Understanding the Interaction
Test your groundwater IQ.
1. Which ways can groundwater move?
a. Up / b. Down / c. Sideways / d. All of the above
2. How is the speed of groundwater movement measured?
a. Feet per day / b. Feet per week / c. Feet per month / d. Feet per year
3. How is stream flow usually measured?
a. Feet per second / b. Feet per minute / c. Feet per hour / d. Yards per hour
4. What determines how fast groundwater moves?
a. Temperature / b. Air pressure / c. Depth of water table / d. Size of materials
5. Can the water table elevation change often?
a. Yes / b. No
Does aquifer storage capacity vary?
a. Yes / b. No
1. d. All of the above Although most movement is lateral (sideways), it can move straight up or down. Groundwater simply follows the path of least resistance by moving from higher pressure zones to lower pressure zones.
2. d. Feet per year Groundwater movement is usually measured in feet per year. This is why a pollutant that enters groundwater requires many years before it purifies itself or is carried to a monitored well.
3. a. Feet per second Water flow in streams/rivers is measured in feet per second.
4. d. Size of materials Coarse materials like sand and gravel allow water to move rapidly. (They also form excellent aquifers because of their holding capacity.) In contrast, fine-grained materials, like clay or shale, are very difficult for water to move through. Thus, water moves very, very slowly in these materials.
5. a. Yes Water table elevations often fluctuate because of recharge and discharge variations. They generally peak in the winter and spring due to recharge from rains and snow melt. Throughout the summer the water table commonly declines due to evaporation, uptake by plants (transpiration), increased public use, industrial use, and crop, golf course and lawn irrigation. Elevations commonly reach their lowest point in early fall.
6. a. Yes Just like the water level in rivers and streams, the amount of water in the groundwater supply can vary due to seasonal, weather, use and other factors.
Groundwater: A Hidden Resource
Groundwater is a hidden resource. At one time, its purity and availability were taken for granted. Now contamination and availability are serious issues. Some interesting facts to consider… Scientists estimate groundwater accounts for more than 95% of all fresh water availablefor use.Approximately 50% of Americans obtain all or part of their drinking water fromgroundwater.
Nearly 95% of rural residents rely on groundwater for their drinking supply. About half of irrigated cropland uses groundwater. Approximately one third of industrial water needs are fulfilled by using groundwater. About 40% of river flow nationwide (on average) depends on groundwater. Thus, groundwater is a critical component of management plans developed by an increasing number of watershed partnerships.
Groundwater is the water that saturates the tiny spaces between alluvial material (sand, gravel, silt, clay) or the crevices or fractures in rocks.
Aeration zone: The zone above the water table is known as the zone of aeration (unsaturated or vadose zone). Water in the soil (in the ground but above the water table) is referred to as soil moisture. Spaces between soil, gravel and rock are filled with water (suspended) and air.
Capillary water: Just above the water table, in the aeration zone, is capillary water that moves upward from the water table by capillary action. This water can move slowly in any direction, from a wet particle to a dry one. While most plants rely on moisture from precipitation that is present in the unsaturated zone, their roots may also tap into capillary water or into the underlying saturated zone. Aquifer: Most groundwater is found in aquifers-underground layers of porous rock that are saturated from above or from structures sloping toward it.
Aquifer capacity is determined by the porosity of the subsurface material and its area. Under most of the United States, there are two major types of aquifers: confined and unconfined. Confined aquifers (also known as artesian or pressure aquifers) exist where the groundwater system is between layers of clay, dense rock or other materials with very low permeability.
Water in confined aquifers may be very old, arriving millions of years ago. It’s also under more pressure than unconfined aquifers. Thus, when tapped by a well, water is forced up, sometimes above the soil surface. This is how a flowing artesian well is formed. Unconfined aquifers are more common and do not have a low-permeability deposit above it. Water in unconfined aquifers may have arrived recently by percolating through the land surface. This is why water in unconfined aquifers is often considered very young, in geologic time. In fact, the top layer of an unconfined aquifer is the water table. It’s affected by atmospheric pressure and changing hydrologic conditions. Discharge and recharge rates depend on the hydrologic conditions above them.
