EARTH’S FRESH WATER The Water Cycle The constant moving of water from Earth’s surface into the atmosphere, back to the Earth’s surface, and into bodies of water is called the water cycle. Water evaporates from bodies of water (lakes, oceans and rivers) and is transpired by plants. This water vapor condenses to form clouds. This water precipitates out of the clouds as rain, snow, sleet or hail (or a combination of these) and returns to the Earth’s surface. Some of this precipitation falls directly back into oceans, lakes and rivers. The rest falls on the land. Runoff is water that moves over the land to return to rivers and, ultimately, the ocean. How Is Water Distributed Across the Globe? The majority of water on Earth is held in the oceans. The second largest water resource is the ice held in the polar icecaps. The world’s fresh water is held in two sources, surface water and groundwater. The source for all surface water and groundwater is precipitation. Surface water Fresh water moves over the surface by an interconnected system of streams and rivers. Small streams that feed water into larger rivers are called tributaries. Rivers are closely interconnected and flow from higher to lower elevations where the water collects in larger and larger rivers until it ultimately flows into the oceans. A geographical region of land which includes its main river or rivers and all the tributaries that feed the rivers is called the drainage basin or watershed. High geographical regions, such as mountain chains, separate drainage basins from one another. A geographical feature separating drainage basins is called a divide. The Rocky Mountains in western United States is known as the Continental Divide. Precipitation that falls to the east of the Rockies drains to the East, and that which falls to the west of the Rockies drains to the West. Lesson Checkpoint: Which water features does the Continental Divide separate? © Copyright NewPath Learning. All Rights Reserved. Permission is granted for the purchaser to print copies for non-commercial educational purposes only. Visit us at www.NewPathLearning.com.
Groundwater Water that collects in the soil, sand and rocks below the Earth’s surface is called groundwater. The force of gravity pulls water down into the soil and the underlying rocks. The water moves through spaces between rocks and particles of soil and sand. Two factors affect water’s ability to move through the soil and ground. Porosity is a measure of the amount of space between particles. The greater the amount of space (that is, the higher the porosity) the more water the ground can hold. This water cannot move through the ground, however, if the spaces are not connected to each other. The measure of the connectedness between spaces is permeability. A layer of rock that can both store ground water and allow it to move through the rock is called an aquifer. For a rock to be considered an aquifer, it must be both porous and permeable. © Copyright NewPath Learning. All Rights Reserved. Permission is granted for the purchaser to print copies for non-commercial educational purposes only. Visit us at www.NewPathLearning.com.
The top of the body of water held in an aquifer is the water table. The water table marks the boundary between the zone of aeration above and the zone of saturation below. The water table rises and falls with the amount of precipitation in a region over a period of time. During drought seasons, the water table falls; during rainy seasons, the water table rises. Water is always moving in an aquifer. As water leaves the aquifer, either by natural flow or from human use, it needs to be replaced. The recharge zone is the ground surface where water comes into an aquifer. Agriculture uses water from aquifers to water the crops. In the Midwestern United States, the Ogallala aquifer provides water to 8 states! Though enormous in size, but it is used so extensively that it is losing water faster than it is recharging. Lesson Checkpoint: What are the necessary characteristics of rock in an aquifer? Water Quality and Water Pollution Though traditionally thought of as a renewable resource, overuse and unmanaged pollution can eradicate a water resource, either depleting it completely or making it unusable for very long periods of time, turning it into a nonrenewable resource. Consequently, water preservation and pollution management is essential. © Copyright NewPath Learning. All Rights Reserved. Permission is granted for the purchaser to print copies for non-commercial educational purposes only. Visit us at www.NewPathLearning.com.
There are two categories of pollution, point-source pollution and nonpoint-source pollution. Point source pollution is pollution that comes from a specific, identifiable source, like a leaky underground fuel tank or a wastewater pipe from a factory. Nonpoint-source pollution is pollution that comes from an unidentifiable region. The pollution is transported by the runoff of groundwater. Runoffs from city streets, fertilizers from farmland, seepage from septic tanks in housing tracts are all examples of nonpoint-source pollution. If polluted water is returned to the ground, it becomes very, very difficult to clean. In fact, it moves so slowly through the aquifer that it will take extraordinary measures to get it out of the aquifer and make the aquifer usable again. Consequently, it is preferable to collect wastewater and transport it to a water purification facility where it can be properly treated so that clean, usable water can be returned to the environment. © Copyright NewPath Learning. All Rights Reserved. Permission is granted for the purchaser to print copies for non-commercial educational purposes only. Visit us at www.NewPathLearning.com.
Water treatment facilities clean water in two steps. The first is primary treatment in which particles are physically filtered out of the wastewater. The solid material that is removed from the water is called sludge. The remaining water enters secondary treatment where it is aerated with oxygen and treated with bacteria that consume dissolved waste and very small waste particles. In the end stages of secondary treatment, chlorine is added to the water to kill any remaining bacteria. At this point the water is returned to the environment. In some cases water is polluted with industrial chemicals and other byproducts that cannot be easily removed and, at times, cannot be removed at all. Eliminating such scenarios is an imperative step of conservation and environmental preservation. Lesson Checkpoint: What is the key difference between the two categories of pollution described here? © Copyright NewPath Learning. All Rights Reserved. Permission is granted for the purchaser to print copies for non-commercial educational purposes only. Visit us at www.NewPathLearning.com.