Pre-Visit Activities : Groundwater & Runoff : Background Sixth - Eighth Grade Online Curriculum : Watersheds

Key Points
Key Points will give you the main information you should know to teach the activity.

• Rainwater that seeps into the ground is known as groundwater. Groundwater is the water below the surface that fills the spaces between soil and rocks. Groundwater is divided into the zone of saturation, the spaces that are completely filled with water, and the zone of aeration, the spaces that are partially filled by water and partially filled by air. The boundary between the zone of saturation and the zone of aeration is known as the water table.
• Porosity is the amount of water a substance can hold or, in other words, the amount of space available in the substance that can hold water. It is expressed as a percentage.
• Permeability refers to the ease at which water passes through a substance.
• In soils, large sediment sizes tend to have more spaces between them and so will have a higher porosity and permeability. On soils with large sediment sizes the water in rain is more likely to become groundwater. On soils with small sediment sizes, the water is more likely to become surface runoff. 
• Sand has much larger particle sizes than clay.
• Because the Sandhill soils are made primarily of sand and have high porosity and permeability, the rain that falls on them is most likely to soak into the ground and become groundwater. Because the Piedmont soils are made primarily of clay and have low porosity and permeability, the rain that falls on them is most likely to become surface runoff.

Top

Detailed Information
Detailed Information gives more in-depth background to increase your own knowledge, in case you want to expand upon the activity or you are asked detailed questions by students.

When it rains, the water dissipates in various ways. Some of the water seeps into the ground while some of it runs off to nearby rivers and streams, some of the water is absorbed by plants and some of it evaporates back into the atmosphere. Depending on where you are, the visible results of this will vary. In some areas of South Carolina, such as the Sandhills, it can rain all day and when it stops there are very few signs of the recent deluge of water. In other areas, such as downtown Charleston, it rains just a little and cars have to plow through three feet of water. What is the difference? Though there are many factors involved, one of the main things that affects the ratio between water saturation and water runoff is the porosity and permeability of the soil.

When it rains, the water is pulled down by gravity through the spaces between the soil particles and the cracks and fissures in the rocks. This water is known as groundwater. It is estimated that 95% of all freshwater available and approximately 50% of the water Americans drink is found in groundwater. Generally, groundwater is found within 100 meters of the surface.

As water seeps downward to the bedrock (the solid rock that underlies the soil) it begins to fill the spaces between soil particles and rock. The spaces that are completely saturated with water are known as the zone of saturation. This extends from the bedrock upward. The size of the zone of saturation is not permanent but changes when rainfall fluctuations and other factors add or remove water from the groundwater. Sometimes the zone of saturation can be a very thin layer and sometimes it can fill the soil to the surface. If large amounts of rain cause it to reach the surface, any excess rain cannot infiltrate the ground. The rain remains on the surface, sometimes causing flooding.

Generally, the zone of saturation does not reach the surface. Another layer of soil exists between the two. This layer has water in it, but is not completely saturated with water so that air may be contained within it as well. This layer is known as the zone of aeration.

The boundary between the zone of saturation and the zone of aeration is known as the water table. The water table does not hold a permanently stable position in the soil, but is constantly moving up and down as rainfall amounts increase and decrease and water is drawn out of the soil by wells and natural processes.
The water underneath the water table in the zone of saturation is not permanently in one place either, but follows the sloping of the bedrock, moving slowly downwards in a lateral direction towards streams, rivers, lakes or the ocean. This is not a quick process, though. The rate of movement for this water resembles glacial speeds, generally being measured in meters per year and sometimes even less.

The amount of water that can be held by the soil is known as its porosity. Porosity is the ratio between empty spaces in the soil and the soil itself. It is expressed as a percentage. For example, if 30% of a volume of soil is open space then 30% of it can contain water and thus it has a porosity of 30%. A liter (1000ml) of soil with a porosity of 30% can contain approximately 300ml (30% of a liter) of water.

Permeability is the rate at which water will flow through the soil to become part of the zone of saturation. Soils with high permeability allow water to flow through them very quickly. Soils and surfaces with low permeability do not allow water to flow through them very well, and have higher amounts of surface runoff.

Both porosity and permeability are affected by the particle size of the soil. Though all soil particles are small, there are microscopic differences in size. Very coarse grained- particles, 60 mm in diameter or larger, are characterized as cobbles and boulders. Coarse-grained particles, 2 to 60 mm, are classified as gravel. Medium-grained particles, 0.06 to 2 mm, are classified as sand. Fine-grained particles, 0.002 to 0.06 mm, are classified as silt. Very fine-grained particles, less than 0.002 mm, are classified as clay. Soils with relatively large particle sizes, such as sand and gravel, have larger gaps between their particles and thus have a higher porosity. These gaps also allow a quicker transfer of water leading to a higher permeability. Soils with small particle sizes, such as clay, have very tiny open spaces and thus have low porosity and permeability. Because soil is often a mixture of different sediment sizes, this can affect the porosity of the soil. Small particle size sediment can sometimes fill the gaps between large particle size sediment, thus lowering porosity.

