The layer of gases surrounding Earth; composed mainly of nitrogen and oxygen.

The downward force of water upon itself and other materials; caused by the pull of gravity.

Any substance that has a pH level below 7, or that has more free hydrogen ions (H+

The amount or degree to which a substance is an acid.

An acre-foot is a unit of volume commonly used in the United States in reference to large-scale water resources, such as reservoirs, aqueducts, canals, sewer flow capacity, and river flows. One acre-foot is equivalent to 325,900 gallons.

The attraction of water molecules to other materials as a result of hydrogen bonding.

Able to live only in the presence of air or free oxygen; conditions that exist only in the presence of air or free oxygen.

Organisms that are closely related to higher plants that perform photosynthesis and are commonly found in or near water, such as seaweed and pond scum.

Able to live and grow only where there is no air or free oxygen; conditions that exist only in the absence of air and free oxygen.

An underground layer of water-bearing permeable rock or unconsolidated materials (gravel, sand or silt) from which groundwater can be extracted using a water well.