Groundwater Hydrology

What is Groundwater?

- Water that comes from the ground

- Water beneath the surface of the earth which saturates the pores and fractures of

sand, gravel, and rock formations.

G

R

O

U

N

D

W

A

T

E

RAgriculture

G

R

O

U

N

D

W

A

T

E

RIndustry

G

R

O

U

N

D

W

A

T

E

RDrinking

• Groundwater is stored in, and moves slowly through, moderately to highly permeable rocks called aquifers.

• Aquifers literally carry water underground. An aquifer may be a layer of gravel or sand, a layer of sandstone or cavernous limestone, a rubbly top or base of lava flows, or even a large body of massive rock, such as fractured granite, that has sizable cracks and fissures.

• In terms of storage at any one instant in time, groundwater is the largest single supply of fresh water available for use by humans.

An estimated one million cubic miles of the world's groundwater is stored within one-half mile of the land surface.

Only a fraction of this groundwater, however, can be practicably tapped and made available on a perennial basis through wells and springs.

The amount of groundwater in storage is more than 30 times greater than the nearly 30,000 cubic-miles volume in all the fresh-water lakes and more than the 300 cubic miles of water in all the world's streams at any given time.

http://www.eoearth.org/image/Fresh_water_storage_on_earth.gif

How does groundwater occur?

• Groundwater is replenished by precipitationand, depending on the local climate andgeology, is unevenly distributed in bothquantity and quality.

• When rain falls or snow melts, some of thewater evaporates, some is transpired byplants, some flows overland and collects instreams, and some infiltrates into the pores orcracks of the soil and rocks.

• The first water that enters the soil replaceswater that has been evaporated or used byplants during a preceding dry period.

• Between the land surface and the aquiferwater is the unsaturated zone. In thisunsaturated zone, there usually is at least alittle water, mostly in smaller openings of thesoil and rock; the larger openings usuallycontain air instead of water.

• After a significant rain, the zone may bealmost saturated; after a long dry spell, it maybe almost dry. Some water is held in theunsaturated zone by molecular attraction, andit will not flow toward or enter a well.

http://www.eoearth.org/image/How_groundwater_occurs.gif

All geologic material beneath the earth’s surfaceis either a potential aquifer or a confining bed.An aquifer is also a saturated geologic formationthat will yield a usable quantity of water to awell or spring. A confining bed is a geologic unitwhich is relatively impermeable and does notyield usable quantities of water. They restrict themovement of ground water into and out ofadjacent aquifer.

Recharge is the process by which ground water isreplenished. A recharge area is where water fromprecipitation is transmitted downward to anaquifer.

Most areas, unless composed of solid rock orcovered by development, allow a certainpercentage of total precipitation to reach thewater table. However, in some areas moreprecipitation will infiltrate than in others. Areaswhich transmit the most precipitation are oftenreferred to as "high" or "critical" recharge areas.

How much water infiltrates depends on vegetationcover, slope, soil composition, depth to the watertable, the presence or absence of confining bedsand other factors. Recharge is promoted by naturalvegetation cover, flat topography, permeable soils,a deep water table and the absence of confiningbeds.

AQUIFER

Confined Aquifer Unconfined Aquifer

- Overlain by a confining bed -Has no confining bed

- Are recharged through cracks in impermeable layer

- Open to infiltration from surface

The water level in a confined aquifer does not rise and fall freely because it is bounded by the confining bed. Being bounded causes the water to become pressurized. In some cases, the pressure in a confined aquifer is sufficient for a well to spout water several feet above the ground. Such wells are calledflowing artesian wells. Confined aquifers are also sometimes called artesian aquifers.

Discharge areas are the opposite of rechargeareas. They are the locations at which groundwater leaves the aquifer and flows to the surface.

Ground water discharge occurs where the watertable or potentiometric surface intersects theland surface, or when we apply mechanical workto pump out the water (wells). Where thishappens, springs or seeps are found. Springs andseeps may flow into fresh water bodies, such aslakes or streams, or they may flow into saltwaterbodies.

A water well is an excavation or structurecreated in the ground, by digging, or drillingto access water in underground aquifers.

The four most important parts of a water well

Casing is used to maintain anopen access in the earth whilenot allowing any entrance orleakage into the well from thesurrounding formations. Themost popular materials used forcasing are black steel,galvanized steel, PVC pipe andconcrete pipe.

Grout is a sealant that is used to fill inthe spaces around the outside of thewell. It protects the well against theintrusion of contaminants. A groutmixture can be made of cement, orconcrete.

Screen keeps sand and gravelout of the well while allowinggroundwater and water fromformations to enter into thewell. The most popular screenused are the stainless steel andslotted PVC pipe. Screen isused when wells are drilledinto unconsolidated materials.

Gravel pack is placedaround the outside of thescreen to prevent sandfrom entering the well orclogging the screen and tostabilize the well assembly.

When water is withdrawn from a well, its water leveldrops. When the water level falls below the waterlevel of the surrounding aquifer, ground water flowsinto the well. The rate of inflow increases until itequals the rate of withdrawal.

The movement of water from an aquifer into a wellalters the surface of the aquifer around the well. Itforms what is called a cone of depression. A cone ofdepression is a funnel-shaped drop in the aquifer'ssurface. The well itself penetrates the bottom of thecone. Within a cone of depression, all ground waterflows to the well.

As ground water flows downwards in an aquifer, its uppersurface slopes in the direction of flow. This slope is knownas the hydraulic gradient and is determined by measuringthe water elevation in wells tapping the aquifer.

For confined aquifers, the hydraulic gradient is the slope ofthe potentiometric surface. For unconfined aquifers, it isthe slope of the water table.

A well can easily be contaminated if it is notproperly constructed or if toxic materials arereleased into the well. Toxic material spilled ordumped near a well can leach into the aquiferand contaminate the groundwater drawn fromthat well.

Contaminated wells used for drinking water areespecially dangerous. Wells can be tested tosee what chemicals, pathogens and othercontaminants may be in the well and if they arepresent in dangerous quantities.

Things you can do to protect your groundwater and water well:

1. Maintain your well and test the water quality annually.

2. Keep household chemicals, paint and motor oil away from your welland dispose of them properly by taking them to a recycling center orhousehold hazardous waste collection site.

3. Limit your use of pesticides and fertilizers.

4. Install a well cap and keep it clear of leaves, mulch, dirt, snow andother materials.

5. Use caution when mowing around your well so you don't damage thewell casing.

6. Practice water conservation measures in your home and install lowwater use appliances.

7. Learn more about well testing.

Groundwater Treatment Technology

Extraction, Treatment, Reinjection

Re-circulating Well Technology

Natural Attenuation

• Natural attenuation refers to the strategy ofallowing natural processes to reducecontaminant concentrations to acceptablelevels. Natural attenuation involves physical,chemical and biological processes which act toreduce the mass, toxicity, and mobility ofsubsurface contamination These processes arealways occurring and in many cases mayreduce risk to human health and theenvironment to acceptable levels.

end