GROUNDWATER

Water cycle, water budget, water table, wells, formations and

characteristics of groundwater

Where is the world’s water?

• 97% of Earth’s water is ocean

• Less than 3% is freshwater (1/2 % is usable)– more than 2/3 is locked in ice– a small percent of it flows on land– 100 times that much in lakes and swamps as in

rivers– 50 times as much in groundwater as in lakes

and rivers combined

Water Cycle

• Hydrosphere: the water of Earth’s surface

• Water cycle: movement of water around the hydrosphere– E from sunlight causes

evaporation (and transpiration) vapor rises, cools and causes condensation when water falls to surface it is precipitation

– excess precipitation = Runoff- the movement of water across the surface

Runoff

• affected by the: – amount of rain– the shape of the land (steep = more flatter = less)– surface material (harder, well-cemented rocks = more,

looser, softer, porous rocks = less) – vegetation (reduces runoff)

• Increases during short, heavy rains, in deserts and in urban areas

• long periods of steady rain- may cause flooding and runoff– Runoff creates gullies and streams in deserts

Water Budget

• The balance of water received (precipitation) and used or lost (use, runoff, and evapotranspiration) in an area

Water budget factors

• Recharge: water that is stored and not needed by plants

• a time when plants need little moisture

• may cause water table to rise or become runoff

• Surplus: precipitation is more than need for moisture and soil water storage is filled

Water budget factors

• Usage: if moisture need is more than rainfall plants can draw water from surplus

• Deficit: when moisture need > precipitation and storage water is gone

Calculating a Water Budget• Start by labeling Usage (U)

– Usage is the first negative number of the year• U continues until a positive number or -100 is

reached• -100 equals Deficit (D)

– Deficit might not be reached

• D or U continues until you reach a positive number– This equals Recharge (R)

• R continues until +100 is reached– This equals Surplus (S)

• Surplus might not be reached• S or R continues until a negative number (U) is

reached– The budget ends here if it is a one year- budget

Journal Entry (Porosity)• Porosity depends on particle shape (size), sorting, and space filling

– round (pebbles) vs. flat (clay)• Draw four circles and four boxes (2x2 setup and equal sized)• Which have more space between? How would angular vary?

– well sorted vs. poorly sorted• Draw four more circles (2x2 and same size) and put dots (sand) in

empty space between circles• Were the first or second set of circles better sorted? Which have

more pore space? Why?– compaction and cement

• Draw four ovals (same size as above and 2x2) on top of each other• Why do they have less pore space than the circles?• Shade in the space above and below where they to touch to show

cement… What is the impact of cement?• Write a one sentence summary and then list the ideal conditions for

porosity

Water in the ground• Porosity- open spaces

between sediment grains (vol. water added/vol. dry material) x 100 = % porosity

A has best porosity; B less to to mixed sorting; C less because compacted; D less from cement

• Depends on particle shape and sorting– rounded particles more

space than angular or flat– well sorted more pore

space than poorly sorted- because smaller grains fill pore space b/t larger grains

– compaction and cement reduce porosity

• Higher porosity sediments make better aquifers: (rocks underground that are reservoirs for water storage )

Water in ground 2• Permeability- the

connectedness of pore space• Impermeable: fine-grained

material that water cannot pass through- clay and shale– also pumice- porous but not

permeable• capillary water- the film of

water that sticks to particles– Capillary action: the ability

of a substance to draw a liquid upward against gravity due to adhesive and intermolecular attractive forces

– Groundwater Flow Tutorial

• Increases with larger grain sizes

• Cracks mean permeable but not necessarily porous

Groundwater Levels• water table levels are replenished in

recharge zones – water filters through dry section of

soil/rock to the level of groundwater in your area OR

– it enters into the ground and flows down slope for miles to become part of your groundwater system

Water Table Location

• zone of aeration (unsaturated zone)- sediment above the water table that is not saturated- 3 parts– soil water just below surface

– a dry section except during rainy season

– capillary fringe- just above the water table

– water table

Water Table

• the surface of the water is called the water table – a zone of saturation exists beneath the

water table • forms from the infiltration and storage of

rainwater • all pore space is filled with water

Depth of Water Table• depends on rainfall (high↑, low ↓) • season • (slope ↓, flat ↑) • soil thickness (thin ↑, thick ↓) • climate• time between rainfall

• seepage keeps streams flowing, maintains lake and swamp levels, supplies drinking water to springs and wells

Water System

Water Table Depth Change• Journal Activity: Use an up-arrow or down-arrow to show if the water

table comes closer to the surface or gets deeper with the following conditions

• rainfall (high, low) • • season (rainy, dry)• • (slope, flat)• • soil thickness (thin, thick) • • climate (tropical, desert)• • time between rainfall (a few days, many days)•

Ordinary Wells and Springs• Ordinary well-

contains water up to level of water table

• Hillside spring – water table intersects hill slope

Aquifer Characteristics

• Aquifers: water-containing rock layer– Must be permeable- easily conducts water-

and porous (sand or gravel)– usually confined above and below by

confining beds (impermeable layers) of clay or hard or well-cemented rock)

– have recharge zones- water usually travels a few cm/day- may travel up to 100’s of km form aquifer-

