GROUNDWATER. KEY GROUNDWATER TERMS: §Pore §Porosity §Groundwater §Infiltration §Water Table §Discharge §Recharge §Aquifer §Well §Artesian Well.

GROUNDWATER

KEY GROUNDWATER TERMS:

PorePorosityGroundwater InfiltrationWater Table

DischargeRechargeAquiferWellArtesian Well

Earth's Water: Ground water

Ground water is the part of precipitation that seeps down through the soil until it reaches rock material that is saturated with water. Ground water slowly moves underground, generally at a downward angle (because of gravity), and may eventually seep into streams, lakes, and oceans.

Earth's Water: Ground water

Here is a simplified diagram showing how the ground is saturated below the water table (the purple area). The ground above the water table (the pink area) may be wet to a certain degree, but it does not stay saturated. The dirt and rock in this unsaturated zone contain air and some water and support the vegetation on the Earth. The saturated zone below the water table has water that fills the tiny spaces (pores) between rock particles and the cracks (fractures) of the rocks.

Permeability For water to move in an aquifer, the pores between rock materials and fractures in rock must be connected. If there is a good connection among pore spaces and fractures, water can move freely and we say that the rock is permeable. The capacity of rock material to transmit water is called permeability. Water moves through different materials at different rates - faster through gravel, slower through sand, and much slower through clay. Therefore, gravel is more permeable than sand, which is more permeable than clay.

Why is there ground water?

A couple of important factors are responsible for the existence of ground water:(1) Gravity Nothing surprising here - gravity pulls water

toward the center of the Earth. That means that water on the surface will try to seep into the ground below it.

Why is there ground water?A couple of important factors are responsible for the

existence of ground water: (2) The Rocks Below Our Feet

The rock below the Earth's surface is the bedrock. If all bedrock consisted of a dense material like solid granite, then even gravity would have a hard time pulling water downward. But Earth's bedrock consists of many types of rock, such as sandstone, granite, and limestone. Bedrocks have varying amounts of void spaces in them where ground water accumulates. Bedrock can also become broken and fractured, creating spaces that can fill with water. And some bedrock, such as limestone, are dissolved by water -- which results in large cavities that fill with water.

INFILTRATIONWater can infiltrate faster from the land surface into sandy soils than silty or clay soils. Water infiltrates into the soil and the unsaturated zone. The unsaturated zone occurs immediately below the land surface and contains both water and air in the pores and fractures in the rock materials. Water moves, or percolates, down through the unsaturated zone to the saturated zone. The saturated zone is where all the pores or fractures in rock materials are filled with water. The top of the saturated zone is called the water table.

Why is there ground water?

Try as it might, gravity doesn't pull water all the way to the center of the Earth. Deep in the bedrock there are rock layers made of dense material, such as granite, or material that water has a hard time penetrating, such as clay. These layers may be underneath the porous rock layers and, thus, act as a confining layer to retard the vertical movement of water. Since it is more difficult for the water to go any deeper, it tends to pool in the porous layers and flow in a more horizontal direction across the aquifer toward an exposed surface-water body, like a river.

Ground-water flow

Water is recharged to the ground-water system by percolation of water from precipitation and then flows to the stream through the ground-water system.

Why is there ground water?

Visualize it this way: get two sponges and lay one on top of the other. Pour water (precipitation) on top and it will seep through the top sponge downward into the bottom sponge. If you stopped adding water, the top sponge would dry up and, as the water dripped out of the bottom sponge, it would dry up too. Now, put a piece of plastic wrap between the sponges, creating your "confining layer" (making the bottom sponge an impermeable rock layer that is too dense to allow water to flow through it). Now when you pour water on the top sponge, the water will seep downward until it hits the plastic wrap. The top sponge will become saturated, and when the water hits the plastic wrap it won't be able to seep into the second sponge. Instead, it will start flowing sideways and come out at the edges of the sponge (horizontal flow of ground water). This happens in the earth all the time -- and it is an important part of the water cycle.

Why is there ground water?