Soils are mixtures of gravel, sand, silt, clay and organic material. The texture of soils is characterized by the percentages of sediment particle sizes in its make-up. It can be predominately sand, predominately clay, fairly even mixtures of both or various other combinations. Soils in South Carolina are composed primarily of sand or clay, and so are sandy or clay soils.

Sandy and clay soils have very different properties. Because of their larger particle size, sandy soils have a higher porosity and permeability. Water drains through them quickly, and they have more capacity to contain water. Because of the relatively large gaps between their grains, though, water loses its cohesive property and sand cannot hold water in its zone of aeration. For this reason, the topsoil of sandy soils tends to be well aerated, but very dry.

Clay soils have a smaller particle size, and thus have a lower porosity and permeability. Water drains very slowly, most of it running off before it can seep through. Because of the tiny gaps between the small clay particles, water that does seep through is held in the clay by cohesion. This is what gives clay its slimy, sticky feel. Because clay soils hold water so well, they become almost completely saturated with water and there is little space for air to infiltrate the clay. For this reason, clay soils are very moist soils, but not well aerated.

In South Carolina, the sediment types found in major regions affect saturation to runoff ratios. Each of these regions can be encountered in the Santee River watershed. In the Blue Ridge Mountain region of the state, the soil layer is very thin, with impermeable rock close to the surface. The Mountain region has the highest rainfall rate in the state, averaging over 60 inches a year. Because of the high amount of rainfall and the small amount of soil that can be saturated, most of the rain that hits the mountains becomes run off. This run off manifests itself in the streams and waterfalls found throughout the mountain region.

The streams of the Mountain region flow into the rivers of the Piedmont region. The soil layer in the Piedmont region is much thicker than that of the Mountain region. It is soil made up of sediment eroded from the mountains and is composed primarily of clays. Clays have the smallest particle size, and for this reason are virtually impermeable to water. Because of the impermeability of the soil, almost all rain that falls in the Piedmont region runs off. This, combined with the large amounts of water flowing in from the Mountain regions of both South and North Carolina, creates the large rivers that characterize this region. These rivers include the Savannah, Broad, Saluda and Catawba rivers. Each of these rivers flows eastward towards the Atlantic Ocean.

Midway through the state, these rivers cross the Sandhills, the boundary between the Piedmont and the Coastal Plain. The Sandhills are the remnants of ancient sand dunes, formed when the ocean reached this part of the state about 9 to 12 million years ago during the Miocene Epoch. As can be gathered from the name, Sandhill soils are composed primarily of sand. Sand has a much larger particle size than clay, approximately the same size difference as a basketball has to a golf ball. Because of the large size of its particles, the soil is looser and has more open space for water transport and so has a very high permeability. When it rains in the Sandhills, almost all of the water quickly soaks into the ground. Because the soil cannot hold water for very long, it resembles a desert because the topsoil is so dry. Only vegetation adapted to this type of habitat, such as cacti and briars, grows there.

Past the Sandhills, the rivers enter the Coastal Plain. The Coastal Plain is a region of sand, clay and limestone characterized by large winding rivers and large areas of wetlands. The soil is the result both of the sediment brought in by the ancient oceans, and the sediment brought in and deposited by the rivers from the Mountain and Piedmont region that is still occurring today. The sand and limestone both have a high porosity, and with the impermeable clays around them keeping the water in place, they act as aquifers, porous bodies of rock that allow water to pass through easily, for the majority of South Carolina’s groundwater.

The Coastal Plain is also characterized by many wetlands. Wetlands and groundwater have an important relationship. Wetlands collect and hold water that recharges the groundwater. They also return the favor by being receptors for large amounts of water discharged from the groundwater.

With the spread of human populations, and the increase in construction of buildings, roads and parking lots, humans have thrown a wrench into the groundwater/runoff works. Almost any human construction creates an impermeable surface, a surface that water cannot run through, decreasing the amount of surface area that water can infiltrate, and causing rainwater to run off. As pavement is often covered with contaminants such as garbage and oil spilled by cars, these contaminants are picked up by the runoff water and carried to streams and other bodies of water, and are a major source of non-point pollution.

Runoff of impermeable surfaces is a particular problem in urban areas. The concentration of development not only increases the problems of water contamination, but also creates problems with flooding. Having no permeable surfaces causes the rainwater to run in the streets. To prevent flooding in the streets, cities construct storm sewers that drain the streets and discharge the water into nearby streams. This sudden expulsion of urban water in the streams often leads to flooding, contamination by pollutants and rapid temperature increases from water off of sun-heated roads. In Charleston, SC, the high water table, brought on by the close proximity of the city to the ocean and to two major rivers (the Ashley and the Cooper) and the fact that most of the city was built on wetland areas, causes the storm sewers to be ineffectual, as they are already filled with water. When it rains, many streets fill up with one, two and even three feet of water and suddenly amphibious cars do not seem like such a bad idea.

This activity is designed to allow you to teach students about how the particle size of soil affects porosity and permeability and thus groundwater. It provides a visible hands-on demonstration of this. With this background information, you can discuss with the students how the concepts learned in this activity can be applied to the real-life habitats of South Carolina.