4.3EARTH’S GROUNDWATER

Surface Water vs. Groundwater

It’s obvious water exists on the surface of the Earth; evident by streams, lakes, and oceans. However, a large amount flowing underground and beneath

Earth’s surface is not visible to us.

The water located within the rocks below the Earth’s surface is called groundwater. Plays an important role in erosion and deposition as well.

Porosity The total volume of open space in a

rock or sediment sample is known as porosity.

Permeability The ability of a rock or sediment sample

to let fluids pass through its pores, or open spaces, is known as permeability.

The Water Table Through infiltration and percolation, surface water seeps

underground and into the soil. As it travels underground, it passes through two distinct zones:

Zone of Aeration (upper zone) Zone of Saturation (lower zone) groundwater

The boundary between the zone of aeration and the zone of saturation, which is the upper surface of underground water, is known as the water table.

The Water Table

Wet seasons The water table

can be at or just beneath the soil’s surface.

Dry seasons The water table

may be hundreds of meters beneath the ground.

Aquifers

A body of rock or sediment storing groundwater and allowing the flow of groundwater is called an aquifer. Described by its ability

to hold water and its ability to allow water to pass freely through it (the zone of saturation).

Aquifers Two types of aquifers:

Unconfined: permeable rock unit replenished (recharged) directly by rainfall and surface water infiltrating/percolating down through and whose upper surface is the water table.

Confined: also known as “artesian”; a sloping layer of permeable rock sandwiched between two layers of impermeable rock and is exposed at the surface.

Recharge Zones Like rivers, aquifers depend on the water

cycle to maintain a constant flow of water. The ground surface where water enters an

aquifer is called the recharge zone. Size of the zone depends upon the permeability of

the rock at its surface.

Wells A hole dug below the level of the

water table in order to bring groundwater to Earth’s surface is called a well. Must be drilled deep enough so when

the water table drops, the well still contains water.

Pumping water from a well lowers the water table around the well and forms a cone of depression. If too much water is taken from the well,

it may drop below the level of the well and the well will go dry.

An artesian well is a well through which water flows freely without being pumped.

Springs A spring is a natural flow of

groundwater to Earth’s surface in places where the ground surface dips below the water table. In a confined (artesian) aquifer, the

top layer of impermeable rock is called the cap rock. An artesian spring is a spring

whose water flows from a crack in the cap rock of the aquifer.

Most springs have cool water, but ones having water with a temperature of 37 ° Celsius which rises to the surface before cooling are called hot springs. Can periodically erupt from surface

pools through small vents geysers.

Old Faithful Geyser, Yellowstone National

Park

Cave Erosion As water moves through the rocks of caves

(mainly limestone), it combines with carbon dioxide (CO2) to produce carbonic acid (H2CO3). This weak acid chemically weathers the rock by

breaking down and dissolving minerals in the rock.

Cave Deposition Caves show signs of both

erosion and deposition. Water dripping from

cracks in a cave’s ceiling leaves behind deposits of calcium carbonate.

Sharp, icicle-shaped features forming on cave ceilings are known as stalactites.

Water falling to the cave’s ground floor builds up to cone-shaped features known as stalagmites.

Cave Deposition Often, a stalactite and a

stalagmite will grow until they meet, forming a calcite deposit known as a dripstone column.