In many places, if you looked at a vertical cross-section of the earth you would see that rock is laid down in layers, especially in areas of sedimentary rocks. Some layers have rocks that are more porous than others, and here water moves more freely (in a horizontal manner) through the earth. Sometimes when building a road, the layers are revealed by road cuts, and water can be seen seeping out through the exposed layers.

Ground-water flow

Water pumped from the ground-water system causes the water table to lower and alters the direction of ground-water movement. Some water that flowed to the stream no longer does so and some water may be drawn in from the stream into the ground-water system, thereby reducing the amount of streamflow.

Water Levels in WellsGround-water users would find life easier if the water level in

the aquifer that supplied their well always stayed the same. Seasonal variations in rainfall and the occasional drought affect the "height" of the underground water level. If a well is pumped at a faster rate than the aquifer around it is recharges by precipitation or other underground flow, then water levels around the well can be lowered. The water level in a well can also be lowered if other wells near it are withdrawing too much water. When water levels drop below the levels of the pump intakes, then wells will begin to pump air - they will "go dry."

Ground-water flow

Contaminants introduced at the land surface may infiltrate to the water table and flow towards a point of discharge, either the well or the stream. (Not shown, but also important, is the potential movement of contaminants from the stream into the ground-water system.)

Ground-water flow

Water-level declines may affect the environment for plants and animals. For example, plants in the riparian zone that grew because of the close proximity of the water table to the land surface may not survive as the depth to water increases. The environment for fish and other aquatic species also may be altered as the stream level drops.

The left-hand side of the picture shows conditions in a ground-water discharge area. Discharge areas are where ground water flows to the land surface or to surface-water bodies such as the pond and river shown on the poster. Ground water also can discharge by pumping a well. The titles of these ground-water discharges are yellow.

This picture depicts an unconfined or water-table aquifer. An unconfined aquifer is an aquifer where the water surface - water table - is free to rise and decline as water moves from recharge areas to discharge areas. Recharge areas are places where an aquifer receives its water. The recharge sources shown are a reservoir, a stream, and precipitation. Their titles are red. The arrows represent the movement of water from the recharge sources to the discharge areas. The large circle on the right-hand side of the poster is a

representation of how recharge occurs from precipitation.

This picture depicts an unconfined or water-table aquifer. An unconfined aquifer is an aquifer where the water surface - water table - is free to rise and decline as water moves from recharge areas to discharge areas. Recharge areas are places where an aquifer receives its water. The recharge sources shown are a reservoir, a stream, and precipitation. Their titles are red. The arrows represent the movement of water from the recharge sources to the discharge areas. The large circle on the right-hand side of the poster is a representation of how recharge

occurs from precipitation.

Recharge Areas Recharge is the addition of water to the ground-water system. The recharge of freshwater begins as precipitation. Precipitation occurs in several forms, including rain, snow, and hail, but only rain is displayed on the poster. Some of the rain infiltrates into the soil. If the rate of the rainfall exceeds the rate of infiltration, surface water will flow over the land surface to surface-water bodies such as rivers and streams.

RECHARGING GROUNDWATERBecause surface-water and ground-water systems are connected, surface water can recharge ground water. Aquifers can obtain water from such surface-water bodies as reservoirs and streams when and where the water table is lower than the surface-water body. Recharge areas usually are higher in elevation than discharge areas.

DISCHARGE OF GROUNDWATER

Places where ground water flows from aquifers to springs, seeps, wetlands, ponds, or streams are called ground-water discharge areas. Ground-water discharge to these natural areas occurs when the water surface of the aquifer (water table on the poster) is at or above the elevation of the discharge area (river and pond on the poster). Surface-water and ground-water systems are interconnected. The flow of most streams is sustained by ground water seeping into the stream. The water surfaces of many ponds and wetlands are an extension of the local ground-water table. Springs occur where ground water flows from an aquifer to the land surface.

DISCHARGE BY WELLSGround water can be brought to the land surface by pumping from a well. A well is an opening that has been drilled or dug into an aquifer below the water table. Water from the aquifer flows into this opening to replace water removed by pumping water from the well. The water table slopes from areas of recharge to discharge areas like rivers, ponds, wells, and springs.

