Waterworks**SCIENCE

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groundwater_class_notes.pdf

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3/22/2016

• Groundwater is water found in the pores of

soil and sediment, plus narrow fractures in

bedrock.

• Groundwater is the largest reservoir of

fresh water that is readily available to

humans

• Fig. 14. 3

Groundwater (Ch 13.1 and Ch14)

• As an erosional agent, dissolving

groundwater produces:

– Sinkholes

– Caverns

• Groundwater serves as an equalizer of

streamflow (fig 14.8)

Geological role of groundwater

– Gaining streams – gain water from the inflow of groundwater through the streambed

– Losing streams – lose water to the ground-water system by outflow through the stream-bed

Water Beneath Earth’s Surface

 The water table forms the boundary between the Saturated Zone and the Unsaturated Zone

–Depth of water table varies seasonally and from year to year

Precipitation variations affect the depth of the water table

• Porosity

– The percentage of pore (open) spaces in a rock or sediment is

called porosity

• Depends on the size and shape of the grains, how well they are

sorted, and how tightly they are packed

– Determines how much groundwater can be stored

Factors Influencing the Storage and

Movement of Groundwater

Permeability is the connectedness of pores and

fractures.

• Permeability is low in

material that have small

pores

• Permeability is high in

materials that have

large pores

How Groundwater Moves • Groundwater moves very slowly

– Average rate is 4 centimeters per day

The force of gravity and pressure differences move groundwater

Fig 14.10

• Groundwater is replenished in areas of recharge

• Groundwater flows back to the surface in a discharge area

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Wells – Most common methods for

removing groundwater

– Drawdown: As water is

withdrawn from the well, the

surrounding water table is

lowered

A cone of depression (cone-shaped

depression in the water table) forms around

–An aquitard is an impermeable layer that

hinders or prevents water movement

• Example: Clay

–An aquifer is permeable rock strata or

sediment that transmits groundwater freely

• Example: sands and gravels

Factors Influencing the Storage and

Movement of Groundwater

Types of aquifers • Unconfined – shallow aquifers at Earth’s surface,

usually made of sand and gravel, no capping

impermeable layer.

• Confined (or artesian)– deep bedrock aquifer

(sandstone, limestone) “sandwiched” between

two impermable layers (shales). Filled with water

under pressure.

• Artesian well Fig. 14. 16 and 14.18

Groundwater under pressure rises above the level of the aquifer

• Types of artesian wells: – Nonflowing – pressure surface is below

ground level

– Flowing – pressure surface is above the ground

• Not all artesian systems are wells, artesian springs also exist

Features Associated with Groundwater

• Springs

•Occur where the water table intersects Earth’s

surface

•Can be caused by an aquitard creating a

localized zone of saturation which is called a

perched water table

• Hot springs

– Water is 6°C to 9°C warmer than the mean

annual air temperature of the locality

– Water is heated by the cooling of igneous rock or

• Geysers

– Intermittent hot springs in which columns of

water erupt with force

• Occur where extensive underground chambers

exist within hot igneous rock

• Groundwater heats, expands, changes to steam,

and erupts

• Chemical sedimentary rock accumulates at the

surface: Siliceous sinter or geyserite form from

dissolved silica

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Environmental Problems • Mining groundwater: Treating groundwater as

a nonrenewable resource

The water available to recharge the aquifer is

significantly short of the amount being

withdrawn

Example: High Plains Aquifer (Ogallala)

• Subsidence

Ground sinks when water is pumped from wells faster than natural recharge processes can replace it (San Joaquin Valley of California)

Groundwater Contamination • Sewage

Extremely permeable aquifers,

Sewage often becomes purified as it passes

through a few dozen meters of an aquifer

composed of sand or permeable sandstone

• Substances such as:

• Highway salt

• Fertilizers

• Pesticides

• Chemical and industrial materials

Geothermal Energy

• Tapping natural underground reservoirs

of steam and hot water

• Favorable geologic factors include

• A potent source of heat

• Large and porous reservoirs with channels

connected to the heat source

• A cap of low permeability rocks

• Geothermal energy not inexhaustible

Geologic Work of Groundwater • Groundwater dissolves rock

• Groundwater is often mildly acidic

– Contains weak carbonic acid

– Forms when rainwater dissolves carbon

dioxide from the air and from decaying

plants

• Carbonic acid reacts with calcite in

limestone to form calcium

bicarbonate, a soluble material

• Caverns and karst topography

Geologic Work of Groundwater

• Karst topography

• Some common features include

– Irregular terrain

– Sinkhole or sinks (formed by groundwater

slowly dissolving the bedrock often

accompanied by collapse)

– Striking lack of surface drainage (streams)