Environmental Science: Earth's Water

Zone of Aeration

An area above the water table where the spaces between rock and soil particles are filled with air and water. it is not completely saturated with water.

infiltration

the process by which water on the ground surface enters the soil

salinity

the salt content of a liquid

saltwater

water that has a high salt content; ocean water is considered saltwater

freshwater

water that has a low salt content; springs and lakes are considered freshwater

surface water

water that has collected on the surface of the Earth

groundwater

water within the soil that supplies wells and springs

groundwater overdraft

when humans exhaust groundwater supplies before the supply can be replenished naturally

What You Should Know

Before you begin, you should: have a firm understanding of the importance of water to all living things.

Your Daily Water Use Notes

Take a moment to think about how much water you use every day. In your notes, briefly list your daily activities that require the use of water. In summary, hypothesize the total amount of water you use in gallons per day. To calculate your daily water use, visit the Water Conservation website. Watch the gallons add up on the right as you complete the areas on the left. How does this calculation compare to your original hypothesis?

The Ocean

Even though ocean water is too salty to drink, the oceans play many important roles on Earth. Oceans are the habitat for thousands of species, including fish, crustaceans, coral, and plants. The oceans also provide humans with food, transportation, and recreation. Finally, oceans help to moderate Earth's climate, which allows many species to survive through the change of seasons. So how did Earth's oceans form? The oceans are thought to be a result of volcanic activity from early earth, some 380 million years ago. As volcanoes erupted, they released many gases such as nitrogen, carbon dioxide, hydrogen, and water vapor. These gases were contained in the Earth's atmosphere. Clouds formed and rain fell as the water vapor became too heavy to hold in the atmosphere. Over millions of years, this rain filled the basins of the Earth, which we now call oceans. Volcanoes also released chlorine gas, which formed hydrochloric acid. This acid combined with sodium, which eroded from the Earth igneous rocks to form sodium chloride (NaCl) or salt. This salt, as well as other salts—such as magnesium chloride—eventually make their way to the ocean where they are stored. It is in this way that the oceans became salty and remains salty to this day.

OBJECTIVES

Explain why water is an important natural resource. Describe the differences between freshwater and saltwater. Explain how the majority of Earth's water is not available for human use because it is either saltwater in the oceans or locked up in the polar ice caps. Describe how freshwater is distributed worldwide.

Properties of Groundwater

Groundwater is found deep underneath the Earth's surface. Layers of sand, gravel, and rock underground that can hold and store groundwater are called aquifers. An aquifer always has a layer of impermeable rock underneath that keeps the water from rapidly seeping out of the bottom of the aquifer. Instead, the water seeps small amounts over hundreds or thousands of years. This impermeable layer is made of rock or very thick clay, which is called a confining bed. If an aquifer lies between two confining beds, it is called a confined aquifer. An unconfined aquifer only has a confining bed beneath it. Water infiltrates the ground and enters an aquifer in places called recharge zones, while a place where groundwater meets the surface is called the discharge area, and it may be a natural spring, a wetland, or a stream. The diagram shows you how water enters the unconfined aquifers within days, but may take centuries or millennia to enter the confined aquifers. Humans use wells to pump groundwater out of the ground for our use, and about half of the world's population relies on groundwater for drinking.

Groundwater

Groundwater is water that has flowed from the surface of Earth to deep under the Earth's surface. Precipitation that does not evaporate into the air or that runs over the surface of the Earth travels down through the soil and accumulates underground. The process of water moving downward into the ground is called infiltration. The diagram shows you some basic terminology related to groundwater. You can click on each term to see its definition.

Properties of Ocean Water

In order for the oceans to be such an integral part of the Earth's ecosystems, the oceans must have important and unique properties. The most important properties of ocean water are salinity, temperature, and density. Salinity, or the amount of dissolved salt, is the number one determining factor in the types of living things that can survive in a body of water. The salinity of ocean water is typically 3.5 percent, whereas the salinity of freshwater is usually less than 0.05 percent. Salinity also affects the temperature and density of water, which in turn affects how ocean water moves throughout the world. Recall from earlier lessons that the Earth's oceans are constantly moving. Winds blow ocean surface waters in patterns similar to atmospheric air movements, and deep ocean water also moves in response to differences in salinity, temperature, and density. Density is directly affected by the salinity of something. Density is a measure of the amount of mass per unit volume of a particular object.( D= m/v) As more dissolved salts are added to water, its mass increases, and therefore—if the amount of water remains the same—the density also increases. Salinity also has an effect on temperature. Freshwater freezes at approximately 0 degree Celsius or 32 degrees Fahrenheit. However, if you add salt to water, you will find that the water needs to be colder to freeze. Some of the coldest water by the Earth's poles remains unfrozen even when the water is below 1.8 degrees C. Cold water is denser than warmer water. So, when salts are added to it, you have very cold, dense salt water. This is what occurs in the oceans. The cold, dense ocean water sinks. Warmer, less dense water from tropical regions moves in to take its place. This sets up a circulation system, known as thermohaline circulation, which moves water and the heat contained within it, worldwide. Thermohaline circulation is very important for marine organisms as well as for heat transfer across the globe.

