.

Leaking Fuel Tanks

Underground storage tanks (USTs) are used to store fuel at gas stations, industrial sites, airports, and anywhere that large volumes of fuel are used. They do not last forever, and eventually, they start to leak their contents into the ground. This is a problem at older gas stations, although it may also become a future problem at newer gas stations. Sometimes a gas station can be seen that is closed and surrounded by a chain-link fence. In virtually all such cases, the discovery of leaking USTs and the requirement has triggered the closure to cease operations and remediate the site.

Petroleum fuels are complex mixtures of hydrocarbon compounds and the properties of their components – such as density, viscosity, solubility in water, and volatility – tend to vary widely. As a result, a petroleum spill is like several spills for the price of one. The petroleum liquid slowly settles through the unsaturated zone and then tends to float on the surface of the groundwater. The more readily soluble components of the spill dissolve in the groundwater and are dispersed along with the normal groundwater flow, and the more volatile components of the spill rise toward the surface, potentially contaminating buildings. (Earle, 2019)

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Physical Geography and Natural Disasters by R. Adam Dastrup, MA, GISP is licensed under a Creative Commons Attribution-NonCommercial-ShareAlike 4.0 International License, except where otherwise noted.

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