Groundwater: Soil, Porosity, Aquifers, and Weathering

Sediment

Material that forms from the weathering and erosion of rocks.

Water Table

The uppermost part of the zone of saturation.

Zones of Aeration

The zone above the water table where soil pores can be filled with a mixture of air and water (but < 100% water).

Zones of Saturation

The zone below the water table where soil pores are 100% filled with water.

Soil Texture

The classification of soil based on the proportions of sand, silt, and clay.

Textural Triangle

A tool used to determine the textural classification of soil based on its sand, silt, and clay content.

Porosity

The measure of the void spaces in a material, indicating how much water it can hold.

High Porosity Material

Materials that have a large number of pore spaces, allowing them to hold more water.

Low Porosity Material

Materials that have few pore spaces, limiting their ability to hold water.

Permeability

The ability of a material to allow fluids to pass through it.

Soil Texture Classification Example 1

Soil with 40% sand, 40% silt, and 20% clay.

Soil Texture Classification Example 2

Soil with 50% sand, 25% silt, and 25% clay.

Soil Texture Classification Example 3

Soil with 25% sand, 20% silt, and 55% clay.

Soil Texture Classification Example 4

Soil with 10% sand, 55% silt, and 35% clay.

Soil Texture Classification Example 5

Soil with 10% sand, 65% silt, and unknown clay content.

Capillary Fringe

The area located above the water table where water wicks up towards the surface due to adhesion and cohesion properties.

Adhesion

The property of water that causes it to stick to solid surfaces.

Cohesion

The property of water that causes it to stick to itself.

Igneous and Metamorphic Rocks

Types of rocks that are usually solid and have little to no pore spaces, thus exhibiting low porosity.

Sedimentary Rocks

Rocks made of compacted weathered and eroded rock particles that have empty spaces between grains, thus exhibiting porosity.

Mechanical Weathering

The process of physically breaking down rocks, such as through frost wedging.

Chemical Weathering

The erosion or disintegration of rocks caused by chemical reactions, chiefly with water and substances dissolved in it.

Hydrolysis

A chemical weathering process that occurs when acid rain reacts with rock-forming minerals to produce clay and salts.

Well-sorted clastic sediment of high primary porosity

Great aquifer material.

Poorly sorted clastic sediment of restricted primary porosity

Not the best.

Well-sorted clastic sediment with extremely high primary porosity

Due to the porous character of the grains (good).

Well-sorted clastic sediment with cement infill of the primary porosity

Poor.

Secondary porosity due to dissolving of rock

Example: limestone (can be good).

Secondary porosity along fractures in a systematically fractured rock

Can be good if lots of fractures.

Particle Size

Does NOT affect porosity alone; shape, packing, and sorting do.

Spherical particles

Have a higher porosity than angular particles.

Well sorted particles

Have a higher porosity than angular particles.

Sorting

If all particles are the same size, they are sorted; if different sizes, they are unsorted (poorly sorted).

Packing

The more closely packed the particles are, the lower the porosity.

Shape

Well rounded particles have greater porosity than angular particles.

Porosity of sorted particles

The same for sorted particles, but not for unsorted particles.

Big spaces

Not a lot of them.

Little spaces

But lots of them.

Groundwater availability

Determined by the amount of porosity in the sediment.

Capillarity

The process of water adhering to rock particles (cohesion) in small spaces, resulting in an upward movement of the water through rock sediment.

Zone of Aeration

Zone above the water table where the soil pores can be filled with a mixture of air and water (but < 100% water).

Zone of Saturation

Zone below the water table where the soil pores are 100% filled with water.

Aquifer

An underground deposit of sediment or material that is capable of holding and transmitting water.

Aquitard/Aquiclude

A material that does not allow water to pass through it readily (e.g., clay, granite).

Recharge Area

Area of land above an aquifer that allows water to infiltrate through the soil to fill up the soil pores with water.

Unconfined Aquifer

An aquifer that is open to the atmosphere and only has one confining layer.

Confined Aquifer

An aquifer that is not open to the atmosphere and is confined by two aquitards.

Artesian Well

A well dug into a confined aquifer that has its water table at a higher elevation than the top of the well so that it flows freely without pumping.

Cone of Depression

A lowering of the water table due to excess pumping.

Groundwater Contamination

Wells can get contaminated by sucking up pollutants from far away areas.

Land Subsidence

Land can sink over large areas due to groundwater withdrawal.

Groundwater Depletion

Groundwater can be depleted due to overuse.

Salty Wells

Wells can become salty if they are near ocean areas.

Recharge and Gaining Stream

When the water table is at a higher elevation than the lake or stream, water passes from the groundwater into the lake or stream.

Soil Texture and Capillary Water

The relationship between soil texture and the capillary water content of the soil.

Effect of Soil Permeability

The effect of soil permeability on the amount of runoff in an area.

Groundwater Movement

Describes how groundwater moves in different experiments.