Transcript CapuanoEnvGeolLec

Groundwater
Groundwater
• Water underground is not in underground
lakes and rivers but stored in pore and
fractures
Source of groundwater is infiltration
of 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 table
all pore spaces are
filled with water
:
Saturated
Zone
© 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
Unsaturated
Zone
Above the water table
water does not fill
most of the pore spaces
© 2005 Pearson Prentice Hall
Saturated
Zone
Soil-water
Zone
Unsaturated
Zone
Water Table
Saturated
Zone
© 2005 Pearson Prentice Hall
Water table changes seasonally
and daily
Wet Period
Dry Period
Level of water in lakes and rivers is
often the level of the surrounding
water table
1207
Unconfined aquifer
Water table
Confined aquifer
Potentiometric surface
Groundwater Recharge
Confined aquifer –
Recharge only at
the limited outcrop
area
Unconfined aquifer –
recharge everywhere
the aquifer occurs
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.
Mapping an aquifer
Elevations of the
Water Table
(head) at each
well
Map of Water
Table Surface –
from contours of
water level
elevation (head)
© 2005 Pearson Prentice Hall
Direction of groundwater flow
is perpendicular to the head
(groundwater elevation) contours
© 2005 Pearson Prentice Hall
Velocity of Groundwater Flow
• Average linear groundwater velocity
vx = K (dh/dl)
q
Velocity of Groundwater Flow
• Average linear groundwater velocity
vx = K (dh/dl)
q
K = hydraulic conductivity
q = 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
Confining
layers
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
http://www.cee.vt.edu/ewr/environmental/teach/gwprimer/amd/amd2.html
• 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.
Examples of Groundwater
pollutants
LNAPL (Light Non-Aqueous Phase Liquid)
• 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.
http://oceanworld.tamu.edu/resources/environment-ook/groundwaterremediation.html
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/environmentbook/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
Bacteria living in the aquifer degrade toxic contaminants
into less toxic compounds.
• 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
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.
New
American 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.