Earth's Water Lab

In this exploratory activity, you will discover how Earth's water sources are distributed. You will need the following material: two 100-milliliter (mL) graduated cylinders (If you don't have a graduated cylinder, look for a liquid measuring cup that is marked in milliliters.) an eyedropper (a standard medicine dropper) a small drinking cup water masking tape with markers for identification of beakers Label one of the graduated cylinders "A" and the other one "B." Start by measuring 100 mL of water into graduated cylinder A, which represents all the water on Earth. Next, pour 97 mL of the water from graduated cylinder A into graduated cylinder B. You should now have one cylinder with 97 mL of water and one cylinder with 3 mL of water in it. Next, use the eyedropper to remove just one drop of water from graduated cylinder A. Place the one drop of water in the drinking cup. Results Of all the water on the Earth, the drinking cup represents the fraction of Earth's water that is freshwater available for our immediate use. Graduated cylinder A represents the unusable freshwater that is underground or frozen. Graduated cylinder B represents the amount of Earth's water that is saltwater, which cannot be used as drinking water. Compare how much water you started with in graduated cylinder A to how much is in your drinking cup. The fraction of readily available freshwater on Earth is very small. Since we cannot drink saltwater, access to clean freshwater is extremely important for all life on Earth.

LET'S REVIEW!

In this lesson, you have covered the following points: Water is necessary for all life on Earth, and it provides humans with a means for washing away wastes and for transportation and recreation. Water is also an important habitat for many of the Earth's species. Most of Earth's water is saltwater that cannot be used as drinking water. A very small fraction of Earth's water is liquid freshwater readily available for our use. The oceans were formed when ancient volcanoes erupted, which caused water vapor to cover Earth's surface. This water vapor was kept close to Earth by gravity, and it eventually condensed to form Earth's oceans. Groundwater and surface water are important sources of freshwater, but their availability varies throughout the world. The amount of precipitation that an area has, as well as its population and economic resources, will determine the availability of freshwater.

The Water Planet

Planet Earth is sometimes called the Water Planet because of its abundance of liquid, gaseous, and solid water. Earth is positioned in just the right spot in the solar system so that it can support liquid water. If Earth was closer to the sun, all the water would evaporate away; if it was farther away from the sun, most of the water would only exist as ice. Without water and its amazing properties, life would not exist on Earth. Water has many unique properties that make it important for life on Earth. Water is a polar molecule, meaning it has a positive side of the molecule—the hydrogen, and a negative side of the molecule—the oxygen. Water is actually a dipolar molecule, since it has two positive hydrogen ends. Water molecules are attracted to other water molecules because of its configuration. This attraction forms a hydrogen bond. Millions of water molecules attached together will fall down on your head as water droplets! Water is a universal solvent, meaning it is able to break apart most substances over time. For instance, water will break apart salt, an ionic substance, very easily be separating the sodium (Na) from the chloride (Cl). Water also has a high freezing and melting point. In addition, water is less dense when it is a solid, meaning ice will float on top of liquid water. Because of these properties of water, we have life on Earth. Without it, our planet would not be a hospitable place to live.

Ice and Snow

Since freshwater is the water upon which we most depend, let's look now at a few of Earth's freshwater sources. By far, most of Earth's freshwater is found in solid ice form in glaciers, ice caps, and snow fields. Glaciers are rivers of slowly flowing ice that glide across land at high elevations or high latitudes. The Earth has large polar ice caps, which are floating sheets of floating ice. The famous North Pole has no land mass; instead it is just a big block of ice. Together, polar ice sheets and glaciers contain more than three times the amount of freshwater found in lakes and streams on Earth. Because these water sources are frozen, the water is not readily available to humans or other animals.

Water Use Facts

The average American uses more than 800 gallons of water every day. A person needs twelve to thirteen gallons a day to have a decent standard of living, but in some countries, like Ethiopia, people only have one gallon a day. People in developing countries need at least twenty-five gallons of water a day to improve their standard of living and help bring them out of poverty.

