Groundwater and surface water: Groundwater basics a single ...web.sahra.arizona.edu/education2/.../lesson9/...ho.pdf · 1 Arizona Water Issues © 2010 - The University of Arizona

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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

1 Arizona Water Issues © 2010 - The University of Arizona – HWR203

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


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


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


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


Basin and Range

capp.water.usgs.gov/gwa/ch_c/C-basin.htmlB. Scarborough, Desert Museum, 2002

Aquiclude

Aquifer

Aquitard

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3



ConfinedAquifer

Aquitard


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.


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Formation of “cone of depression”

Arizona Water Issues © 2010 - The University of Arizona – HWR203 16

Note: only occurs in

unconfined aquifer


How does groundwater flow?

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Flow: Elevation Pressure, ∆head


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


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.


Groundwater and surface water:Travel times

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Geology break: stream cross-section


More simply:

stream

bench

terrace


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?


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


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


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


Subsidence

WRRC, University of Arizona, 199932


Impacts:Groundwater-stream interactions

1942 1989

Drawdown of aquifer causes Santa Cruz River to dry out and destroys riparian vegetation along stream banks


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


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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Documents

Groundwater and surface water: Groundwater basics a single ...web.sahra.arizona.edu/education2/.../lesson9/...ho.pdf · 1 Arizona Water Issues © 2010 - The University of Arizona