Groundwater. In the U.S. we use: 400 million gallons of water a day to drink 450 billion gallons of water a day in our homes, agriculture and industry

Groundwater

Groundwater

In the U.S. we use: 400 million gallons of water a day to drink

450 billion gallons of water a day in our homes, agriculture and industry

Groundwater provides:

>50% of our drinking water40% of irrigation water25% of industrial water

Groundwater is not usually in the form of lakes or rivers, it mostly occupies the small pore spaces in rocks and between sediment

Similar to water in a sponge.

Groundwater

We withdraw groundwater from reserves that have been accumulating over thousands of years.

We pull it out faster than it gets in there = many reserves are decreasing and will eventually run out

Critical issues related to groundwater:

Where do we find it?How do we keep it clean?Who owns it?

We need to understand:

How it gets into the Earth? Where is it stored? How does it travel?

Groundwater

A typical oasis in Oman

We withdraw groundwater from reserves that have been accumulating over thousands of years.

We pull it out faster than it gets in there = many reserves are decreasing and will eventually run out

Critical issues related to groundwater:

Where do we find it?How do we keep it clean?Who owns it?

We need to understand:

How it gets into the Earth? Where is it stored? How does it travel?

Groundwater

Rainwater or snowmelt = infiltration into ground, runoff or evaporation

I. Infiltration(Recharge of groundwater)

Controlled By:

A. Surface materials

1. Easy to get in: loose soils, unconsolidated sediment, highly fractured rock

2. Hard to get in: clay, unfractured rock

Groundwater


Controlled By:

B. Vegetation

1. Some plants are good because roots provide pathways for water

2. Too much vegetation - hard for water to get to ground, stuck in trees

C. Topography

1. High infiltration: gentle slopes2. Low infiltration: steep slopes

Groundwater


Controlled By:

D. Precipitation

1. Moderate precipitation good, not too much, not too little

2. Droughts: ground gets too hard, impermeable

3. Heavy rainfall: ground gets saturated with surface runoff

Groundwater

II. Groundwater Distribution

Where do we find groundwater?

Gravity pulls water downward

A. Zone of aeration (unsaturated zone)

Pore spaces in sediment and rocks are water and air

Some water bonds to clay particles, or is taken up by roots

Other water is moving downward toward water table




D. Capillary Fringe

Small volume of water rises upwards against gravity

Capillary action—attraction of water molecules to mineral surfaces

Similar to meniscus in a graduated cylinder or paper towel in water




B. Zone of saturationWater fills every available pore

space

Usually only extends to ~1000 meters because pressure too high below that - no pores

C. Water Table

Upper surface of the zone of saturation (separates the two).


Unsaturated zone:

As water begins to seep into the ground, it enters a zone that contains both water and air.

Below lies an intermediate zone, followed by a saturated capillary fringe, which results from the attraction between water and rocks.

As a result of this attraction, water clings as a film on the surface of rock particles.


Saturated Zone:

All the interconnected openings between rock particles are filled with water.

It is in the saturated zone that "ground water" exists.




E. Location of the Water Table

Generally mimics the surface topography, but subdued

Groundwater


Moist Climates: Less than few meters

Deserts: 10’s metersVaries from season to seasonAlso changes during usually wet or dry spells

Groundwater

III. Availability of Groundwater

A. Porosity

Determines how much water the material can hold

The volume of pore space compared to material

Percentage of open space

well rounded = more porositywell sorted = more porosityno cement = more porosity

Groundwater

III. Availability of Groundwater

B. Permeability

The capability of a substance to allow the passage of a fluid

Controlled by amount of pore spaces and whether they’re connected

Larger pores = higher permeability

Sand is much better than clay

Basalt usually has many fractures so water flows through basalt

Groundwater

Pore Connection and Permeability

IV. Groundwater Flow

Flows under influence of gravity, high ground to low ground

A. Flow Rate dependent on:

1. Hydraulic conductivity

Reflects sorting, roundness, and sizes of grains

Depends on porosity and permeability

High hydraulic conductivity: well sorted, well rounded, coarse grains

Low hydraulic conductivity: poorly sorted, angular, fine-grained

Groundwater



A. Flow Rate dependent on:

2. Hydraulic gradient

Essentially the slope of the water table

Steep hydraulic gradient faster flow

Gentle hydraulic gradient slower flow

Groundwater

Groundwater velocity Depends on permeability and

hydraulic gradient (slope of water table)

Ranges from 100 m/day to mm/day

A good round number: 1 ft/day



B. Speed: 0.2 to 0.6 cm per day in permeable rock

Tens of cm per year in unfractured igneous rock

C. Discharge

Water table intersects the surface = streams, lakes, swamps, springs (confined)

Groundwater

Groundwater

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Groundwater. In the U.S. we use: 400 million gallons of water a day to drink 450 billion gallons of water a day in our homes, agriculture and industry