Chapter 14Chapter 14GroundwaterGroundwater

Importance of groundwaterImportance of groundwater• Groundwater is water found in

sediment, plus narrow fractures in bedrock

• Groundwater is the largest reservoir of fresh water available to humans

Sources of Earth’s waterSources of Earth’s water

Distribution of Groundwater

The Water TableThe Water Table

• Water table is the upper limit of saturation

• Variations in the water table

• Depth is highly variable

–Varies seasonally and from year to year

• Shape is usually similar to surface topography

Factors influencing movement of Factors influencing movement of groundwater - 1groundwater - 1

• Porosity – Volume of Pore Space

• Permeability – the ability of a material to transmit a fluid through pore space

Factors influencing movement of Factors influencing movement of groundwater - 2groundwater - 2

• Aquifer – a permeable rock layer or a sediment that transmits groundwater freely (such as sands and gravels)

• Aquitard – an impermeable layer that hinders water movement (such as mudstone - clay)

Springs may result from a perched water tableSprings may result from a perched water table

Rollie’s Well

Features associated with Features associated with Groundwater - 1 Groundwater - 1

• Hot springs

•Water is 6-9oC warmer than the mean annual air temperature of the locality

•The water for most hot springs is heated by cooling of igneous rock

Features associated with Groundwater -2 Features associated with Groundwater -2 • Geysers

• Intermittent hot springs

• Water erupts with great force

• Occur where extensive underground chambers exist near hot igneous rock

• Groundwater heats, expands, changes to steam, and erupts

Yellowstone Geyser erupting

Yellowstone Geyser Pool prior to eruption

Groundwater flows back in, lowest chamber flashes to steam, erupts again.

Features associated with Features associated with groundwater - 3groundwater - 3

• Wells

•To ensure a continuous supply of water, a well must penetrate below the water table

Artesian Well Model

Demo: Artesian

Discussion:Quicksand

Hydraulic head and urban water supplies

Just like an artesian, saves money

Features associated with Features associated with groundwater - 3groundwater - 3

• Wells (con’t)• Pumping of wells can cause

–Drawdown (lowering) of the water table

–Cone of depression in the water table

Formation of a cone of Formation of a cone of depression in the water tabledepression in the water table

Discharge exceeds recharge

Other problems associated with Other problems associated with groundwater withdrawalgroundwater withdrawal

• Treating groundwater as a nonrenewable resource

•In many places water available to recharge aquifer is less than amount being withdrawn

•“Discharge greater than Recharge

Other problems associated with Other problems associated with groundwater withdrawal groundwater withdrawal

(Continued)(Continued)

• Subsidence !

•Ground sinks when water is pumped from wells faster than natural recharge replaces it

Other problems associated with Other problems associated with extreme groundwater withdrawalextreme groundwater withdrawal

(Continued)(Continued)

• Dry Wells – cone of depression already seen

• Saltwater contamination near coast

• Contamination of Wells with sewage

• Formation of Collapse Sinkholes

Saltwater contamination due Saltwater contamination due to excessive well pumpingto excessive well pumping

Wildwood Crest

Well design requires knowledge of local geology

Don’t fire the town engineer!

Geologic work of groundwaterGeologic work of groundwater

• Caverns• Most caverns are created by acidic groundwater dissolving soluble rock at or just below the surface in the zone of saturation

• Acid forms as Carbonic Acid (next)

Geologic work of groundwaterGeologic 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

Geologic work of groundwaterGeologic work of groundwater

• Groundwater dissolves rock• Carbonic acid reacts with calcite in limestone to form calcium bicarbonate, a soluble material

• Later cements particles - lithification

Geologic work of groundwaterGeologic work of groundwater

• Caverns•Features found within caverns

–Composed of dripstone (travertine)•Calcite deposited as dripping water evaporates

Geologic work of groundwaterGeologic work of groundwater• Caverns

• Features found within caverns

–Composed of dripstone (travertine)• Collectively, they are called speleothems

• Includes stalactites (hanging from the ceiling) and stalagmites (form on the floor of a cavern)

Geologic work of groundwaterGeologic work of groundwater

• Karst topography

•Landscapes that have been shaped by the dissolving power of groundwater on limestone

Geologic work of groundwaterGeologic 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)

–Disappearing (aka sinking) streams

Stream incision, cavedevelopment and karsttopography

Solution

Solution

Sinkholes in Urban Settings

End of Streams and End of Streams and GroundwaterGroundwater