Science of Water : Concepts and Applications

406 The Science of Water: Concepts and Applications
Disinfection A chemical treatment method. The addition of a substance (e.g., chlorine, ozone,
or hydrogen peroxide) that destroys or inactivates harmful microorganisms, or inhibits
their activity.
Dispersion The extent to which a liquid substance introduced into a groundwater system spreads
as it moves through the system.
Dissociate The process of ion separation that occurs when an ionic solid is dissolved in water.
Dissolved constituent Operationally defi ned as a constituent that passes through a 0.45-micrometer
fi lter.
Dissolved oxygen (DO) The oxygen dissolved in water usually expressed in milligrams per
liter, parts per million, or percent of saturation.
Dissolved solids Any material that can dissolve in water and be recovered by evaporating the
water after fi ltering the suspended material.
Diversion A turning aside or alteration of the natural course of a fl ow of water, normally
considered physically to leave the natural channel. In some states, this can be a consumptive
use direct from another stream, such as by livestock watering. In other states,
a diversion must consist of such actions as taking water through a canal, pipe, or
conduit.
Dolomite A sedimentary rock consisting chiefl y of magnesium carbonate.
Domestic withdrawals Water used for normal household purposes, such a drinking, food preparation,
bathing, washing clothes and dishes, fl ushing toilets, and watering lawns and
gardens. The water may be obtained from a public supplier or may be self-supplied. Also
called residential water use.
Drainage area The drainage area of a stream at a specifi ed location is that area, measured in a
horizontal plane, which is enclosed by a drainage divide.
Drainage basin The land area drained by a river or stream.
Drainage divide Boundary between adjoining drainage basins.
Drawdown The difference between the water level in a well before pumping and the water level
in the well during pumping. Also, for fl owing wells, the reduction of the pressure head as
a result of the discharge of water.
Drinking water standards Water quality standards measured in terms of suspended solids,
unpleasant taste, and microbes harmful to human health. Drinking water standards are
included in state water quality rules.
Drinking water supply Any raw or fi nished water source that is or may be used as a public
water system or as drinking water by one or more individuals.
Drip irrigation An irrigation system in which water is applied directly to the root zone of plants
by means of applicators (orifi ces, emitters, porous tubing, or perforate pipe) operated
under low pressure. The applicators can be placed on or below the surface of the ground
or can be suspended from supports.
Drought A prolonged period of less-than-normal precipitation such that the lack of water causes
a serious hydrologic imbalance.
E
Ecoregion An area of similar climate, landform, soil, potential natural vegetation, hydrology, or
other ecologically relevant variables.
Ecosystem A community of organism considered together with the nonliving factors of its
environment.
Effl uent Outfl ow from a particular source, such as a stream that fl ows from a lake or liquid
waste that fl ows from a factory or sewage-treatment plant.
Effl uent limitations Standards developed by the EPA to defi ne the levels of pollutants that
could be discharged into surface waters.