Geology Quiz 6: Groundwater Resources

Porosity

Groundwater resides in subsurface pore spaces, the open spaces within any sediment or rock. The total volume of open space is termed porosity. Porosity can be filled with water or air. Pores can also become filled with mineral cement or other fluids, like oil or natural gas.

Secondary Porosity

A new pore space created after the rock was first formed. Examples include fractures, fault breccia, and solution cavities.

Permeability

The ease of water flow due to pore interconnectedness. High-permeability material allows water to flow readily. Water flows slowly through low-permeability material.

Aquifer

A high-porosity, high-permeability rock that transmits water easily.

Aquitard

A lower-permeability rock that hinders water flow.

Perched Water Table

A perched water table occurs where a mound of groundwater becomes trapped above a discontinuous aquitard that lies above the regional water table.

Hydraulic Head

The potential energy driving flow, is due to elevation above sea level and the pressure exerted by the weight of overlying water.

A Piezometer

A small device used to measure hydraulic head. Flow always moves from high to low hydraulic head.

Recharge

Groundwater flows from recharge areas to discharge areas along curving flow paths. Groundwater infiltrates at recharge areas, which are higher elevations. Groundwater exits the subsurface at discharge areas, which occur at lower elevations.

Groundwater Flow

Groundwater flow occurs a variety of time and spatial scales. Some groundwater may flow hundreds of kilometers across sedimentary basins. The transit time depends on the flow path. Deeper flow paths take longer .

Cone of Depression

Pumping groundwater affects the water table. If groundwater is extracted faster than it can be replaced, a cone of depression develops around the well. Pumping by a large well may lower the water table enough to cause a nearby small well to go dry.

Structural Springs

Springs form under a variety of conditions. Where flowing groundwater reaches a steep, impermeable barrier, the pressure pushes it up to the surface. A seep develops where a perched water intersects the surface.

Caves and Karst

Caves develop when weakly acidic groundwater dissolves limestone.

Caves and Jointing

Joints, which are conduits for water, are dissolved by the flow, creating a network of caves and passageways. Soluble beds dissolve more rapidly.

Speleothems

Formed from precipitation of dripstone, which develops when groundwater entering a cave degasses CO2. CaCO3 is precipitated as a coating on interior surfaces and grows over time into spectacular forms. Stalactites hang down, stalagmites point up.