Groundwater and surface water: Groundwater basics a single...
Transcript of Groundwater and surface water: Groundwater basics a single...
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Arizona Water Issues © 2010 - The University of Arizona – HWR203
Groundwater and surface water:a single resourceSanta Cruz River, Tucson
1942 1989
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Groundwater basics
Groundwater is water found within the pore spaces of geologic material beneath the surface of the Earth. It exists in saturated layers of sands and gravels, in certain types of clay material, and in cracks within crystalline rock. -Cech, 2002
Are all pore spaces created equal?
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Porosity in different sediments. A. A porosity of 30 percent in a reasonably well-sorted sediment. B. A porosity of 15 percent in a poorly sorted sediment in which fine grains fill spaces between larger grains. C. Reduction in porosity in an otherwise very porous sediment due to cement that binds grains together. http://www.usd.edu/esci/figures/BluePlanet.html
http://www.usd.edu/esci/figures/158406.JPG
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A B C
D E F
Pore Spaces
Typical grain packing showing: A and B) well sorted, well rounded grains; C) poorly sorted, well rounded grains; D) moderately well sorted, angular grains; E) platy uncompressed grains; and F) platy compressed grains . –Ferre, 2005.
Clay
Sand Gravel
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• Typically, the water table separates the zone of aeration (vadose zone) from the saturated zone and fluctuates in level with seasonal changes in precipitation. • Corresponding fluctuations are seen in the water level in wells that penetrate the water table. • Lakes, marshes, and streams occur where the water table intersects the land surface. • In shape, the water table is a subdued imitation of the overlying land surface.
http://www.usd.edu/esci/figures/158405.JPG
http://www.usd.edu/esci/figures/BluePlanet.html
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Simplified View
Components of subsurface water:
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Which way willwater flow?
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Groundwater Recharge
Precipitation and surface water infiltrate below ground until intercepted by plant roots or slowed down by a less permeable material such as clay, shale or bedrock.
This naturally occurring process of downward water migration that reaches the groundwater table is called groundwater recharge.
Cech, 2002
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Aquifers
An aquifer is a water-bearing geologic formation that can store and yield
usable amounts of water.
Latin orgin:“aqua” = water
“ferre” = to bear or carry
-Cech, 2002
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Aquifer terms
• Unconfined aquifer = Aquifer with no confining bed of material between the saturated zone and the land surface.
• Water table = the surface of the unconfined aquifer
• Confined aquifer = Aquifer that is overlain by a confining bed of geologic material
Cech, 2002
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Aquifer terms
• Permeability = the measure of the ability of a material to transmit fluids through it. – Wikipedia, 2006
• Aquitard = Geologic material with a relatively low permeability that yields low amounts of groundwater. - Freeze & Cherry, 1979
– Examples: clay and bedrock
• Aquiclude = Impermeable geologic material that prevents groundwater movement. - Cech, 2002
– Examples: highly compacted clays & bedrock
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Basin and Range
capp.water.usgs.gov/gwa/ch_c/C-basin.htmlB. Scarborough, Desert Museum, 2002
Aquiclude
Aquifer
Aquitard
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http://www.usd.edu/esci/figures/BluePlanet.html
ConfinedAquifer
Aquitard
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Unconfined Aquifer
Water elevation in unconfined aquifer wells = water table elevation
Water table
Confined aquifer versus unconfined aquifer(plus respective water elevations in wells)
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The High Plains aquifer is an example of an unconfined aquifer. A. This section across southeastern Wyoming and central Nebraska shows the eastward slope of the water table and the relation of the aquifer to underlying rock units.
http://www.usd.edu/esci/figures/BluePlanet.html
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Formation of “cone of depression”
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Note: only occurs in
unconfined aquifer
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How does groundwater flow?
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Flow: Elevation Pressure, ∆head
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No Flow
Flow
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How does groundwater flow?
Above ground: Water flows downhill!Below ground: Water flows down gradient
Groundwater naturally moves to lower elevations due to the force of gravity. However, the direction and rate of movement are determined by lithology, stratigraphy, and structure of geologic deposits.
Cech, 2002
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Darcy Flow and Conductivity
Freeze & Cherry, 1979
Q h--- = -k * -----A l
Q = flowK = hydraulic conductivityh = hydraulic head
= elevation + pressurel = lengthA = cross-sectional area
• Factors that affect flow or conductivity– Sediments and their packing (permeability)– Fluid properties such as viscosity and density
• Orders of magnitude of K (m/s)Clay ~ 10-12, Slit ~ 10-8, Sand ~ 10-4, Gravel ~ 100
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Permeability
Relative Permeability
Pervious Semi-Pervious Impervious
Unconsolidated Sand & Gravel
Well Sorted Gravel
Well Sorted Sand or Sand & Gravel
Very Fine Sand, Silt, Loess, Loam
Unconsolidated Clay & Organic
PeatLayered
ClayUnweathered Clay
Consolidated Rocks
Highly Fractured Rocks
Oil ReservoirRocks
Fresh Sand-stone
Fresh Limestone, Dolomite
Fresh Granite
-Wikipedia, 2006 and Bear, Jacob, 1972. Dynamics of Fluids in Porous Media, Dover.
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Groundwater and surface water:Travel times
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Geology break: stream cross-section
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More simply:
stream
bench
terrace
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Recharge in mountain areasWater from mountaintop precipitation can follow several pathways:
A. Infiltration (slowest)
B. Transport in shallow water zone
C. Overland flow (fastest)
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Water Tables & Flow
Location of wells in map view (above)
Contours of equal hydraulic head may be drawn from water levels in wells
Flow lines are perpendicular to countour lines
Contouring groundwater from well data – how do hydrologists know which way water is flowing?
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Interactions: Gaining Streams
Gaining streams
Contour plot of a gaining streamThis can be determined from water-table contour maps because the contour lines point in the upstream direction where they cross the stream
streams that receive water from the ground-water system
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Interactions: Losing streams
streams that lose water to the ground-water system
Contour plot of a losing stream
can be determined from water-table contour maps because the contour lines point in the downstream direction where they cross the stream
Losing streams
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Groundwater-surface water interactionsInfluence of increased surface water flow on groundwater
During high flow, stream water moves into the unsaturated portion of the stream banks as bank storage
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Groundwater-surface water interactions
A. Initially gaining stream
B. Flooding causes stream to rise, bank storage occurs
C. Additional bank storage water recharges groundwater (losing conditions) increasing height of the water table
Impact of increased surface water flow
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Human impacts on groundwater-surface water resources
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Development of sinkholes, subsidence
Sinkhole in Florida caused by overpumping of groundwater for irrigation
Subsidence fissure in Arizona caused by groundwater pumping
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Subsidence
WRRC, University of Arizona, 199932
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Impacts:Groundwater-stream interactions
1942 1989
Drawdown of aquifer causes Santa Cruz River to dry out and destroys riparian vegetation along stream banks
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Impact:Groundwater / surface water interactions
Gaining stream: g.w. discharges into stream naturally
Installation of a well and pumping may intercept water discharging to stream
Increased pumping rates may draw water from the stream itself, creating losing stream conditions
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GW-SW interaction - San Pedro
Natural cycle: groundwater recharges river
Today: increased groundwater withdrawls influence stream
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Impact: pollution
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GW flow dir
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Impact: saltwater intrusion
Coastal areas: freshwater flows over saltwater because it is less dense
Large draw downs of aquifers can cause underlying saltwater to migrate upwards into wells
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