Ground Water Basics
Section 1: Ground Water Basics
It is important to understand basic concepts about ground water. These concepts are important because they determine the quality and chemical characteristics of the water used, the amount of water available, and how protected the water is from chemical or biological contamination. Ground water concepts include an understanding of the hydrologic cycle and how it affects ground water sources, different types of aquifers used to obtain water for public water supply systems, some characteristics of ground water movement, and basic water chemistry. These concepts are introduced briefly in this section.
The Hydrologic Cycle
Figure 1.1 illustrates the Hydrologic Cycle. This cycle is the overall exchange of water between the earth and the atmosphere.
The Earth’s water is always in movement, and the water cycle, also known as the hydrologic cycle, describes the continuous movement of water on, above, and below the surface of the Earth. Since the water cycle is truly a “cycle,” there is no beginning or end. Water is not gained or lost. This means all the water we have now is all we’ve ever had, and all we’re ever going to have. When we look at all the water on earth, most of it (~97%) is in oceans and salty. Icecaps and glaciers make up 2% of earth’s water supply. Therefore, there is only 1% or less fresh water available on the surface and in the ground to use.
Water changes states among liquid, vapor, and solid at various places in the water cycle continuously. A drop of water:
• Evaporates as water vapor from the heat of the sun or other sources
• Condenses in the atmosphere
• Falls to the earth in the form of precipitation
• Leaches or infiltrates into the ground entering aquifers
• Recharges surface water
• Evaporates again beginning the cycle all over
When rain, snow or other precipitation reaches the land’s surface some of the water renews surface waters such as rivers, lakes, streams, and oceans; some percolates into soils to be absorbed by plant roots; and some evaporates back into the atmosphere from the soil surface, from plant leaves (called evapotranspiration) and from surface water.
Water in the atmosphere accumulates, eventually forming clouds and more precipitation. The rest of the water infiltrates the ground to become ground water.
Most ground water is simply water filling spaces between small grains of rock, or fractures and fissures in solid rock. It may also occur in solution channels which have been formed in limestone deposits. Underground lakes or streams only occur in areas of cavernous limestone or in tunnels from lava flows.
Most ground water moves beneath the land surface. How fast the water moves depends on the nature of the underground rock layer the water must travel through. Most ground water eventually discharges into springs, rivers, the sea, or other surface waters. This discharge may occur within a few days of the water entering the ground or it may take several thousands of years. The movement of ground water is discussed further in Section 1.3.
This cycle of water through precipitation and evaporation or evapotranspiration is called the hydrologic cycle. It is an important concept because it shows how the amount of ground water available to a water well is influenced by the amount of precipitation, percolation and underground water flow which occurs in a given area.
1.2 Types of Aquifers
If you were able to look at a slice of the earth, from the ground surface to two miles in depth most would appear as irregular layers of different colored and textured material. The layers are a result of geologic activity which has occurred since Earth was formed.
Water beneath the land surface occurs in two different zones- the unsaturated zone and the saturated zone.
The unsaturated zone is the area immediately beneath the surface and above the water table that contains both water and air. Water collects in the fractures, intergranular pores, and caverns in some of the rock layers (see Figure 1.2.a). Any water in the unsaturated zone is not officially considered groundwater.
The saturated zone (also known as the phreatic zone) is the area beneath the unsaturated zone where all interconnected openings contain water. The top of the saturated zone is referred to as the water table and any water beneath is officially considered groundwater.
An aquifer, a term that literally means “water bearer”, is a layer that will yield ground water in useful quantities to a well or spring. In an aquifer, all of the voids or openings between the rocks are filled with water.
The saturation zone of an aquifer is composed of either consolidated or unconsolidated materials.
Unconsolidated deposits are composed of loose rock or mineral particles of varying sizes. Examples include clay, silt, sand, and gravel. Alluvial deposits such as stream beds, glacial drifts, and lake deposits are examples of unconsolidated materials.
Consolidated deposits are rocks formed by mineral particles combining from heat and pressure or chemical reactions. They include sedimentary (previously unconsolidated) rocks, such as limestone, dolomite, shale, and sandstone; igneous (formed from molten) rocks, such as granite and basalt; and metamorphic (highly compressed) rocks, such as quartzite and gneiss. Some limestones and sandstones may be only partially cemented and are called semi-consolidated deposits.
Aquifers are classified into two types – confined and unconfined aquifers. They are illustrated in Figure 1.2b.
Unconfined aquifers are also commonly called water table aquifers. They are not confined by an upper layer of rock or clay and are essentially at atmospheric pressure. They are also often shallow, unconsolidated materials composed of sands and gravels. The water level in these types of aquifers will rise and fall with seasonal changes of recharge rate. The water level in a well located in a water table aquifer will not rise above the initial point of encounter.
