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Guide to Water Quality Reports

The monthly water quality report includes data from each of the three water treatment plants that the district operates. The guide below defines the terminology used in the report. For more information about our water quality laboratory, contact Jim Scott at (408) 265-2607 extension 2119.

Water quality terminology
Coliform bacteria: Microscopic organisms in water that are used as indicators of possible contamination.

CFU/mL (colony-forming units per milliliter): A calculated number derived from the number of colonies counted, divided by the actual volume of sample.

DLR (detection limit for reporting purposes): In order to compare results from different laboratories, the California Department of Health Services established DLRs. This provides a common basis for reporting purposes. For example, if a laboratory reports that copper is not detectable, it is understood that the amount of copper in the water is less than 0.05 mg/l (the DLR for copper). In most cases, laboratories including those of our water district, can actually measure at lower levels.

Hardness: Hardness of natural waters is caused by dissolved mineral compounds. Hardness in water is expressed as milligrams per liter (mg/l) of calcium carbonate (CaCO3). Ion-exchange softening systems often express hardness as grains per gallon (grains/gal).

The measurement 1 grain/ gallon is equivalent to 17.12 mg/l of CaCO3

HPC (heterotrophic plate count): HPCs are a measure of a broad group of a bacteria. The significance of the HPC lies as an indication of the general bacteriological quality of the water, and how efficiently the treatment process is working in terms of the reduction of these bacteria in the treated water.

MCL (maximum contaminant level): MCLs are the maximum permissible levels of contaminants in water which enter the distribution system of a public water system, except in the case of bacteriological quality and trihalomethanes where the MCLs are measured within the distribution system. The federal and state MCLs are enforceable and must be met by public drinking water systems.

mg/L: Milligrams per liter or parts per million.

ug/L: Micrograms per liter or parts per billion.

MFL: Million fibers per liter

umhos/cm2: Microhms per square centimeter is a measure of the electrical conductance of water which depends on the mineral content.

NA: Not analyzed. Samples were not analyzed because they were not required or because they are scheduled to be sampled in the near future.

ND: Not detected at the DLR. Samples were analyzed and the constituent was not found at the detection limit for reporting purposes.

NS: No standard exists.

NTU: Nephelometric Turbidity Units.

Primary standard: Health related standards established by federal and state agencies. Primary standards are enforceable.

Secondary standards: Standards for constituents that effect the aesthetics of water quality such as, taste, odor, and appearance. They are not considered a health concern and are nonenforceable.

TDS (total dissolved solids): An overall indicator of the minerals dissolved in water.

TON (threshold odor number): A measure of the intensity of an odor in water.

THMs (trihalomethanes): THMs are by-products of chlorine disinfection. When chlorine reacts with certain naturally occurring organic substances in water, trihalomethanes are formed. Total Trihalomethanes (TTHMs) are composed of four compounds; chloroform, bromodichloromethane, chlorodibromomethane, and bromoform. Individual values for each of the four THMs are added together to give a TTHM result. The MCL for trihalomethanes is based on a continuous averaging of four quarters of sampling.

Turbidity: Turbidity in water is caused by suspended matter such as clay, silt, finely divided inorganic and organic matter, and biological material. The turbidity measurement provides information as to how effective the treatment process is at removing the suspended matter.