Lecture 18: Groundwater

Define groundwater

Underground water that is flowing through saturated media (soil)

What percent of Earth’s freshwater is stored in groundwater? What is the residence time?

11%

8 years

What is an aquifer?

Porous and permeable rocks that store enough water and transmit it at a rate fast enough to be hydrologically significant

What is an Aquitard?

Impermeable layers that hinder or prevent water movement

Define primary and secondary porosity?

Primary porosity: Pores originally formed with the rock

May decrease with burial compaction

Secondary porosity: Pores formed due to fracturing, faulting, and dissolution

What are unconfined aquifers?

The upper boundary of groundwater is the water surface at atmospheric pressure.

Water is recharged from above, and the elevation can vary.

An unconfined aquifer is exposed to and recharged from the surface.

The flow and storage are correlated

What are confined aquifers?

Aquifers bounded above and below by aquitards

The pressure is greater than the atmospheric pressure

Recharge occurs upstream

Flow and storage are not correlated.

What are pros and cons of pumping from unconfined aquifers?

Pros:

Don’t need to drill well as deep

cheaper

Recharge is over a larger area, so faster to replenish

Cons:

Water table is deeper (larger depth) so requires more pumping

Pollutants can easily infiltrate due to the aquifer being in a larger area

What are pros and cons of pumping from confined aquifers?

Pros:

Shallow water table so you don’t need to pump water as far

Less likely of pollutants since recharge occurs in one area

Cons:

Need to drill deeper wells

Harder to recharge

How does groundwater flow?

From an area of greater head (recharge area) to areas of less head (discharge area)

What is a hinge line?

The line that separates recharge and discharge areasa

Where do lakes, springs, and wetlands occur?

Where the water table is at or above ground level

How can streams and lakes be changing?

Gaining: Discharge increases due to net groundwater inflow

Losing: Discharge decreases due to net flow into groundwater

What are springs? What are some characteristics of them?

Springs: Locations of natural groundwater discharge

Marked by:

Hydrophilic vegetation

Perennial wetlands

Saturated soils

Non-freezing ground

Stream flow