Saturation zone: The portion that’s saturated with water is called the zone of saturation. The upper surface of this zone, open to atmospheric pressure, is known as the water table (phreatic surface). How Groundwater and Surface Water connect. It’s crystal clear. Groundwater and surface water are fundamentally interconnected. In fact, it is often difficult to separate the two because they “feed” each other. This is why one can contaminate the other.
A closer look.
To better understand the connection, take a closer look at the various zones and actions. A way to study this is by understanding how water recycles … the hydrologic (water) cycle. As rain or snow falls to the earth’s surface: Some water runs off the land to rivers, lakes, streams and oceans (surface water). Water also can move into those bodies by percolation below ground.
Water entering the soil can infiltrate deeper to reach groundwater which can discharge to surface water or return to the surface through wells, springs and marshes. Here it becomes surface water again. And, upon evaporation, it completes the cycle. This movement of water between the earth and the atmosphere through evaporation, precipitation, infiltration and runoff is continuous.
How groundwater “feeds” surface water.
One of the most commonly used forms of groundwater comes from unconfined shallow water table aquifers. These aquifers are major sources of drinking and irrigation water. They also interact closely with streams, sometimes flowing (discharging) water into a stream or lake and sometimes receiving water from the stream or lake. An unconfined aquifer that feeds streams is said to provide the stream’s baseflow. (This is called a gaining stream.) In fact, groundwater can be responsible for maintaining the hydrologic balance of surface streams, springs, lakes, wetlands and marshes.
This is why successful watershed partnerships with a special interest in a particular stream, lake or other surface waterbody always have a special interest in the unconfined aquifer, adjacent to the water body.
How surface water “feeds” groundwater.
The source of groundwater (recharge) is through precipitation or surface water that percolates downward. Approximately 5-50% (depending on climate, land use, soil type, geology and many other factors) of annual precipitation results in groundwater recharge. In some areas, streams literally recharge the aquifer through stream bed infiltration, called losing streams.
Left untouched, groundwater naturally arrives at a balance, discharging and recharging depending on hydrologic conditions. Common boundaries.
Aquifers are often difficult to delineate. It requires someone with an understanding of the aquifer, the geology, the surface above it, and the land that drains toward the surface. An unconfined aquifer area often extends to the surface waterbody’s (i.e. lake, river, estuary) watershed. When determining an aquifer protection area, pumping (working) wells are not considered. The biggest risk to an unconfined aquifer is contaminated water moving through the permeable materials directly above it. This area is known as the primary recharge area. Depending on the depth and overlying geologic characteristics, travel time from the surface to the aquifer can be relatively short.
When pumping wells are located near a stream or lake, infiltration can be increased. Infiltrating streams typically provide an aquifer with large quantities of water and a pathway for bacteria, viruses and other contaminants.
A confined aquifer area may be limited to the outcrop of the aquifer unit and its immediate contributing area. This area may actually be isolated from the location of water supply wells within the aquifer. Semi-confined aquifers may receive water from both outcrop areas and overlying aquifers. Delineating the aquifer protection area can be extensive and complex. Sole-source aquifers are delineated based on aquifer type – confined, semi – confined or unconfined – and local geologic and hydrologic conditions. Defined as providing a minimum of 50% of the water for its users, sole-source aquifers usually exist only where there simply are no viable alternative water sources.
Wellhead protection areas (also known as zone of contribution and contributing areas) are the surface and subsurface areas surrounding a well or field of wells (wellfield) supplying a public water system.
Threats to quantity.
When an increased quantity of groundwater is being withdrawn to meet the demands of a growing population, typical threats such as overdraft, drawdown and subsidence can occur. Overdraft occurs when groundwater is removed faster than recharge can replace it. This can result in a permanent loss of a portion of its storage capacity. A change that can cause water of unusable quality to contaminate good water. Generally, any withdrawal in excess of safe yield (the amount that can be withdrawn without producing an undesirable result) is an overdraft.
Drawdown differs significantly from overdraft. It results in a temporarily lowered water table generally caused by pumping. In this situation, the water table recovers when the supply is replenished. Subsidence is one of the dramatic results from overpumping. As the water table declines, water pressure is reduced. This causes the fine particles that held water to become compacted. In addition to permanently reducing storage capacity, the land above the aquifer can sink … from a few inches to several feet … causing a sinkhole. This can damage property and fields.