A B C D E F G H I J K L M N O P Q R S T U V W X Y Z

Observation Well
A well constructed in a specific location for the purpose of observing(measuring) changes in water level. An existing well perhaps drilled for a different purpose may also be used to observe water level changes. Observation wells are typically used for short duration data collection such as before, during and after an aquifer test. Wells that are used to collect data on a long term basis are usually referred to as monitoring wells. See also monitoring wells.

Ombotrophic
A condition in which a wetland is hydrologically independent of surface water or ground water, and is almost exclusively supplied with water from precipitation.

Organic Matter
Carbon compounds usually associated with plant and animal biomass and residues, or substances made by living organisms.

Osmosis
The movement of water molecules through a thin membrane. Reverse osmosis is a water treatment process used to remove or reduce salts from saline water.

Outfall
The place or the outlet or structure, where a sewer, drain, or channel discharges (usually) treated waste water to a river, lake or the ocean.

Outwash Sediments
Glacial sediments of stratified sand and gravel formed by glacial meltwater streams.

Overburden
Originally a mining term, overburden is now used to describe any unconsolidated material overlying bed rock.

Overdraft
Pumping of ground water in excess of rates of recharge. Short term overdraft pumping may be part of a management strategy to conjunctively use surface and ground water resources.

Overland Flow
Flow of water over the land surface originating from snowmelt or precipitation. Overland flow is not concentrated in defined channels.

Ox-bows
Abandoned river meanders on flood plains. Usually curved and now occupied by lakes and wetlands unless drained for cultivation.

Oxygen Demand
A measure of the need for molecular oxygen to meet the needs of biological and chemical processes in water. Even though very little oxygen will dissolve in water, it is extremely important in biological and chemical processes. Changes in oxygen levels in aquifers can result in changes in ground water chemistry.