The water in lakes, rivers, and groundwater all together only account for about 0.6% of the total water on Earth. Of these, groundwater is the most important fresh water supply. About half of the United States depends on groundwater for their drinking water. An expanding population and economy has prompted a dramatic increase in the demand for water from groundwater supplies. Many thought such water was immune from contamination. However, as more incidents of groundwater pollution occur, public interest in protecting our groundwater has increased as well as legislative protection of this resource.
Although the technical aspects of groundwater and groundwater pollution are beyond the scope of this report, the basic concepts are easy to understand and can help in evaluating other information in this report.
How Groundwater Reservoirs Form
In order to understand the basic principles of groundwater, it is necessary to appreciate the “hydrologic cycle.” This is the system by which nature circulates water. In short, water evaporates from the ocean and rises to form clouds. Water in the air condenses and eventually falls back to the earth in the form of rain, snow, sleet, hail etc. which then eventually flows back to the ocean.
We depend on precipitation that falls on land to replenish our fresh water supply. Precipitation can fall directly into water bodies, from run-off, or soak into the soil. Much is absorbed by plants or evaporates, but a portion seeps down into the groundwater. Technically, “groundwater” is water located in water-saturated zones below the surface. Water located in unsaturated zones is simply called “soil water.”
Groundwater is primarily stored in “aquifers.” Usually, aquifers consist of gravel, sand, clay or fractured rock. Groundwater is stored in the pores and cavities of sediments or in rock fractures. It does not occur as an underground “ocean” or “river” except perhaps in a rare cave-type environment. Aquifers may consist of one continuous “layer” or as several layers stacked-up like pancakes.
How Groundwater Moves
Groundwater is added to (recharged) by water seeping down through the ground and is released (discharged) when it intersects a surface body of water such as a lake or stream or when it is pumped out. The surface area below the ground where the soil or rock is water saturated is called the “water table.” The water table is not fixed. It can move up and down as water is removed from or added to the aquifer.
In general, groundwater moves slowly. This is an important concept when considering groundwater contamination. The water can move as slowly as a few inches per year in clays. However, if the aquifer consists of loose sand and gravel, the groundwater can move hundreds of feet per year. Like water at the surface, groundwater moves with the gradient (i.e. downhill). However, groundwater can also move as a result of being under pressure. Pressurized water will move from high to low pressure areas. This works under the same principle as any material under pressure, such as fluid in an aerosol can.