The total water resources of the earth equal 326 million cubic miles, with each cubic mile equal to 1 trillion gallons of water. Only 2.5 percent% of water is freshwater; 97.5 percent is saltwater. Almost 69 percent of freshwater resources are tied up in glaciers and ice caps; about 30 percent is groundwater, and a mere 0.27 percent is surface water. Water resources are important for the survival of the planet.
Saltwater is abundant in the world. However, saltwater is not useful when it comes to potable water supplies. Desalination plants, while they do exist, are scarce because the energy required for desalination makes the process extremely expensive. However, there are saltwater resources from which we benefit. Saltwater fish are a staple in much of the world's diet, but overfishing has put much of the fishing population at risk. Furthermore, tidal waters are being used as a source of hydroelectric energy. So, while saltwater is not helpful in dealing with scarce water supplies, it's inherent resources are being used.
Groundwater is the most plentiful of all freshwater resources. As water soaks into the ground, some of it adheres to clay and soil to provide water to the plants. This water is in the unsaturated, or vadose, zone. Most of the pores in the vadose zone are filled with air, rather than water. Gravity continues to move the water down through the ground. Eventually, the water reaches the saturated zone, where all the pores are filled with water. The separation between the saturated and unsaturated zone is called the water table.
Aquifers are areas of permeable rock that yield water. Aquifers are typically made of bedrock that has many fractures and connected pores, such as limestone, sandstone and gravel. Shale and clay layers are impermeable, and therefore make poor aquifers. An aquifer is recharged through precipitation. There is significant interaction between surface water and ground water. Groundwater feeds surface water through springs. Surface water can also recharge groundwater supply.
Groundwater is accessed by wells. A well is drilled down past the water table. A pump is placed in the bottom of the well, and it is pumped into homes, businesses and water treatment plants where it is dispersed. As water is pumped from the ground, a cone of depression forms around the well. The groundwater from the surrounding area moves towards the well. Wells can run dry during times of drought or if surrounding wells are pumping too much water, causing the cone of depression to be large.
Water pumped from wells is generally very clean. The ground acts as a natural filter. However, contaminants from nearby contaminated soils, leaky underground tanks, and septic systems can contaminate a well, rendering it unusable. Furthermore, salt water intrusion can occur when the rate of pumping near a shoreline exceeds the rate of recharge. Saltwater gets pulled from the ocean into the cone of depression, and enters the well.
Subsidence has also become an issue as groundwater is mined. Subsidence is the gradual settling of the land due to continuous pumping and development. Subsidence occurs when groundwater is pumped out faster than it can be replenished and the sediments become compacted. It is a permanent phenomenon. Subsidence can cause structural problems to foundations, can cause sinkholes to increase in number, can cause flooding problems and is extremely costly. In some areas, such as the San Joaquin Valley in California, the land has subsided over 9 meters due to groundwater withdraw.
Surface water is the water that exists in streams and lakes. This water is used for potable water supply, recreation, irrigation, industry , livestock, transportation and hydroelectric energy. Over 63 percent of the public water supply is withdrawn from surface water. Irrigation gets 58% of its water supply from surface water. Industry gets almost 98 percent of its water from surface water systems. Therefore, surface water conservation and quality is of utmost importance.
Surface water quality and stream flow
Watershed organizations, the USGS, NOAA and the EPA continuously measure the water quality and stream flow of surface water. Water quality is so important as the majority of the water used in the United States comes from surface water. Stream flow is also monitored to warn of flooding and drought conditions.
Water quality is the measure of how suitable the water is from a biological, chemical and physical perspective. Water quality can be affected negatively by natural and man made causes. Water that runs off into the stream naturally carries sediment, debris and bacteria. Turbidity is the measure of suspended sediment in a stream. The more turbid the water, the lower the water quality. In addition, man-made contaminants such as gasoline, solvents, pesticides, and nitrogen from livestock, wash over the land, degrading the quality of the stream. Electrical conductivity, pH, temperature, phosphorus levels, dissolved oxygen levels, nitrogen levels and bacteria are tested as a measure of water quality. The Clean Water Act protects the quality of the stream and issues fines to those contributing to the degradation in water quality. By protecting and conserving the water supply, there is a greater guarantee of future water use.