Confined aquifers are commonly called artesian aquifers. They are confined or trapped between impermeable layers (impervious strata) of rock or clay. It is difficult for water or other materials to flow through this layer. Confined aquifers generally contain water that is under pressure greater than atmospheric pressure. Because as the water flows between these strata it becomes confined and as recharge continues, the water backs up, creating pressurized conditions in the aquifer. If the pressure is great enough, a well drilled into the aquifer and open to the atmosphere, will cause the water to rise above the initial point of encounter or water table. The maximum level that the water in the well will rise to is known as the potentiometric surface, or potential water level. If this is higher than the top of the well, the well will overflow and create a free-flowing artesian well. If not, it is called a non-flowing artesian well.
Aquifers can range from several acres to thousands of miles wide and from a few feet to hundreds of feet thick. In some areas, the ground water table is less than 10 feet below ground surface. In other areas, systems must rely on wells drilled more than 1,000 feet deep.
Wells constructed in relatively shallow alluvial aquifers (less than 50 feet deep) have a greater potential for biological and chemical contamination than do wells constructed in deep or confined aquifers.
1.3 Ground Water Movement
1.3.1 Effect of Aquifer Characteristics
The ability of an aquifer to receive, store, or transmit water or contaminants depends on the characteristics of the aquifer. This includes the characteristics of the confining layers of a confined aquifer or the overlying unsaturated zone of an unconfined aquifer.
Hydraulic gradient, potentiometric surface, porosity and hydraulic conductivity are important concepts which determine ground water movement. Each of these are discussed briefly.
Ground water generally moves quite slowly – from about several feet per day to several feet per year – although it can move much faster in very permeable soils or in certain geologic formations, such as cavernous limestone. Gravity and pressure differences are also important factors in ground water movement. The direction and speed that ground water and accompanying contaminants flow are to a large degree determined by the hydraulic gradient.
The hydraulic gradient is the slope of a water table, or in a confined aquifer, the slope of the potentiometric surface (the surface defined by the elevation to which water rises in a well open to the atmosphere – also called the piezometric surface). In many cases in unconfined aquifers, the hydraulic gradient parallels the slope of the land surface.
Porosity refers to the amount of space between soil or rock particles and reflects the ability of a material to store water. Soils are said to be porous when the percentage of pore space they contain is large (such as a soil with porosity of 55 percent).
Hydraulic conductivity is a term that describes the ease with which water can pass through deposits and thus transmit water to a well. Generally, the larger the pores, the more permeable the material, and the more easily water can pass through.
Coarse, sandy soils are quite porous and permeable, and thus ground water generally moves through them rapidly. Bedrock is often not very porous, but may contain large fractures through which ground water passes quickly. Clay soils are quite porous but not very permeable and water moves through clay very slowly.
Fractures in consolidated rock play an important role in ground water movement. The fractures allow water to flow through them in many directions. This makes it difficult to predict and measure ground water flow in these formations.
Aquifers composed of limestone and other water-soluble rocks often have fractures which have been widened by physical or chemical erosion to form sinkholes, caves, tunnels or solution channels. Water and any accompanying contaminants often move very rapidly in these aquifers.
1.3.2 Effect of Well Pumping
As shown in Figure 1.3.2a, well pumping alters the natural movement of ground water. The depth from ground level to the top of the aquifer in a well not being pumped is called the static water level. When pumped, ground water around the well is pulled down and into the well. The depth from ground surface to the water level in the well during stabilized water withdrawal is called the pumping water level. The difference between the static water level and the pumping water level is called the drawdown. The greater the discharge of water from a well, the greater the drawdown experienced by the well.
The underground area affected by pumping is in the shape of an inverted cone and is called the cone of depression; the same area as viewed on a map of the ground surface is known as the zone of influence. The cone of depression may extend from a few feet to many miles, depending on local hydrogeological conditions. Locating additional wells within this zone of influence will cause the wells to compete with each other for water (Figure 1.3.2b).
The zone of contribution is the area of the aquifer that recharges the well. The zone of contribution can also be altered by increased or decreased pumping. Any contaminants located in the zone of contribution might be drawn into the well along with the water.
1.3.3 Recharge of Aquifers
Replenishment of aquifers is known as recharge. Unconfined aquifers are recharged by precipitation percolating down from the land’s surface. Confined aquifers are generally recharged where the aquifer materials are exposed at the land’s surface called an outcrop.
In many areas, surface waters also provide ground water recharge under certain conditions. When a surface water loses water to the adjacent aquifer, the stream is called a losing stream. When the opposite occurs and water flows from the ground water to the stream, it is called a gaining stream.
