get the facts . . . Volume 2
Water in Action
All of us have had firsthand experience with water in its many forms - rain, hail, snow, ice, steam, fog, dew. Yet, in spite of our daily use of it, water is probably the natural resource we least understand. Today there is much concern over the contamination of this resource.
To understand how water becomes contaminated, it is necessary to understand the hydrologic cycle (water cycle) - the endless circulation of water between the ocean, atmosphere and land. By understanding how water moves on and through the soil, it becomes how contaminants can move with it.
The water cycle begins with precipitation in the form of rain, snow, sleet or hail falling on the surface of the earth. As precipitation falls, some of it may evaporate directly into the atmosphere from bodies of water, and a portion may fall on vegetation. The remainder reaches the ground where it can enter the soil by a process called infiltration.
Some of the infiltrating water stays near the soil surface and evaporates into the atmosphere. Another portion is taken up by plant roots and transported to leaves where it is lost to the atmosphere as a vapor. The process is called transpiration.
Still another portion of water that enters the soil can move out of the plant root range. Significant lateral movement of water through soil is called throughflow or interflow. Downward movement of water through the soil is referred to as percolation.
Percolating water eventually makes its way to a saturated zone, where all spaces between rock and soil are filled with water. The water filling the spaces between soil particles and rock in the saturated zone is called groundwater.
Gravity causes water to move within the saturated zone from areas where the water table is high toward areas where the water table is lower. As it moves groundwater may flow into surface water, such as a lake or river. In fact, this process, known as baseflow, accounts for most of the water that recharges rivers, lakes, and perennial streams (streams which flow all year).
Water will soak into the soil until precipitation comes down faster than it can be absorbed by the soil. The excess water builds up on the soil surface and moves by overland flow called surface runoff. If surface runoff is excessive or concentrated, erosion can occur.
Recharge is the process in which water percolating through the soil replenishes groundwater. Because the amount of precipitation and evaporation varies seasonally, the rate at which groundwater is recharged will vary seasonally as well.
In turn, the depth of the water table will vary. The water table is the upper boundary or the top of the groundwater.
One common misperception, however, is that groundwater moves somewhat rapidly. In reality, groundwater moves much more slowly than water in a river or stream. This is because groundwater must overcome friction to move through the small spaces between soil particles and rocks. While water in a stream may move several inches or feet per second, groundwater generally moves several inches or feet per month.
Because groundwater flows slowly, most wells draw on the groundwater that lies close to the point where it is withdrawn. This is why activities taking place on the land surface closest to the well have the greatest impact on water quality.
Aquifers and Aquitards
An aquifer is defined as permeable, geologic material through which significant quantities of water can move. Geological materials that serve as aquifers include sand and gravel; permeable bedrock, such as sandstone, limestone and dolomite; and fractured crystalline bedrock.
Geologic material through which significant materials of water cannot move is called an aquitard or aquiclude. Clays, shales, dense bedrock and other very slowly permeable materials are common aquitards.
Aquifers and aquitards vary in their depth, thickness and continuity. A confined aquifer is bounded on the top and bottom by aquitards. Unconfined aquifers have a permeable layer above them, and they are generally found close to the land surface.
Although the total amount of water circulating in the hydrologic cycle remains constant, the amount of clean water available to us is not constant. We tend to return water to the cycle dirtier than it was when we received it. Knowledge of the water cycle’s processes (transpiration, throughflow, surface runoff, etc.) gives us a better understanding of how water becomes contaminated and how we can reduce this contamination.
Published by the South Dakota Association of Conservation Districts in cooperation with the Nonpoint Source Task Force and Groundwater Research and Advisory Group. Funded by the US Environmental Protection Agency and the South Dakota Board of Water & Natural Resources.
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