• Dakota Sandstone: Rockies to Great Plains

Types of aquifers and formation• Confined aquifer: sandwiched by impermeable rock

from above and below– cap rock: impermeable rock below unconfined aquifer

or atop of an artesian aquifer• Artesian aquifer: (naturally “pumping” and confined

from above and below by confining beds (artesian wells might form)

– confining bed: impermeable beds above and below an artesian aquifer (a confined aquifer)

– Bedrock: rock that soil is formed from (might be a confining bed)

• (cap rock + artesian aquifer + confining beds) = artesian formation

• Unconfined aquifer: not topped by an impermeable rock from above (rock above is permeable)

Artesian Aquifer System

Artesian Wells and Springs

• flow from confining pressure of overlying sediment and water plus the effect of gravity– (as distance from source increases, depth increases)

• wells hundreds of km from mountains may go down more than 1000 m to reach water table

• fissure springs: artesian formations broken by cracks in cap rock- allow water to come through (e.g. desert oasis)

Groundwater Temperature

• Groundwater temperature- Usu. cool… at depths up to 20 meters soil and rock are

protected from weather change Ground temp. is same as average annual temp. at that location-

about 5-15 degrees Centigrade as is groundwater

• Cool in summer, no-freeze in winter except in polar regions where water in ground is always frozen- permafrost Below 20 meters depth, internal heat raises the ground

temperature about 1 C/40 meters of crust

Hot Springs Hot springs- (must be at least 37 C (body temp.))

water heated from recent volcanic activity or from its proximity to pockets of molten rock… or heat source could be from 1000’s of meters depth Water may boil

if gases bubbles through the thick clay slurry (volcanic gas induced chemical weathering of nearby rock)

the feature is called a mud pot… if it is highly colored by mineral and organic matter it

is called a paint pot

Hot Spring photo

Fumaroles and Geysers

Fumaroles- fissures from which water vapor and other hot gases emanate Fumarole fields

“grow” in some places- Might provide energy

for geysers

• Geysers:– boiling hot springs

that periodically erupt as gushes of hot water steam- shoot up sheets of water

Geyser formation cartoon and Old Faithful

Geyser Formation

• Crooked geyser chambers and tubes• Water is superheated by hot rock• immense pressure- causes higher boiling point

however, water above the bottom is under less pressure so it boils and forces out water above- superheated water changes to steam and

explodes at surface due to sudden lessening of pressure

most of chamber is emptied process often occurs at regular intervals as groundwater re-collects causing process to repeat

Karst Topography• Occurs in subsurface due to chemical weathering in well-drained landscapes by groundwater or by over- pumping wells

TYPES: • Above ground karst towers as shown

– Formed by groundwater dissolving the rock between the towers

• Caverns: a large cave often with smaller connecting chambers formed by underground erosion of limestone by carbonic acid– Slightly acidic rainwater soaks into the ground, gets

in to cracks and cavities that are easily attacked by chemical weathering

Cave foto/formation image

Cave features• Stalactites vs. Stalagmites

– Cone-shaped deposits of calcite that form from dissolved minerals precipitating (turning solid) out of solution when water drips from a cave roof

• StalaCtites: hang from cave ceilings

• StalaGmites: build upward from ground of cave floor

– Column: a stalactite and stalagmite connect

• Other mineral deposits by groundwater– travertine, dripstone, geyserite (dissolved silica),

petrified wood- silica replaces organic matter, and natural cement

Sinkhole photo/formation image

• Sinkholes– A circular

depression that forms when part of a cave roof collapses beneath the ground

Groundwater minerals As rainwater passes through the ground, it

dissolves minerals which may remain in groundwater Content depends on kind of rock water passes through

(limestone = more) distance water has traveled underground

(farther = more) water temperature

(higher = more)

Hard water contains more ions in solution than soft water- (usu. Ca, Mg, and Fe ions)– Hardness of artesian water > ordinary water > river

water

Mineral Springs

Mineral content is too high to be used for ordinary drinking and washing WHY do they exist?

Water passes through very soluble rock such as limestone

water contains large quantities of gases that form acids when mixed with water

– CO2 (g) + H2O(l) – carbonic acid, also hydrogen sulfide

– Also because water is hot and capable of dissolving more minerals

Mineral Spring Benefits and Cautions

Benefit: can be used as a “health resort”- similar to mineral bath salt some people buy

Caution: in desert regions, alkali (bitter) mineral springs may be poisonous (borax, sodium carbonate, and sodium sulfate solutions)

Groundwater Problems and Conservation

Over-pumping can cause a drop (cone of depression) in the water table level drops the level of water in

wells/springs (Recharge < Use)

inland, wells may go dry seaside, there may be

saltwater intrusion Natural recharge (from

rainwater and snow melt) is slow may take > 1000

years to completely replenish some aquifers

Communities monitor the level of local water table- may need to regulate their water use

recycle used water after drainage into water treatment plants- its purified and pumped back into ground.

Groundwater Pollution Sources of pollution:

toxic chemicals and hazardous materials from waste dumpsites

accidental spills corrosion of underground storage containers highway salt agricultural fertilizers and pesticides salt water

all seep into the ground and may mix with groundwater

• A difficult problem- no simple or inexpensive way to purify polluted water is known

Pollution Reduction

restrict and/or reduce the use of pesticides and fertilizers)

find safer ways to dispose of harmful wastes (i.e. more durable/less corrosive storage containers)

harvest bacteria that breakdown wastes