Ground-water aquifers

One of our most valuable resources is the water beneath our feet - something you can't see and may not even know is there! As you may have read, most of the void spaces in the rocks below the water table are filled with water. But rocks have different porosity and permeability characteristics, which means that water does not move around the same way in all rocks.

Ground-water aquifers When a water-bearing rock readily transmits

water to wells and springs, they are called aquifers. Wells can be drilled into the aquifers and water can be pumped out. Precipitation eventually adds water (recharge) into the porous rock of the aquifer. The rate of recharge is not the same for all aquifers, though, and that must be considered when pumping water from a well. Pumping too much water too fast draws down the water in the aquifer and eventually causes a well to yield less and less water and even run dry. In fact, pumping your well too fast can even cause your neighbor's well to run dry if you both are pumping from the same aquifer.

Ground-water aquifers In the diagram below, you can see how the ground below the water table (the

blue area) is saturated with water. The "unsaturated zone" above the water table (the greenish area) still contains water (after all, plants' roots live in this area), but it is not totally saturated with water. You can see this in the two drawings at the bottom of the diagram, which show a close-up of how water is stored in between underground rock particles.

Ground-water aquifers Sometimes the porous rock layers become

tilted in the earth. There might be a confining layer of less porous rock both above and below the porous layer. This is an example of a confined aquifer. In this case, the rocks surrounding the aquifer confines the pressure in the porous rock and its water. If a well is drilled into this "pressurized" aquifer, the internal pressure might (depending on the ability of the rock to transport water) be enough to push the water up the well and up to the surface without the aid of a pump, sometimes completely out of the well. This type of well is called artesian. The pressure of water from an artesian well can be quite dramatic .

Ground-water aquifers Maybe you've heard advertisements by water

companies wanting to sell you "artesian-well drinking water." Is this water different from other bottled water taken from springs?

The water may not be different, but it comes to the earth's surface a bit differently. A flowing well has water that comes up to the surface because of internal pressure in the underground aquifer containing the water. An artesian aquifer is confined by rock layers that restrict water flow, resulting in an aquifer that is "pressurized." Water is virtually squeezed to the surface by underground pressure. This picture shows how strong artesian pressure can be!

Ground water: Wells

There's a good chance that the average Joe who had to dig a well in ancient Egypt probably did the work with his hands, a shovel, and a bucket. He would have kept digging until he reached the water table and water filled the bottom of the hole. Some wells are still dug by hand today, but more modern methods are available. It's still a dirty job, though !

Ground water: Wells

Types of wells

Digging a well by hand is becoming outdated today (would YOU want to do it?). Modern wells are more often drilled by a truck mounted drilling rig. Still, there are many ways to put in a well -- here are some of the common methods.

Ground water: Wells

Dug wells

Hacking at the ground with a pick and shovel is one way to dig a well. If the ground is soft and the water table is shallow, then dug wells can work. They are often lined with stones to prevent them from collapsing. They cannot be dug much deeper than the water table -- just as you cannot dig a hole very deep when you are at the beach... it keeps filling up with water!

Ground water: WellsDriven wells

Driven wells are still common today. They are built by driving a small-diameter pipe into soft earth, such as sand or gravel. A screen is usually attached to the bottom of the pipe to filter out sand and other particles. Problems? They can only tap shallow water, and because the source of the water is so close to the surface, contamination from surface pollutants can occur.

Ground water: Wells

Drilled wells

Most modern wells are drilled, which requires a fairly complicated and expensive drill rig. Drill rigs are often mounted on big trucks. They use rotary drill bits that chew away at the rock, percussion bits that smash the rock, or, if the ground is soft, large auger bits. Drilled wells can be drilled more than 1,000 feet deep. Often a pump is placed at the bottom to push water up to the surface.

The messy job of drilling a well

Now, not all wells are drilled in an area this wet (most are drilled in rock). Notice the big screw used to do the drilling. In the left-side picture, the driller is adding another length of pipe to extend the length of the drilling screw. This drilling apparatus only works when drilling shallow wells in soft soil. And drilling in soft soil is much quicker and easier than drilling through solid granite, where a rock-eating drill bit must be used.