Properly identifying the recharge area of an aquifer is critical because the introduction of contaminants within the recharge area can cause aquifer contamination. Knowing if the aquifer is influenced by a gaining or losing stream helps identify periods when biological contaminants from the surface water might reach the well water. Periods of gaining and losing stream flow may change seasonally, depending on the level of the ground water table. Monitoring the surface water level or stream stage and comparing it to the static water level in the well can give an indication of the direction of water flow.
Groundwater Wells
There are both underground and above ground components to a well. These components channel the water into a pipe, lift the water up from below the ground, and allow the water to flow above ground and into a distribution system. First we will look at the above ground surface features of a well, these are:
• Well casing vent
• Gravel tube
• Sounding tube
• Pump pedestal
• Pump motor base
• Sampling taps
• Air release vacuum breaker valve
• Drain line (Discharge to waste)
Well Casing Vent
When a well is in operation, the water in the well starts to rise in the column pipe of the well. However, there is an air space between the water and where the water is being pumped. This “air” needs to be released from the column pipe. The well casing vent provides this release. It prevents vacuum conditions inside a well by admitting air during the drawdown period when the well pump is first started and it prevents pressure buildup inside a well casing during by allowing excess air to escape during the well recovery period after the well pump shuts off.
Gravel Tube
When a well is drilled, a pipe is lowered into the ground. Water enters in to this pipe from the surrounding soil. We want water to enter the well, but we do not want the surrounding soil (sand) to enter the well. Gravel is used as a barrier between the surround soil outside of the casing and the water entering the casing. A gravel tube is installed to monitor the level of gravel within the well and to add additional gravel as necessary.
Sounding Tube
A sounding tube is a tube (pipe) that is installed into the well casing to allow for the measurement of groundwater levels within a well. There are several methods of measuring the depth to groundwater, which include automatic measuring devices and manual methods. The simplest means of measuring the level of groundwater below ground surface (bgs) is with a cable lowered into a sounding tube, which has markings identifying distances in various increments (inches.) The cable is connected to a light or signal and when the bottom of the cable touches the water a sound or light signal will occur. Other means of measuring the depth to groundwater include electronic transducers and airline water level measuring. Airline tube measuring is accomplished by lowering a tube in to the well, supplying air pressure to the tube and measuring the pressure with a gauge. Each pounds per square inch of pressure equates to 2.31 feet. In addition to measuring groundwater levels, sounding tubes can be used to add chlorine or other disinfecting or treatment chemical agents in to a well.
Pump Pedestal
The well casing vent, gravel tube, and sounding tube are encased within a concrete pump pedestal. A pump pedestal is designed to support the entire weight of the pumping unit. The concrete should be continuously poured with steel reinforcement in order to minimize cracks and breaches in the concrete. Fractures in the concrete could expose the inside of the well to surface water and other potential contaminants. A pump pedestal must also be a minimum of 18 inches above the finished elevations of the well pad.
Pump Motor Base
At the point where the motor rests on the pedestal, it must have a watertight seal. This seal is commonly provided by a neoprene rubber gasket. This establishes a barrier seal between the motor base and the concrete pump pedestal.
Sampling Taps
Drain Line (Discharge to waste)
These are only some of the more common above ground components of a well. There are other features, but they will not be covered in this text. The following items are a list of some below ground features of a well.
Casing
A casing is an impervious durable pipe placed in a well to prevent the walls of the surrounding soil from caving in on the well. A casing is also designed to seal off water from draining into a well from specific depths.
Conductor casing
When a well is drilled the upper portions of the surrounding soil tends to be loose and a conductor casing is used to support the drilling operations. It is a tubular structure between the drilled hole and the inner casing completed in the upper portion of a well.
Annular seal
Another means to prevent surface water from entering a well is the annular seal. An annular or sanitary seal is a cement grout installed between the well casing and the conductor casing, the space between the conductor casing and the borehole, or the space between the well casing and the borehole depending on the well. This seal also protects the well casing or conductor casing against exterior corrosion. Three types of grout are used; neat cement grout, sand-cement grout, and bentonite clay.
Intake section
Water enters a well through an intake section. This portion of a well is designed to allow water to enter the well casing and prevent the surrounding soil from entering. The following items are general characteristics of a properly designed intake section.
• Non clogging slots/screen
• Resistant to corrosion
• Sufficient collapse strength
• Resistant to encrusting
• Low head loss
• Prevent sand from entering
There are five common types of intake sections of a well. These include:
• Well screens
• Mill-cut slots
• Formed louvers
• Torch-cut/chisel-cut slots
• Mechanical slots
Well screens are generally constructed of stainless steel, monel metal, special nickel alloy, silicon red brass, red brass, special alloy steel, and plastic. They are broken into three main categories which include continuous slots, bar, and wire-wound screens.
