Groundwater

Groundwater

• Water underground is not in underground lakes and rivers but stored in pore and fractures

Source of groundwater is infiltration Source of groundwater is infiltration of rain or surface waterof rain or surface water

http://jnuenvis.nic.in/subject/freshwater/groundwater.htm

Water table

• Is the top of the zone of saturation where the water pressure is equal to atmospheric.– Because of this equal

pressure if a hole Is dug water will flow freely into that hole up to the top of the water table.

– water at the water table will flow freely into a well up to the height of the water table.

http://www.educaid.org.uk/gal_well.htm

Below the water tableall pore spaces are filled with water

:

SaturatedZone

© 2005 Pearson Prentice Hall

Unsaturated zone

• Above the water table, water in the unsaturated zone is under negative pressure (less than atmospheric)

• Water in this zone will not flow freely into a well.

• This zone can contain air

Above the water table water does not fillmost of the pore spaces

UnsaturatedZone

SaturatedZone

© 2005 Pearson Prentice Hall

© 2005 Pearson Prentice Hall

UnsaturatedZone

SaturatedZone

Soil-waterZone

Water TableWater Table Capillary fringe

Water table changes seasonally and daily

Wet PeriodDry Period

Level of water in lakes and rivers is often the level of the surrounding

water table

1207 Unconfined aquifer Water table

Confined aquiferPotentiometric surface

Unconfined aquifer –recharge everywhere the aquifer occurs

Confined aquifer – Recharge only at the limited outcrop area

Groundwater Recharge

Gravity controls groundwater flow.

• Groundwater flows from high head to low head.

Flow can be complex.

What is a water well?

• Hole drilled and lined with PVC or stainless steel pipe.

• Bottom 1 to 2 feet of the pipe is slotted for the water to flow in.

Hydraulic Head (h)

• The level that water rises in a well.

• In an unconfined aquifer it is the elevation of the water table.

Map of Water Map of Water Table Surface – Table Surface – from contours of from contours of water level water level elevation (head)elevation (head)

Elevations of the Elevations of the Water Table Water Table (head) at each (head) at each wellwell

© 2005 Pearson Prentice Hall

Mapping an aquifer

Direction of groundwater flowis perpendicular to the head

(groundwater elevation) contours

© 2005 Pearson Prentice Hall

Velocity of Groundwater Flow

• Average linear groundwater velocity

vx = K (dh/dl)

Velocity of Groundwater Flow

• Average linear groundwater velocity

vx = K (dh/dl)

K = hydraulic conductivity= porosity

dh/dl = hydraulic gradient- change in head with distance. Calculated from the hydrologic map.

The greater the hydraulic gradient (dh/dl) the faster the groundwater velocity.

Slow

Fast

Confininglayers

How long does it take a contaminant to travel.

• Travel time T = vx L vx = average linear velocity (previous slide)

L = distance of flow path

Importance of Groundwater

• Nearly 50% of the population of the US uses groundwater for its drinking water supply.

• 30 % of stream flow in the US is derived from groundwater discharging to the stream.

• Many aquifers are being mined – (that is withdrawal is greater than natural recharge)

• Reduced rainfall from global warming is predicted to increase the need for groundwater.

Groundwater Pollution

• Any degradation of water quality as measured by biological, chemical or physical criteria

• with respect to health or ecology– A pollutant is any substance in which an

identifiable excess is known to be harmful to a desirable living organisms.

http://www.cee.vt.edu/ewr/environmental/teach/gwprimer/amd/amd2.html

Examples of Groundwater pollutants

LNAPL (Light Non-Aqueous Phase Liquid)

http://oceanworld.tamu.edu/resources/environment-ook/groundwaterremediation.html

• Non-aqueous phase is organic liquid instead of water.– Common LNAPL – gasoline

• LNAPL floats on water at the top of the water table– Smears across the zone of water table fluctuation.

