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Monitoring Glossary

Below are common terms used regarding water sampling and monitoring.

Dissolved Oxygen: DO (mg/L)

  • Dissolved Oxygen (DO) is the amount of oxygen dissolved in the water, measured in concentrations of milligrams per liter (mg/L).
  • DO is one of the most important parameters in determining the water quality for aquatic life.
  • DO is essential for all plants and animals inhabiting the Chesapeake Bay.
  • Measuring the DO tells us how healthy the environment is for aquatic life. 
  • When DO levels in the water fall below about 3-5 mg/L, fish and many other aquatic organisms cannot survive.

Dissolved Oxygen: % (percentage)

  • DO percentage is a measure of how much oxygen can be dissolved in the water at a given temperature.
  • The colder the water, the more oxygen that can be held and used by aquatic animals to breathe.
  • The warmer the water, the less oxygen that can be held and used by aquatic animals to breathe.
  • A number above 100 means the water is saturated with oxygen at that specific temperature.

E. coli: A type of Coliform bacteria (MPN)

  • E. coli is found in the intestinal tract of warm blooded animals and is often associated with other fecal-related disease carrying organisms.
  • E. coli is used as an indicator organism in fresh water only since it does not survive well in salt water.
  • Starting in the 2008 sampling season, all non-detectable sample results (<1) for E. coli will be shown as (1) in calculating the geometric mean (Geo. Mean).

Enterococci: A type of fecal Streptococci bacteria (MPN)

  • Enterococci are a type of fecal Streptococci present in the intestinal tract of warm-blooded animals.
  • Enterococci, when associated with sewage, have been found to have a significant correlation with swimming-related gastrointestinal illness. Enterococci are less likely to die off in salt water, and therefore are used for tidal water monitoring.
  • Starting in the 2008 sampling season, all non-detectable sample results (<10) for Enterococci will be shown as (1) in calculating the geometric mean
    (Geo. Mean).

Geometric Mean (Geo. Mean):

  • The geometric mean is a calculation typically used to determine an average when a given set of numbers covers a wide range (small and large numbers) when used in bacteriological water testing. The advantage of using the geometric mean instead of the arithmetic mean is that it reduces the effect of very high and low values in a number set. Starting in the 2008 sampling season, all non-detectable sample results (<1) for E. coli and (<10) for Enterococci will be shown as (1) in calculating the geometric mean (Geo. Mean).

Indicator Organism:

  • Indicator organisms are a basic monitoring tool used to measure both changes in environmental water quality or conditions, and the potential presence of hard-to-detect pathogenic organisms. An indicator organism provides evidence of the presence or absence of a pathogenic organism that survives under similar physical, chemical, and nutrient conditions.
  • It is important to note—an indicator is not necessarily a pathogen. Although some strains of E. coli are pathogenic, the reasons E. coli and Enterococci are used are, because they have been shown to be indicative of recent fecal contamination. In addition, their behavior (viability, longevity, movement) in the environment is assumed to be similar to actual pathogens of concern, and there is a relatively fast method of analysis available.

Most Probable Number (MPN):

  • MPN is the bacteriological analysis designator for the probable number of bacteria colonies expected to be found in a 100-milliter sample of water.  Example: Enterococci 10 MPN


  • A disease causing agent or microorganism.
  • Many organisms found in sewage are pathogenic in humans.


  • pH is a measurement of how acidic or basic (alkaline) a solution is.
  • pH is shown in a scale from 0-14.  A pH of 7.0 is said to be neutral.  Distilled water has a pH of 7.0, Blood has a pH of 7.5.
  • If the pH is less than 7.0 it is said to be acidic. Examples: lemon juice pH 2.2, beer pH 4.4, battery acid pH 0.3.
  • If the pH is greater than 7.0 it is said to be basic. Examples: baking soda pH 8.4, bleach pH 12.6, and ammonia pH 11.4.
  • Generally, the ability of aquatic organisms to complete a life cycle diminishes greatly as the pH becomes greater than pH 9.0 and less than pH 5.0.


  • Parts per thousand

Public Bathing Beach:

  • Defined in the Baltimore County Code as “a natural bathing beach together with any appurtenances, or other improvements, at a pond, lake, quarry, stream, or bay, intended to be used collectively by numbers of individuals for swimming or recreational bathing, regardless of whether a fee is charged for use."
  • A permit is required to operate or maintain a public bathing beach.       

Salinity - PSS (Practical Salinity Scale) displayed in ppt:

  • Salinity is the measure of dissolved salts in the water expressed in ppt. (parts per thousand).
  • Seawater is between 30-35 ppt.
  • Brackish water zones in the Bay range between 1-5 ppt.
  • Typically, the salinity will be low in the spring with more rainfall occurring, and higher in late summer/fall when it is dryer.
  • County tidal rivers typically fall into the brackish water zone of the Chesapeake Bay throughout the year with salinity readings ranging between 0-5 ppt.

Specific Conductance - (SpC) mS/cm (Millisiemens per centimeter):

  • Conductivity is a measure of the ability of an aqueous solution to carry an electric current.
  • Inorganic compounds (mineral) are good conductors.  Conversely organic compounds (living organisms) conduct very poorly.

Symbols - (<) less than and (>) greater than:

  • When the symbol proceeds the numerical bacteria sample count, it indicates the degree the symbol expresses. Example: (< 10 MPN Enterococci) and
    (> 2,000 MPN Enterococci).

Temperature - Temp. (F) Degree Fahrenheit:

  • Water temperatures are taken at 1.5 feet of depth and at the bottom.

Turbidity - NTU (Nephelometric Turbidity Units):

  • Materials that become mixed and suspended in the water column will reduce its clarity and make the water look turbid (cloudy). This turbid condition will also reduce sunlight penetration that aquatic plants and animals need to grow and survive.
  • Examples of materials that can cause turbid conditions in the water are: Silt-laden sediments caused by heavy rain events that wash off the land and into the streams and rivers, and very large populations of single celled algae. Wind and boat generated waves that break on the shoreline release sediment, also contributing to turbidity.
  • The higher the number the more turbid or cloudy the water is.

Water Contact Alert:

  • A regulatory notice regarding unsafe recreational water quality.
  • Water Contact Alerts are typically associated with sewage overflows and are issued when dangerous levels of bacteria are present.
  • Water Contact Alerts prohibit activities at regulated beaches and indicate a definite public health risk in open non-regulated waters.

Water Quality Advisory:

  • A non-regulatory public information notice.
  • Water Quality Advisories provide precautionary water quality information to help the public make informed decisions regarding recreational activities. 
Revised July 2, 2018         


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