Units of Measure Typically Used to Express Test Results

Most analyses for contaminants provide results in terms of concentration, which are usually expressed in units of either parts per million (ppm) or milligrams per liter (mg/l). These two units are used interchangeably by most persons, but are technically different. For the range of concentrations found in most water supplies, the difference is negligible. However, for uniformity in reporting milligrams per liter is used. Concentrations greater than 10,000 mg/l are commonly expressed in percentage by weight.

In the US domestic water treatment industry, water hardness is often reported in grains per gallon. One grain per gallon is equal to 17.1 mg/l.

"Acidity" of water is expressed in pH units. It is the logarithm of the reciprocal of the hydrogen ion concentration [H + ] in the solution. For pure water the hydrogen concentration is 1 x 10 ^-7 moles per liter and the solution can be characterized as pH 7. The pH can range from 0 to 14, but most potable water will range from 6.5 to 8.5. Any solution with a pH below 7 is acidic; any solution with a pH above 7 is alkaline.

If you have your water tested for a broad range of substances, do not be surprised if a lot of things are found and reported. Compare results with accepted standards and nuisance levels discussed previously before becoming overly concerned. If a problem is found or confusion as to the meaning of the results develops, then a water quality treatment expert should be consulted. Your local health department office should be notified if a standard MCL is exceeded. These agencies as well as private water treatment companies can be contacted for specific treatment recommendations.