Many substances that are hazardous to human health can enter water supplies. Chemical waste from factories is sometimes dumped into rivers and lakes, or directly into the ground. Pesticides (chemicals that kill insects) applied to farmland enter surface water and groundwater, often in large quantities. Leaks from underground storage tanks for liquids like gasoline go directly into groundwater. Salt put on icy roads in winter pollutes water also, although it is not as hazardous to health.

Everybody knows that water flows downhill. The reason is that the force of gravity pulls everything downward toward the center of the Earth. If a material is resting on a sloping surface, part of the force of gravity acts in the downslope direction. If that does not make sense, think about leaning a board against a wall at an angle and placing a brick on the board. You need to exert a force to keep the brick from sliding down the board. The steeper the board, the more force you have to exert to keep the brick from sliding.

In rural areas, most homes and businesses get their water from groundwater. Long ago, wells had to be dug by hand and reached only shallow groundwater. Now wells can be drilled by machinery to as deep as several hundred feet. In urban and suburban areas, most water is piped in from a central water supply. The water supply might be a river, a natural lake, a reservoir behind a dam, or a number of deep wells. In many big cities, reservoirs are located far away, and the water is brought to the city through aqueducts.

In most parts of the world, water is a scarce resource. That might seem strange, because there is so much water on Earth. Almost all of the water on Earth, more than 97 percent of it, is seawater in the oceans. The rest is called fresh water, because it does not have a high salt content. Most of the world’s fresh water is frozen solid in large glaciers in Antarctica and Greenland. Almost all of the fresh water that is available for human use is either contained in soil and rock below the surface, called groundwater, or in rivers and lakes.

Water or air that moves over a soil surface applies forces to the soil particles on the surface. If the forces are large enough, they move the particles. The stronger the current or wind, the more particles are put into motion. Larger and heavier particles tend to roll or hop near the soil surface. Finer and lighter particles are carried upward from the soil surface. In nature, the finest particles may be carried for hundreds or even thousands of kilometers high in the atmosphere before they fall out!

Knowing how water passes through soil is very important. For example, engineers need to know how quickly water will drain away from buildings or bridges. Drainage rates are also important to farmers, landscapers, gardeners, environmental scientists, and other professionals. Water is one chemical found in soil. There are many others. Some of the chemicals in soil are very important for plant growth. Farmers and gardeners often test their soil for these chemicals using soil-testing kits. These kits can be bought at gardening or hardware stores.

Soil scientists separate soil using a stack of several sieves. The sieves have holes with slightly different sizes. The coarsest sieve is at the top of the stack, and the finest sieve is at the bottom. Most soils contain many kinds of material. All soils consist mainly of two kinds of material: particles of minerals and rocks, and organic matter. Organic matter is any matter that is or once was living.

There are many kinds of soil. A group of soil scientists from the U.S. made up a way of grouping soils that is used around the world. This grouping has hundreds of named soil types! All soils, however, are made of just a few main things. Soil consists of fine particles of minerals and rocks, decaying plants, and living plants and animals. You can easily see the larger plants and animals. There are even more tiny plants and animals that you can only see with a microscope.