Wilkes University Center for Environmental Quality Environmental Engineering and Engineering Department Pennsylvania Groundwater

• Hydrologic Cycle
• Occurrence of Groundwater
• How Groundwater Moves
• How Do Wells Work?
• Factors Affecting Groundwater Declines
• Definitions

Clean water is one of the most valuable and under appreciated resource of our planet.  Approximately 70 percent of the earth's surface is covered with water, but only a small fraction of this water is fresh water that is actually available for consumption and productive use.  Because of the unique properties of water, a dynamic cycle, known as the Hydrological Cycle, has developed on this planet.  This cycle provides for the continuous movement, transformation, and remediation of  water as it most through a series of solid, liquid, and vapor phases.  One of the most confusing parts of this cycle is when the water becomes classified as groundwater.  This is because it is not easy to visualize the actual flow and transformations (chemical and biological) that is going on as the water moves through the soil and rock. 

The primary sources of water is Pennsylvania include: rainwater, stream inflow from other states, surface water (stored in lakes, streams, and ponds), and groundwater.  In 1966, it was estimated that Pennsylvanians use approximately 6.6 billion gallons of water per day.

Because of the rural nature of Pennsylvania, groundwater provides approximately 85 percent of the water used for human consumption, but because it is difficult to set how water moves through the soil, unconsolidated  material (sand and gravel) and bedrock.  For some homeowners, they believe that the groundwater comes from a vast underground lake or from underground streams that come from Canada, Vermont, or even Maine.  Even through there is a large database of information on groundwater in Pennsylvania, it still is difficult to really document the total available resource and actual movement and quantity without implementing a very elaborate system of monitoring wells, observation points, and background water quality data.

Hydrologic Cycle

The hydrologic cycle describes the constant movement of water above, on, and below the earth's surface. As part of this cycle, water is transformed  between liquid, solid and gases states.  Condensation, evaporation and freezing of water occur in the cycle in response to the earth's climatic conditions. Figure 1 presents components of the hydrologic cycle that directly affect Pennsylvania.

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Figure 1. Components of the Hydrologic Cycle.

The hydrologic cycle begins with water evaporation from the earth's soil, plant and water surfaces to form water vapor. The energy required to evaporate water is supplied by the sun.  The vast majority of evaporation occurs from the oceans.  It is estimated that 39 inches of water annually evaporate from each acre of ocean (Water of the World, US Geological Survey).   Water vapor is drawn into the atmosphere by temperature gradients and can be transported over hundreds of miles by large air masses.  When water vapor cools, it condenses to form clouds. As water condenses within clouds, water droplets increase in size until they fall to the earth's surface as precipitation such as rainfall, hail, sleet or snow.

Approximately 50 to 90 percent of the water that falls to the earth's surface enters the soil.  This water can become groundwater but most of it evaporates from the soil surface, used by vegetation via evapotranspriation, or flows to streams and springs as interflow. Water that passes through the root zone may continue to move downward to reach the groundwater. In soils with fragipans, clay pans or other low permeable strata of a limited extent, this water may create a seasonal high or perched water table.   The distance water has to travel to reach groundwater can range from a few feet to hundreds of feet. Water movement toward groundwater may take hours or  years, depending on the depth to the aquifer and the characteristics of the unsaturated zone.

Once in the groundwater, water will slowly move to discharge points which may include: springs, streams, lakes, wetlands, or even the ocean.  Groundwater moves slowly within the groundwater aquifer, often remaining in storage for 100s of years.

For Pennsylvania, the annual precipitation ranges from 30 to 60 inches per year with a mean rainfall of approximately 41 inches.  Approximately 55 to 60 percent of the precipitation occurs during the warmer months.  Of this approximately, 20 inches is returned to the atmosphere via evapotranspiration or evaporation, 12 to 15 inches infiltrates into the groundwater system, and direct runoff accounts for approximately 6 to 12 inches of water.  Groundwater storage in Pennsylvania is equivalent to approximately 60 inches of water or a conservative estimate of  over 47 trillion gallons, of which, 9 to 12 trillion is naturally discharged to springs, seeps, streams, and lakes. 

