groundwater i 2014 - university of colorado...

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N. Fork Stillaguamish River

On March 22, 2014, a major landslide occurred near Oso, Washington. Death toll currently at 30, with 15 still missing.

Before and After Swipehttp://bit.ly/PeN1JT

Groundwater

Thunder River, Grand Canyon Oct 2006

Groundwater: Outline1. Groundwater intro and activity

2. Water table and groundwater flow

3. Darcy’s Law and the driver for groundwatermovement

4. Groundwater storage: aquifers & aquitards

4. Groundwater problems (e.g., contamination, pumping, etc.)

The Water Table

Top of the saturated zone

Configuration similar to ground surface

Water level of most lakes and rivers corresponds to the water table

Varies with time (season, rainfall, ...

Gradient: change in water level elevation/change in distance

Groundwater Movement

Water table mimics topography, it is not flatIts elevation varies with time

Recharge: the infiltration of water into any subsurface formation

Discharge: the opposite of recharge; the exit of groundwater to the surface

Groundwater Movement

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Draw the water tableHow might you draw the water table in this region? What drives groundwater flow?

Hydraulic gradient

?

Horizontal hydraulic gradientWell 1 Well2

200 ft

610 ft

h2=500 ft

Hydraulic gradient = water level difference / distance= (550 – 500) / 200 = 0.25

Flow from well 1 to well 2

60 ft100 ft

(depth to water)

610 ft

h1=550 ftdh L

dh/L

Friday’s in-class activity






How fast does groundwater move?Rate controlled by 2 things:1. Driving force “gradient of potential energy” think of the water table’s slope

2. permeability How easily water moves through subsurface

Rates extremely variable

< 1 cm/year to km’s/year

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Porosity - the percentage of rock or sediment that are voids or openings…A measurement of a rock’s ability to hold water

Permeability – rocks’ ability to transmitfluid…degree of interconnectedness of pore spaces

Porosity - the percentage of rock or sediment that are voids or openings

A measurement of a rock’s ability to hold water

– Loose sand may have 30-50% porosity

– Compacted sandstone may have only 10-20% porosity

– Low in metamorphic and igneous rocks

Permeability – rocks’ ability to transmit fluid

Depends ongrain size/arrangementinterconnectedness of pore spaces

High permeability: gravels, sands, limestones with dissolution channels, fractured rocks

Low permeability: clays, unfractured metamorphic and igneous rocks, limestones without dissolution

Porosity (and permeability) can be increased by fracturing and dissolution

- secondary porosity Vesicular rhyolite is likely to be ________.

Clicker question

A. Both porous and permeable.

B. Porous but not very permeable.

C. Permeable but not very porous.

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How fast does groundwater move?Rate controlled by 2 things:1. Driving force “gradient of potential energy” think of the water table’s slope

2. permeability How easily water moves through subsurface

Rates extremely variable

< 1 cm/year to km’s/year

Darcy’s Law Q = discharge (per unit area) (m/day)

K = coefficient of permeability (m/day)

h1 = beginning height (m)

h2 = ending height (m)

L = distance (m)

L

Q = K(h1– h2)

LFlow rate






How does most groundwater exist within the subsurface?

A. As large pools of water within large pockets (caverns) in rocks.

B. Within cavernous regions occupied by rivers of water that flow underground.

C. Within the small open spaces between the grains of rocks and sediments.

D. Both A and BE. None of the above

Aquifer: geologic unit that stores and is capable of transmitting significant water, must be both porous and permeable

Aquitard: low permeability rock formations that retard (or slow down) the flow of water to or from an adjacent aquifer

Unconfined AquiferAt shallow depth

Water table is the top boundary

Recharged by precipitation infiltrating

Confined Aquifer Deeper

Confined on top by a less permeable layer

Recharged slowly

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An aquifer must be:a. both permeable and porous.b. neither permeable nor porous.c. permeable, but not necessarily porous.d. porous, but not necessarily permeable.

Perched water table Above the main water table

In unsaturated zones

Due to lenses of less permeable rocks (e.g., shales or clays)

Artesian (confined) aquifer: under high pressure

Water in wells rises above the top of the aquifer.

City water systems can be considered artificial artesian systems.

Well #___ is a flowing artesian well and well #___ is a well that must be pumped

from an unconfined aquifer?(Assume that only the very bottom of the well is open)

A) 1, 2 B) 2, 3 C) 3, 4 D) 1, 4 E) 2, 4

SpringLocation where the water table intersects the surface or where groundwater is discharged in general

Marble CanyonGrand Canyon National Park

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Problems Associatedwith Groundwater

• Overuse• Subsidence• Contamination

- Salt water intrusion- Hard water, pollutants

Drawdown due to Over Pumping Friday’s in-class activity

If pumping > recharge

Subsidence due to groundwater withdrawal in Pahrump Valley, Nevada



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Subsidence: when water is removed, sediment compacts, lowering ground surface.

San Joaquin Valley subsidence

DenverBasin

Aquifers

Barkmann, 2004

As of Feb 2001:~34,000water wells were drilled in the Denver Basin

Non-renewable resource

Denver BasinCross Section

West East

Barkmann, 2004

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Ogallala Aquifer: Largest in the U.S.

First used for irrigation in the late 1800s

1950s: large-scale exploitation

Today: ~170,000 wells used for irrigation

Water table ~200’ lower in some areas!

Setting of the Ogallala Aquifer

Unconfined aquifer; water levels dropping in south; some parts may run dry (graphs below)

17.11.a

Ogallala Water Supplies and Usage

Thickest in northMore rain in east Water table decline greatest in south (Kansas & Texas)

17.11.b-c

Water Quality and Groundwater Movement

Groundwater ContaminationSources:Pesticides/herbicides/fertilizers

Landfill pollutants

Sewage

Acid mine drainage

Radioactive waste

Oil and gasoline

Salt water intrusion

Saltwater Intrusion

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Question: Why is ground water flow direction changing here?

A) Pumping lowers water table so that it is tilted towards community well

B) Bacteria expand and push water away from septic tank

C) Drought causes water tableto become lower

D) Septic wastes always flowtowards wells open to the air

groundwater i 2014 - university of colorado...

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