Surface Water

The opposite of groundwater is surface water. Surface water is water found at Earth's surface in rivers, streams, lakes, ponds, and wetlands. Rivers and streams form when water that has not infiltrated the ground begins to run over the land. The water is drawn by the force of gravity on a downhill path, and this surface water forms freshwater rivers and streams. Most rivers would disappear within a few weeks if not replenished by precipitation and groundwater seepage. Rivers and streams are used for drinking water wherever they are found worldwide. Other forms of surface water are lakes, ponds, and wetlands. Lakes are depressions in the ground that hold water year-round, and ponds are small shallow bodies of standing water. Wetlands are areas where the soil is saturated with water at least part of the year. Examples of wetlands are swamps, marshes, and bogs.

WATER USAGE

Think about all the ways you will have used water by the end of the day today. Include everything from drinking to bathing, flushing toilets, brushing your teeth, washing clothes and dishes, cooking, and watering your lawn or washing your car. Do you think you could fit all the water you use in one day in a one-gallon bottle? For most of us, this task would be about impossible. Just a flush or two of the toilet would use up all the water in the bottle. If a gallon a day was all the water you did have, how would you choose what to use it for? For millions of people throughout the world, this is exactly the choice they are faced with. Most of us here in the United States have reliable and adequate sources of water, but in 2005, the United Nations reported that 1.2 billion people worldwide lack regular access to enough clean water for their drinking, cooking, and sanitary needs. In this lesson, we'll talk about water and how it's distributed throughout the world. Later in this course, we will discuss water pollution and water conservation.

impermeable rock

This is a layer of rock or thick clay that prevents the water from moving down any farther

Water Table

This is the boundary between the zone of aeration and the zone of saturation

soil-moisture belt

This is the where the spaces in between rock and soil particles are completely saturated with water. There is no air between the particles, just water. Groundwater is found in this zone.

River or lake

This is where groundwater may seep up to the surface and become groundwater discharge

zone of saturation

This is where the spaces in between rock and soil particles are completely saturated with water. There is no air between the particles, just water. Groundwater is found in this zone.

Groundwater Pollution

Watch this funny groundwater animation that describes where groundwater comes from and the ways that this unseen water supply can become polluted. We will talk about groundwater pollution in greater depth in a later lesson, but for now, just realize that groundwater can be harmed by human activities above ground. Just because we cannot see groundwater does not mean we don't pollute it.

The Importance of Water:

Water is essential for all life on Earth. It covers about 71 percent of the planet's surface and accounts for approximately 60 percent of the composition of your body, with most of it located inside your cells. The human body can survive for weeks without food, but only for a few days without water. We use it for drinking, growing food, washing away wastes, bathing, cooking, and recreation. Access to available water is associated with several global issues. Click on each picture to learn more.

The Importance of Water

Water is important to all living things. Here are a few reasons why: Water is the primary component of living cells and makes up 60 to 70 percent of the weight of all living organisms. Water is the main component of cells, and it provides the cell with form and support. Water is the medium in which important chemical reactions take place inside an organism's cells. Water absorbs important substances, so they can be transported into cells. These substances include food molecules, gases, hormones, minerals, and other nutrients.

Water Availability

Water is one of the most valuable resources on Earth. As you previously read, less that 1 percent of Earth's water is useable liquid freshwater. The World Resources Institute estimates that about 41 percent of people worldwide live under "water stress" because the regions they live in do not have adequate freshwater sources. The map shows you how water scarcity compares worldwide. What factors contribute to this unequal distribution of water stress? The main problem is that water resources vary worldwide, and differences in population, precipitation, and economic resources divide the world's peoples into haves and have-nots. Recall, from our previous lesson on climate, how precipitation varies worldwide. This has a large impact on how much liquid freshwater is available to a region. Canada, for example, has 20 percent of the world's liquid freshwater, but only 0.5 percent of the world's population. China has 20 percent of the world's people but only about 7 percent of the world's water. These unequal numbers cause extreme stress that can lead to death, illness, and in some cases, war. These inequalities of water are one of the major humanitarian causes of the world, so next time you take a 30-minute shower, think about those who only have one gallon of water a day.

TYPES OF WATER

Water on Earth is categorized as either freshwater or saltwater. While saltwater contains a high concentration of dissolved salt, freshwater has very small amounts of dissolved salt. Salinity is the measure of how much salt is dissolved in a body of water. The saltwater in Earth's oceans has an average salinity of 3.5 percent, and freshwater in Earth's lakes and streams has less than 0.5 percent. Freshwater is a basic need for all animals, including humans, which makes the discussion of water one of the most important in this course. Even though the planet is 71 percent water, the largest percentage of Earth's water is saltwater in the oceans and cannot be used for drinking. The table describes the main locations of water on Earth. Notice that about 98 percent of Earth's water is saltwater. The remaining 2 percent is freshwater, but the majority of that percentage is found in the polar ice caps and glaciers, which is not readily available to us. The only freshwater that can be readily used by animals and humans is the freshwater found in rivers and groundwater.

aquifer

an underground layer of earth, gravel, or sand that yields water