Occurrence of Groundwater

Groundwater is stored in the voids, spaces and cracks between particles of soil, sand, gravel, rock or other materials.   These cracks or space can include fractures, faults, bedding planes, solution channels (limestone formations), dissolution channels associated with more easily weathered material or other structural features such as bed planes or deformation in the bedrock due to folding.  These materials form what is sometimes called the groundwater aquifer or reservoir.  In most areas of the world, and specifically in Pennsylvania, water does not flow in and is not stored in large underground lakes or rivers.  The only exception to this might be the dissolution channels and caverns associated with limestone formations and mine shafts associated with underground mining operations.

How Groundwater Moves
Groundwater Always Move From 
Areas of High Head to Areas of Low Head 

(The Problem: Which way is down?)

To understand how we can remove groundwater using wells, we must understand how groundwater moves. S ome people attempt to associate the flow of water on the earth's surface with groundwater movement.

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Figure 2. Confined and unconfined aquifers and related water tables.

Surface water flows in rivers or streams at velocities of 2-8 miles per hour. Pennsylvania's groundwater moves through the spaces between particles of a saturated material at rates between 0.1 foot per day to 3 feet per day. That translates into movement of 35 to 1,100 feet per year.

Groundwater moves only if sufficient pressure, or head, is available to force water through the spaces between porous aquifer materials. Rate of movement is determined by the hydraulic gradient, permeability, and porosity of the material.  The hydraulic gradient, or slope of the water surface between two points in an aquifer, and the aquifer material determines how rapidly water moves from one location to another.

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Figure 3. Unconfined and perched water table aquifers.

Groundwater moves from high water surface elevations (high pressure or head) to low water surface elevations (low pressure or head). In general, the water flows more rapidly where large differences exist in water surface elevations (steep hydraulic gradients), but this is not always the case.  A large variation in the hydraulic gradient could also mean an lower permeability formation..  Groundwater may move toward or away from streams or lakes, depending on the hydraulic gradient.  As groundwater moves it may be removed by a pumping well, or it may be discharged to the earth's surface as a spring, a lake or stream. Groundwater supplies are recharged by precipitation or from rivers and lakes. Groundwater removed by wells or discharged by springs may have been stored for thousands of years, or may have entered the aquifer quite recently.

How Do Wells Work?

Wells are drilled into a variety of different aquifer formations to supply water for many different uses. Specialized equipment is required to meet the construction standards for drilling a well, installing the well casing and screen, backfilling the well hole, cement grouting, well development and conducting pumping tests. Without proper well drilling and construction techniques, the reliability of the well and protection provided to the aquifer may be questionable.

In most cases, water is removed from an aquifer using pumps. Pumps use mechanical energy supplied by a drive motor or engine to force water toward the land surface. Removing water lowers the water level in the well.  The difference between the initial water level, or static water level, and the pumping water level causes water to move within the aquifer (Figure 4). Since the water level always is lowest in the well, water from the surrounding aquifer flows toward the well to replace the water being removed.

Figure 4. Water level changes associated with groundwater pumping.

When pumping starts in an unconfined aquifer, most of the water is removed from very near the well. With continued pumping, water is removed further from the well, lowering the water level at a greater distance from the well.

Drawdown decreases with the distance from the well until at some distance, the water level remains relatively unaffected by pumping.  Drawdown in the well continues to increase slightly with pumping due to aquifer or pump efficiency. After many hours of pumping the pumping water level nearly stabilizes. The resulting cone-like shape of the water surface is referred to as a cone of depression (Figure 4).

The size and shape of the cone of depression is determined by the aquifer materials and the amount of water being removed from the aquifer. For example, domestic wells generally pump for short periods of time at rates of 5 to 20 gallons per minute. This results in small, poorly defined cones of depression. Even low-yield aquifers often can be developed for domestic uses.

Irrigation and municipal wells typically have pumping rates that range from 100 to more than 300 gallons per minute, and operate for long periods. Aquifers must yield large volumes of water, and much larger and deeper cones of depression result. In some cases, the cone of depression may extend several hundred feet from the well and be up to 100 feet deep. Where there are many wells, like in river valleys or major extraction sites, cones of depression for adjacent wells can overlap, increasing the depth and size of each well's cone of depression.

