Groundwater

Water Beneath the Surface

Groundwater

Largest freshwater reservoir for humans

Distribution of Groundwater

Groundwater: Belt of soil moisture

Zone of aeration • Unsaturated zone • Pore spaces in the material are filled mainly with air

Zone of saturation – All pore spaces in the material are filled with water

– Water within the pores is groundwater

Water table – the upper limit of the zone of saturation

Features associated with subsurface water

Porosity • Amount of pore space available• Determines how much groundwater can be

stored • Depends on:

• Shape, packing and arrangement of grains

• Well-sorted: all grains are the same size (high porosity), examples: sand, poorly cemented coquina.

• Poorly-sorted: finer grains fill spaces between larger grains (low porosity), examples: well-cemented sandstone, shale, mudstone, and limestone.

Permeability• Ability to transmit water through connected pore

spaces • How easily water can flow through a rock or sediment

• Depends on:• Grain size (Coarser-grained sediments are more

permeable than fine-grained sediments because the pores between the grains are larger)

• Grain shape and packing (sorting)

Features Associated with Groundwater

• Aquitard – an impermeable layer of material • Aquifer – a permeable layer of material• Springs

• Hot springs • Water is 6-9ºC warmer than the mean air temperature

of the locality • Heated by cooling of igneous rock

• Geysers • Intermittent hot springs • Water turns to steam and erupts

Old Faithful geyser in Yellowstone National Park

Features Associated with Groundwater

• Wells• Pumping can cause a drawdown (lowering) of the

water table

• Pumping can form a cone of depression in the water table

• Artesian wells• Water in the well rises higher than the initial

groundwater level

Two Types of Aquifers

• Unconfined: water level rises freely as the aquifer recharges

• Confined: rising water reaches impermeable layer, becomes pressurized

Formation of a cone of depression in the water table

An artesian well resulting from an inclined aquifer

Environmental Problems Associated with Groundwater

• Treating it as a nonrenewable resource

• Land subsidence caused by its withdrawal

• Contamination

Geologic Work of Groundwater

• Groundwater is often mildly acidic

• Contains weak carbonic acid • Dissolves calcite in

limestone

• Caverns • Formed by dissolving rock

beneath Earth's surface • Formed in the zone of

saturation

Geologic Work of GroundwaterFeatures found within caverns

• Form in the zone of aeration

• Composed of dripstone • Calcite deposited as

dripping water evaporates • Common features include

stalactites (hanging from the ceiling) and stalagmites (growing upward from the floor)

Dripstone formations in Carlsbad Caverns National Park

• Karst topography • Formed by dissolving rock at, or near, Earth's

surface • Common features

• Sinkholes – surface depressions

• Sinkholes form by dissolving bedrock and cavern collapse

• Caves and caverns

• Area lacks good surface drainage

Geologic Work of Groundwater

Infrared image of karst topography in central Florida