Geology - Groundwater

Where does most groundwater exist?

In the pore spaces of soil, sediment, and rock in the saturated zone.

How fast does groundwater flow compared to surface water?

Much slower—centimeters to meters per day vs. meters per second for rivers.

What is the difference between the unsaturated and saturated zones?

Unsaturated zone: pores partly filled with air and water.

Saturated zone: pores completely filled with water.

What is groundwater recharge? What is groundwater discharge?

Recharge: water added to the aquifer (e.g., precipitation, infiltration).

Discharge: water leaving the aquifer (e.g., springs, rivers, wells).

What is the water table?

The boundary between the unsaturated and saturated zones.

How does the slope of the water table compare to land surface slope?

It generally mimics the land surface but is less steep.

What is an aquifer? Aquiclude? Aquitard?

Aquifer: permeable rock/sediment that stores & transmits groundwater.

Aquiclude: impermeable material (blocks flow).

Aquitard: low-permeability material (slows flow).

What sediment and rock make a good aquifer? A good aquiclude?

Aquifer: sand, gravel, sandstone (high porosity & permeability).

Aquiclude: clay, shale, unfractured igneous/metamorphic rocks.

What are the 3 aquifer types?

Unconfined, confined, and perched.

What is an unconfined aquifer?

Aquifer directly open to recharge at the surface (no confining layer above).

What is the potentiometric surface?

The level to which water would rise in a well drilled into a confined aquifer.

What is an artesian well? Which aquifer produces it?

A well where water rises above the aquifer due to pressure; produced by a confined aquifer.

What is porosity? Permeability?

Porosity: percentage of pore space.

Permeability: ability of material to transmit water.

What is the primary cause of aquifer recharge?

Precipitation and infiltration.

What is a gaining stream vs. a losing stream?

Gaining (effluent): fed by groundwater.

Losing (influent): water seeps from stream into aquifer.

Which way will a pollutant flow once in groundwater?

It flows with the direction of groundwater movement (down-gradient).

Would your well water stay safe if your well is up-gradient from a pollutant?

Yes, because groundwater flows down-gradient.

What should you review from the last lecture slide?

The graph showing groundwater flow, recharge/discharge, and aquifer response.

What are the 3 problems with excessive groundwater withdrawal?

Lowering of the water table (temporary if recharge continues).

Subsidence (permanent).

Saltwater intrusion (can be permanent or long-lasting).

How does grain size affect porosity and permeability?

Larger grains → higher permeability.

Porosity depends more on sorting than grain size.

Poor sorting or lots of cement → lower porosity & permeability.