Mill-cut slots are commonly made of the same type and diameter as the casing. The openings are machine milled (cut) into the wall of the casing pipe parallel to the axis of the casing and uniformly spaced around the casing pipe.
Formed louvers are machined horizontal to the axis of the casing with the openings facing downward. They are shaped to create an upward flow as the water enters a well and they are placed together in vertical rows.
Torch-cut slots are not very common. They are relatively simple to create, but very difficult to control the size of the openings. This has a tendency to produce excessive quantities of sand. Mechanical slots are usually slotted after the well has been drilled. The openings are made opposite the water-bearing formations by means of a casing perforator tool lowered into the well and activated from the drill rig. One main downside of this process is the openings cannot be closely spaced.
1.4 Brief Chemistry of Water
Ground water nearly always contains more minerals than nearby surface water. The main reason for the difference is the slow movement of ground water through soil and the unsaturated zone to underground aquifers. During this movement, ground water becomes slightly acidic as it flows through soils. This acidic solution then dissolves some of the rock through which it travels and picks-up carbonates, iron, manganese, sulfate and other compounds. When tapped by a well, the water may no longer be acidic but may have picked-up sufficient other minerals to have changed its chemical characteristics dramatically.
The slow passage of water through soil and sediment results in some filtration of particulate matter and adsorption of some chemical compounds onto clay minerals. This may reduce the amount of chemical contaminants in ground water and provide some degree of protection against contamination by disease causing organisms from surface water. There are a few significant chemical characteristics of water that are important to know. They may or may not cause problems, depending on the amount present. They include the following:
Hardness – Carbonates and sulfates of calcium and magnesium cause hardness, as do sulfate, chloride and nitrate. Very hard water inhibits lathering by soap and can build up as scale in hot water piping and water heaters.
Alkalinity – Alkalinity is a measure of water’s ability to neutralize acids and is due primarily to the presence of bicarbonates.
pH – pH is a measure of the hydrogen ion concentration of water. It determines if water is acidic, basic or neutral. Even slightly acidic water may be corrosive to pipes, tanks, and home plumbing.
Iron (Fe) – A high concentration of iron causes reddish-brown stains on fixtures and laundry. It may also cause a bad taste and odor in water when associated with growth of iron bacteria. It may be dissolved in ground water and not be evident until oxidized to its insoluble form by exposure to air or an oxidant or disinfectant such as chlorine.
Manganese (Mn) – High concentrations of manganese cause brownish to black stains. Like iron, it may not be apparent until the water has been exposed to oxygen or a disinfectant.
Sulfide – Hydrogen sulfide gas has a distinctive smell of rotten eggs. Depending on the water pH, temperature and hydrogen sulfide concentration, it reacts with chlorine to form sulfuric acid and elemental sulfur – a fine white powder with a bad odor.
Sodium – Sodium is a component of table salt. It may make the water taste bad and can be a health risk for people with heart problems.
Radioactivity – Radioactivity in the form of radium and uranium naturally occurs in ground water in some parts of the U.S. Radon gas is radioactive and has been found in many states. Radioactivity is a concern because of its cancer-causing characteristics.
Nitrate – Nitrate and nitrite occur in some ground water and can cause a health risk for young children and pregnant mothers. These chemicals may interfere with the ability of blood to carry oxygen through the body.
Physical characteristics also affect how water will be used. Important physical characteristics include the following:
Turbidity – Turbidity is a measurement of the light-reflecting properties of water. It is used to indicate the relative amount of suspended particles in water – those which reflect light in the turbidimeter. It is required to be monitored in all surface water systems and some ground water systems suspected of being directly influenced by surface water.
Color
Even pure water is not colorless, but has a slight blue tint to it. Color in water is caused by mineral and organic matter, and a brown shade in water often comes from rust in the water pipes. Organic matter and most contaminants are usually removed by water-supply systems, the plus side is that the water you drink likely contains a number of dissolved minerals that are beneficial for human health. And, if you have ever drunk “pure” water, such as distilled or deionized water, you would have noticed that it tasted “flat”. Most people prefer water with dissolved minerals, although they still want it to be clear.
Temperature – Ground water sources typically have constant temperatures, although some may be warmer than others. Temperature is a useful tool for determining if ground water is directly influenced by surface water.
Taste and Odor – These characteristics are determined by the physical and chemical content of water. However, most contaminants do not impart either and cannot be detected by just smelling or looking at a glass of water.
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