Factors Affecting Groundwater Declines

Under natural conditions, a balance existed between the volume of water entering an aquifer and the volume of water being discharged from an aquifer. With the development of water wells, the natural balance between recharge rates and discharge rates was disrupted. The overall groundwater supply has been depleted due to increased discharge. Groundwater supplies also can be altered due to natural causes. Years of below-normal precipitation can alter the amount of water entering the aquifer. Likewise, seasonal and year-to-year differences in regional stream flow can cause fluctuation in localized groundwater levels. The combination of intensive pumping and several years of below-normal precipitation can accelerate the downward trend in water levels. This is true because below normal precipitation often results in decreased groundwater recharge. More important, below normal precipitation generally results in increased groundwater pumping.  In areas that rely on the groundwater resource, the community should consider the develop of water conservation measures and means of increasing or sustaining groundwater recharge by natural or induced means.

Definitions

• Aquifers are saturated underground formations that will yield water to a well or spring. The water in an aquifer is called groundwater and the two terms often are used interchangeably. A saturated formation that will not yield water in usable quantities is not considered an aquifer. Figure 2 shows various types of aquifers discussed here.
  
• Cone of depression is a depression in groundwater levels around a well or group of wells in response to groundwater withdrawal.
  
• Confined aquifers, or artesian aquifers, are saturated formations between low permeability materials that restrict movement of water into or out of the saturated zone. In some areas of the state, confined aquifers supply water for flowing wells (Figure 2). When pumping from confined aquifers, water levels often change rapidly over large areas. Water levels generally will recover to normal when pumping ceases.
  
• Drawdown is the difference between the level of water standing in a well under non-pumping and pumping conditions. It is the difference between the static water level and the pumping water level.
  
• Groundwater, sometimes written as "ground water," is water that occupies voids, cracks or other spaces between particles of clay, silt, sand, gravel or rock within the saturated zone of a geologic formation.
  
• Groundwater recharge is water that enters the soil and eventually reaches the saturated zone. In Pennsylvania, approximately 20 to 30  percent of the annual precipitation received typically reaches the groundwater (12 to 15 inches annually per unit area). Recharge varies considerably, depending on the total amount of water available (precipitation, lakes, streams, or irrigation), the type and amount of vegetation (grasses, trees, or row crops), the rate at which water enters the soil, the permeability and depth of the unsaturated zone, and the nature of the consolidated aquifer.
  
• Hydraulic conductivity is a term used to describe the rate (distance traveled per unit of time) water will move through soil or a saturated geologic formation. The rate groundwater travels is controlled by the type of material comprising an aquifer, the porosity of the material, the slope of the water table, and the degree to which existing pores are interconnected.
  
• Hydraulic gradient is the slope of the water surface. The hydraulic gradient indicates which direction groundwater will flow, and how rapidly. Water always will flow from higher water surface elevations to lower water surface elevations (Figure 1).
  
• Perched water tables occur when a low permeability material, located above the water table, blocks or intercepts the downward flow of water from the land surface. Water mounds up over the impermeable material creating a water table (Figure 3).
  
• Permeability is a property of porous soils indicating how rapidly water will be transmitted through soils toward the groundwater. Sand and gravel have high permeabilities while clay has low permeability.
  
• Porosity refers to the percentage of the formation that consists of open spaces or voids that could contain air or water. Porosity determines the amount of water that can be stored in a saturated zone. For example, a saturated zone 100 feet thick with a porosity of 20 percent could store an equivalent water depth of approximately 20 feet.
  
• Pumping water level is the water level in a well when the pump is operating and water is being removed.
  
• Saturated zones are the portions of a soil profile or geologic formation where all voids, spaces or cracks are filled with water. No air is present. Figure 3 locates the saturated zone within a typical geologic formation.
  
• Static water level is the water level in a well located in an unconfined aquifer when the pump is not operating and no water has been discharged from the well for several days.
  
• Unconfined aquifers often are called water table aquifers because they have no layers that restrict water movement into the saturated zone from above. The upper water surface of an unconfined aquifer is called the water table (Figure 3).
  
• Unsaturated zones are the soil and geologic materials located between the land surface and the saturated zone. The voids, spaces or cracks are filled with a combination of air and water (Figure 3).
  
• Water table is the level in the geologic formation below which all voids or cracks are saturated. The water table also can be thought of as the upper surface of the groundwater and top of the saturated zone for an unconfined aquifer.

Water Quality in Pennsylvania and Other States

See also the Glossary of Terms

For More information about the Environmental Quality Center, please contact:

 Attn: Mr. Brian Oram, Professional Geologist (PG)
Laboratory Director
Wilkes University
Environmental Engineering and Earth Sciences
PO Box 111
84 West South Street
Wilkes-Barre, PA 18766

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