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The Hydrologic Cycle: How Water
Movement Affects Water Quality
John M. Jemison, Jr., Extension water quality and soil
specialist Introduction To understand how contamination of groundwater
and surface water occurs, it is necessary to understand the hydrologic cycle
— the endless circulation of water from the ocean, atmosphere and land. Then
you can understand how contaminants move with it. The hydrologic 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 land surfaces and a portion may be
intercepted by vegetation. The remainder reaches the ground, where it can
enter the soil by a process called infiltration. Some of the infiltrating water remains near the
soil surface and evaporates into the atmosphere. Another portion is
extracted by plant roots and transported to leaves where it is lost to the
atmosphere as a vapor. This process is called transpiration. When the precipitation rate exceeds the
infiltration rate, excess water builds on the soil surface and moves by
overland flow called surface runoff. If surface runoff is excessive or
concentrated, erosion can occur. Still another portion of water that enters the
soil can move vertically or laterally out of the plant root zone.
Significant lateral movement of water through the 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 particles are filled
with water. The water filling the spaces between soil particles and rock in
the saturated zone is called groundwater. Groundwater Flow 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 ground water is recharged will vary seasonally as well. In turn, the depth to the water table will vary. The water table is the upper boundary or top of the groundwater. Water moves within the saturated zone under the influence of gravity from areas where the water table elevation is high toward areas where the water table elevation is lower. As it does, groundwater may flow into surface water, such as a lake or river. In fact, this process, known as baseflow, accounts for rivers and lakes. One common misconception, however, is that
groundwater moves somewhat rapidly. In reality, groundwater moves 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. Geologic materials that serve as aquifers include unconsolidated material, such as sand and gravel, including tills; permeable sedimentary bedrock, such as sandstone, limestone and dolomite; and fractured crystalline bedrock. Geologic material through which significant
quantities of water cannot move is referred to as an aquitard or aquiclude.
Clays, shales, dense crystalline and sedimentary bedrock, and other very
slowly permeable materials are common aquitards. Aquifers and aquitards vary in their
occurrence, thickness, continuity and depth. A confined aquifer is bounded
on the top and bottom by aquitards. In contrast, unconfined aquifers are
overlaid by permeable layers and are generally found close to the land
surface. Groundwater Contamination Groundwater becomes contaminated as substances are leached downward by percolating water. Some groundwater contamination occurs naturally, but contamination usually is the result of human activities on the land surface. An aquifer overlaid by an aquitard is less susceptible to contamination than one that is not because contaminated water has difficulty percolating through the aquitard. In addition, deep aquifers are relatively less susceptible to contamination than shallow aquifers because of the distance the contaminant must move in the soil before reaching a deep aquifer. However, these relative differences, and differences in percolation rates due to different geological structures, have little practical effect in Maine. Most groundwaters are vulnerable to contamination. Point and Nonpoint Source Pollution Point sources of contamination are identified by a well-defined point of entry where pollutants reach a body of water, such as municipal and industrial treatment facilities, where treated wastewater is discharged into rivers or streams. Nonpoint sources of contamination have no easily identified point of entry where pollutants reach a body of water. Contaminants originate from a wide variety of sources over a wide area, and they enter surface water and groundwater at many locations, by many processes. Pesticides and fertilizers applied to cropland, effluent from septic systems, leaching of highway de-icing salts, landfill leachate and products from leaking underground storage tanks are commonly identified as nonpoint sources of pollution. It is important to consider the cumulative effects of the nonpoint source problem. What's Beneath Your Land? After becoming familiar with these concepts, the next step is to determine the source of your well water. Is it a sand and gravel or a bedrock aquifer and is the aquifer confined or unconfined. What is the potential for contamination? If the company that drilled the well is not known, general information may be obtained from the Maine Geological Survey, Ray Building, Hospital St., Augusta, ME 04333, (207) 287-2801. Protecting Your Water Supply You should pay particularly strict attention to the handling and disposal of potential contaminants, such as pesticides, fertilizers, cleaning solvents and motor oil. Mix and handle chemicals as far from the well as possible. In addition, if large amounts of
pesticides are mixed and handled, consider installing a concrete pad and
rinsate collection system, far from the For more information on water quality, contact your UMaine Extension county office. Reprinted and adapted from Land and Water Fact
Sheet #13, published by University of Illinois at Urbana - Champaign, College of
Agriculture, Cooperative Extension, September 1988. Reviewed by Steve Pinnate
and Paul Dutrum, Maine DEP. This material is based on work supported by
the U.S. Department of Agriculture, Extension Service, under special project
number 92-EWQI-I-9231. © 2002 Extension books and publications homepage Published and distributed in furtherance of Acts of Congress of May 8 and June 30, 1914, by the University of Maine Cooperative Extension, the Land Grant University of the state of Maine and the U.S. Department of Agriculture cooperating. Cooperative Extension and other agencies of the U.S.D.A. provide equal opportunities in programs and employment. Call 800-287-0274 or TDD 800-287-8957 (in Maine), or 207-581-3188, for information on publications and program offerings from University of Maine Cooperative Extension, or visit www.extension.umaine.edu.
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Last
Modified:
08/19/09
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