Ohio Nowcast.
Glossary of Terms

These definitions of water-related terms might help you understand our Web site better. They are not universal definitions, so other definitions may be valid. This list was compiled from a number of sources and should not be considered an official Cuyahoga County Board of Health or U.S. Geological Survey glossary.


Bacterium (bacteria, plural). A small one-celled microorganism that can either be free living or parasitic. Some bacteria are disease causing.

Bathing waters are suitable for swimming and other full-body contact exposure where a lifeguard is present.

Combined sewer. A sewer that carries both sanitary (sewage) and storm water in the same pipe, often in older, urban areas.

Combined sewer overflow (CSO). An event that takes place, often with the aid of a control device, that allows for a combined sewer to overflow into area waterways in order to prevent flooding. A CSO event is most likely to occur during a heavy rain event.

Escherichia coli (E. coli). A rod shaped, gram-negative bacterium, commonly found in the gastrointestinal tract and feces of warm-blooded animals. It is one species within the fecal-coliform group of bacteria and is distinguished from other fecal coliforms by biochemical tests. Although most strains of E. coli are harmless, some strains cause illness. E. coli is an “indicator” of fecal contamination and the possible presence of disease-causing organisms. E. coli is the preferred indicator for freshwater recreational waters.

Explanatory variable. A quantity of something that varies and can be used to explain increases and decreases in another variable. Explanatory variables for recreational waters include rainfall, wave height, and turbidity, and are used to explain increases and decreases in E. coli concentrations.

Fecal coliforms. A group of bacteria that were used in past to establish standards for fecal contamination in recreational waters. Fecal coliforms are still used to establish standards for shellfish-harvesting waters and for some permitting applications. Although they are used to assess fecal contamination of water, some fecal coliforms actually come from nonfecal sources. Fecal coliforms are defined by the methodology used for detection (lactose fermentation at 44.5C).

Foreshore head. Water level data from a piezometer (shallow ground-water well) installed 20 ft inland from the edge of water. The foreshore head measured in this manner can best be characterized as a combination of the pressure changes due to the local water table, fluctuations in barometric pressure, the water level in the lake, and the pressure variance due to wave action (Francy and others, 2009).

Indicator bacteria. A type or group of bacteria that indicate the possible presence of disease causing (pathogenic) microorganisms. Indicator bacteria are not usually pathogenic themselves, but they are often associated with pathogens because they are present in large quantities in the feces of warm-blooded animals.

Membrane filtration. A technique used for the detection and enumeration of bacteria from water. This method provides a direct count of bacteria based on the growth of colonies on the surface of a membrane filter through which sample was passed. To assess the sanitary quality of water using bacterial indicators such as E. coli, the number is expressed as colonies per 100 milliliters (col/100 mL).

Milliliter. A milliliter is a metric unit of volume equivalent to one-millionth of a cubic meter. 100 milliliters is equivalent to about 3.5 fluid ounces or slightly less than cup.

Multiple linear regression (MLR). A standard statistical technique that uses several explanatory variables to predict the outcome of another variable (response variable). The goal of MLR is to select a group of explanatory variables that explain as much as possible of the variation observed in the response variable, leaving as little variation as possible to unexplained “noise.” In the case of predictive models for beaches, rainfall and wave height are examples of explanatory variables and E. coli is the response variable.

Nephelometric turbidity unit (NTU). A unit of measure for the turbidity (cloudiness) of water.

Nowcasting. A “nowcast” of recreational water quality is similar to a weather forecast except it provides an estimate of current conditions instead of future conditions. A nowcast gives the probability that the bathing-water standard will be exceeded and swimming would result in an excessive health risk.

Pathogen. An organism that causes disease.

Photosynthetically-active radiation (PAR). Radiation in the 400-700 nanometer range measured by use of sensor at the beach. PAR from the previous day is negatively related to the concentration of E. coli in water at some beaches.

Predictive models. Mathematical models that use easily measured environmental and water-quality variables (“explanatory variables”), such as wave height and rainfall, to estimate the E. coli concentration or the probability of exceeding the bathing-water standard of 235 col/100 mL E. coli.

Primary-contact waters are suitable for full-body contact such as swimming, canoeing, and scuba diving.

qPCR. Quantitative polymerase-chain reaction. qPCR is a rapid analytical method that can be performed in about 2 hours. It is a molecular method that targets the DNA of a specific organism – for recreational water-quality, qPCR methods were designed to target genes specific for E. coli or enterococci.

Recreational water-quality standards Standards established by the state and based on recommendations and epidemiological studies from the U.S. Environmental Protection Agency, they used to decide when to issue a water-quality advisory. In Ohio, the single-sample bathing-water standard for E. coli is 235 colonies per 100 milliliters (col/100 mL). This means that when the E. coli concentration is equal to or greater than 235 col/100 mL, swimming in the water would pose a high risk of illness.

Sanitary sewer overflow (SSO). An event that takes place in which sewage from a sanitary sewer system overflows, or spills, often from a sanitary manhole. A SSO can occur if a blockage is present within a sanitary sewer system and/or there is too much flow.

Threshold probability. To decided whether or not there’s too great a risk to go swimming on any particular day, beach managers establish a beach-specific threshold probability based on historical data. Probabilities equal to or above the threshold would indicate to the beach manager that E. coli concentrations are most likely above the standard, and an advisory would be issued. Probabilities below the threshold indicate that E. coli concentrations are most likely below the standard.

Turbidity. A quantitative value for the clarity of water sample—the higher the turbidity the more cloudy the water. Turbidity is measured with a turbidimeter, which measures the scattering effect that suspended particles have on light.

Virus. A submicroscopic particle (are too small to be seen by regular light microscopes) that is only able to replicate by taking over the machinery of a living cell. Many viruses cause disease.

Water quality. A term used to describe the chemical, physical, and biological characteristics of water, usually in respect to its suitability for a particular purpose.

Water-quality advisory. A public notification, typically in the form of a sign posted at the beach and (or) on the Internet, to advise the public that current recreational water-quality standards are not acceptable for swimming because of elevated E. coli levels.


For comments or changes regarding this Web page, please contact:
Donna Francy, USGS Ohio Water Science Center, 6480 Doubletree Avenue, Columbus, OH 43229; Phone: (614) 430-7769