USGS Water-Quality Information
USGS personnel collect a water-quality sample on the Missouri River. USGS/Photo by Kelly Brady.
Water Quality Data
National Water Information System (NWIS)
NWIS provides current and historical water-quality data.
Today's Water Conditions
Water-Quality Watch provides continuous real-time water-quality measurements.
Water-Quality Web Services
Web services automate the sharing of USGS and USEPA water-quality monitoring data.
For More Data
Additional water-quality data resources are available.
USGS in Your Area
USGS Water Science Centers are located in each state.
Conversion Factors, Selected Terms and Symbols, Chemical Symbols and Formulas, and Abbreviations
See Errata for Chapter A6 for changes to this section.
Temperature: Water and air temperature are given in degrees Celsius (°C), which can be converted to degrees Fahrenheit (°F) by use of the following equation: °F = 1.8 (°C) + 32
Use the following equation to convert temperature from degrees Celsius to
degrees Kelvin (K):
Sea level: In this report "sea level" refers to the National Geodetic Vertical Datum of 1929 (NGVD of 1929)--a geodetic datum derived from a general adjustment of the first-order level nets of both the United States and Canada, formerly called Sea Level Datum of 1929.
Selected Terms and Symbols
*The definitions identified with an asterisk were adopted by the Water Resources Division of the U.S. Geological Survey in Water Resources Division Memorandum 91.09.
*accuracy: The extent to which the measured value of a quantity agrees with the accepted value for that quantity.
approximately equal to:
*bias: Systematic error that is manifested as a consistent positive or negative deviation from the known or true value. It differs from random error, which shows no such deviation.
dissolved constituent: A solute in a water sample--often operationally defined by the method and media properties used to separate the aqueous solutes from the particulate or colloidal phase.
District: A water-data-collecting organizational unit of the USGS located in any of the states or territories of the United States of America.
filtered sample: A sample passed through a commercial filter membrane of identified media, diameter, and pore size.
Formazin turbidity unit (FTU): (See Nephelometric turbidity unit).
gpm: gallons per minute
greater than: >
greater than or equal to:
less than: <
less than or equal to:
method detection limit (MDL): The minimum concentration of a substance that can be identified, measured, and reported with 99-percent confidence that the analyte concentration is greater than zero; determined from analysis of a sample in a given matrix containing analyte.
micrometer (µm): The millionth part of the meter; the pore diameter of filter membranes is given in micrometer units.
micromoles per liter (µmoles/L): A solution having a concentration of one millionth of a mole of a substance per liter solution (micromolar solution). A mole of substance is its atomic or molecular weight in grams.
µS/cm: microsiemens per centimeter at 25 degrees Celsius.
milliequivalents per liter (meq/L) or microequivalents per liter (µeq/L): One equivalent per liter is equal to one thousand milligram-equivalents per one thousand milliliters (meq/mL). Chemical analyses of solutes in a sample are expressed in unit concentrations that are chemically equivalent in terms of atomic or molecular weight and electrical charge.
milligrams per liter (mg/L) or micrograms per liter (µg/L): Milligrams per liter is a unit expressing the concentration of chemical constituents in solution as weight (milligrams) of solute per unit volume (liter) of water. One thousand micrograms per liter is equivalent to one milligram per liter. For concentrations less than 7,000 mg/L, the numerical value is the same as for concentrations in parts per million.
millivolt (mV): A unit of electromotive force equal to one thousandth of a volt.
minimum reporting level (MRL): The smallest measured concentration of a constituent that may be reliably reported using a given analytical method. In many cases, the MRL is used when documentation for the method detection limit is not available.
molal (m): Moles per kilogram (1,000 grams) of solvent.
molar (M): Concentration in moles per liter of solution, where one mole of a substance is its formula weight expressed in grams.
nanometer (nm): The millionth part of a millimeter.
Nephelometric turbidity unit (NTU): A measure of turbidity in a water sample, roughly equivalent to Formazin turbidity unit (FTU) and Jackson turbidity unit (JTU).
normality (N): The number of equivalents of acid, base, or redox-active species per liter (equivalents/L) of solution. Examples: a solution that is 0.01 F in HCl is 0.01 N in H+. A solution that is 0.01 F in H2SO4 is 0.02 N in acid. Formality (F) is the number of atomic (formula) weights per 1,000 grams of solution.
plus or minus: ±
*precision: The degree of similarity among independent measurements of the same quantity, without reference to the known or true value. It often is presented as the inverse of the standard deviation.
*quality assurance (QA): All those planned or systematic actions necessary to provide adequate confidence that a product or service will satisfy given requirements for quality.
*quality control (QC): The operational techniques and the activities used to fulfill requirements of quality.
recommended (recommend, recommended, recommendation): Pertains to USGS protocols and indicates that USGS Office of Water Quality policy recognizes that one or several alternatives to a given procedure or equipment selection are acceptable on the basis of research and (or) consensus. Specific data-quality requirements, study objectives, or other constraints may affect the choice of recommended equipment or procedures. The recommended equipment or procedures selected must be documented and can be based on referenced research and good field judgment. Departure from or modifications to recommended procedures must be quality assured and documented.
required (require, required, requirements): Pertains to USGS protocols and indicates that USGS Office of Water Quality policy has been established on the basis of research and (or) consensus of the technical staff and reviewed by water-quality specialists and selected District personnel. Departure from or modifications to the stipulated requirements that might be necessary to accomplishing specific data-quality requirements or study objectives must be quality assured and documented.
specific electrical conductance (conductivity) (SC): Conductivity of water is expressed in microsiemens per centimeter at 25 degrees Celsius (µS/cm). This unit is equivalent to micromhos per centimeter at 25 degrees Celsius (µmho/cm), formerly used by the U.S. Geological Survey.
unfiltered sample: Sometimes referred to as a wholewater or raw sample--pertains to a water sample collected for subsequent chemical or physical analysis without undergoing a phase-separation procedure.
Chemical Symbols and Formulas
Last Modified: 27Sept06