LUST (Leaky Underground Storage Tanks)usually LNAPL – gasoline or other fuel

DNAPL (Dense Non-Aqueous Phase Liquid)

• Sinks in water to the bottom of the aquifer

• Common DNAPL is dry-cleaning solvents

http://oceanworld.tamu.edu/resources/environment-ook/groundwaterremediation.html

Modern Municipal Landfills• Landfills are lined with clay or plastic.• A network of drains collects the leachate and

pumps it to the surface for treatment. • Monitor wells installed around the landfill to monitor

groundwater quality and to detect leakage.• Old landfills are serious pollution problem

http://oceanworld.tamu.edu/resources/environment-book/groundwatercontamination.html

Salt Water Intrusion

• Seawater – 35,000 mg/L NaCL

• Drinking water – <250 mg/L NaCL

Salt water intrusion at Long Island New York

Road Salt

http://www.cee.vt.edu/ewr/environmental/teach/gwprimer/roadsalt/roadsalt.html

Septic-tank sewage disposal• Used in areas lacking sewage treatment plants

for household waste• 30% of households in U.S. use septic tanks.• Must remove bacteria, organic compounds and

                                                                                                            

From Thurston County (Washington State) Public Health & Social Services Department web page on Inspecting Your Septic Tank.

How Septic tank systems work• Sewage flows to underground septic tank where solids

drop to the bottom and bacteria breaks it down.• The liquid leaks into the drain field and seeps into the

soil zone where it is purified by bacterial activity in the soil.

From Thurston County (Washington State) Public Health & Social Services Department web page on Inspecting Your Septic Tank.

• Failure of septic system can occur because of– Poor soil drainage– Times of high water table

• Sewage will poor onto surface and into shallow soil zone if system fails.– Can infiltrate into the groundwater and contaminate

the aquifer

• Contaminants released– Nutrients (nitrates/phosphates), metals, synthetic

organic compounds, virus and bacteria.

Groundwater remediation

• Many methods – these are just a few

• Sediment removal and treatment

• Pump and treat

• Vapor extraction

• Enhanced Bioremediation

Pump and Treat• Old preferred remediation method, now seldom used.• It is slow, taking decades to centuries to remove contamination

and often fails to remove all contamination– Some contaminants stick to soil and rock (they are

adsorbed) and they cannot easily be removed (desorbed). – Non-Aqueous Phase Liquids NAPLs cannot be removed.

• It is very costly.

From Environment Protection Agency

Vapor Extraction

• Used to remove volatile organic compounds

http://www.p2pays.org/ref/14/0_initiatives/init/winter98/success.htm

Enhanced Bioremediation

• Growth of the natural bacteria is stimulated by injecting nutrients and/or carbon compounds needed by the bacteria into the aquifer.

• The bacteria break down contaminants such as trichloroethylene TCE into non-toxic compounds such as carbon dioxide

http://www.erc.montana.edu/MultiCellStrat/01-Waiting_UMB/BioremedSchem.htm

Bacteria living in the aquifer degrade toxic contaminants into less toxic compounds.

Who owns the Groundwater?• English Rule (Absolute Ownership)

– Water can be pumped at will by landowner independent of effect on other users of the aquifer

• American Rule (Reasonable Use) – applied in areas were water supplies are limited – Applied in much of the western U.S. – Limits water use to what is reasonable and

beneficial (hard to determine)– Argue over what is beneficial use

• Is supplying water to a river more important than to crops?

– Safe yield calculated and used to regulate aquifers.

Texas groundwater law “Law of the biggest pump”

• Based on the English rule - a landowner has a right to take for use or sale all the water that he can capture from below his land – Once pumped, groundwater may be used or sold as

private property

• Creation of groundwater conservation districts is the only way to limit use.

                                                     

(Texas Groundwater Use,

Well head protection of aquifer recharge zones

http://www.geopanorama.rncan.gc.ca/h2o/okanagan/groundwater_e.php

Federal regulations protecting groundwater quality

Federal “Superfund” legislation

• The Comprehensive Environmental Response, Compensation, and Liability Act (CERCLA), 1980, commonly known as Superfund,

• This law created a tax on the chemical and petroleum industries to be used to clean up releases of hazardous substances that may endanger public health or the environment.

Superfund National Priorities List Sites

http://www.nationalatlas.gov/natlas/Natlasstart.asp

RCRA

• Resource Conservation and Recovery Act.

• It controls all solid waste disposal and encourages recycling and alternative energy sources.

NewAmerican Recovery and Reinvestment Act of 2009• $600 million to EPA to clean up land

contaminated by hazardous wastes.

• Part of Federal stimulus funding program to create jobs.