Have you ever been confused by scientists who seem to speak a language that barely resembles English? Often, technical people are so immersed in their terminology that they forget that not everyone communicates in the same manner. Some terminology is very specific in its meaning. Water quality numerical data can be confusing and difficult to understand. To help you interpret the meaning of data, an overview of how water analysts make measurements follows.

Weight and Volume Measurements
When following a recipe for preparing foods, specific volumes and weights of ingredients are used to create a desired product. In the laboratory, the analyst is given the product and asked to identify and measure the ingredients. The ingredients scientists look for and measure in water are called analytes. Scientists analyzing water perform two basic kinds of tests. One type simply confirms or denies the presence of something in the water, and results in a "yes" or a "no" answer. The other type finds the exact amounts of something in the water, and results in a number.

For consistency it is necessary to express data in a standard manner. Laboratories worldwide use the Metric system. The Metric system allows us to "share our recipes" with other countries and scientists.

The metric unit for weight is the gram. One thousand (1,000) grams is equal to 2.2 pounds. Usually, measurements in the laboratory are very small – one thousandth (0.001) or one millionth (0.000001) of a gram. One thousandth (0.001) of a gram is called a milligram (preceded by the Latin "mill" which means a thousandth) and is abbreviated as "mg." One millionth (0.000001) of a gram is called a microgram (preceded by the Latin "micro" which means a millionth). Many grocery items list their weights in grams. Vitamins and other dietary supplements are often measured in milligrams. A pinch of salt weighs about 100 milligrams.

The metric measurement for a volume of liquid is liters. One liter is equivalent to 0.26 gallons or about one quart. As with weight, volume can be expressed as fractions of a liter. One thousandth (0.001) of a liter is called a milliliter, and is abbreviated "ml." One millionth (0.000001) of a liter is called a microliter.

Concentrations in different units
Most water quality data is expressed as a concentration – the weight of an element or compound (such as lead or calcium carbonate) per volume of the solution. Typically, concentrations are expressed as milligrams per liter or micrograms per liter. Milli and micro are a factor of 1,000 apart. One thousand micrograms is equal to one milligram.

For example, an analysis of city water might detect 0.050 milligrams of copper per liter. This means that the concentration of copper in the sample was 0.00005 grams in one liter, or in other units, 50 micrograms of copper per liter.

One milligram per liter is the same amount as one part per million (ppm). One microgram per liter is equal to one part per billion.

Imagine that one part per million is like one person in a room of a million people, and one part per billion is like that one person in a room of a billion people. In different terms, one part per million is like one minute in two years, and one part per billion is like one minute in 2,000 years. These analogies show just how very small a part per million or a part per billion really is. Some instrumentation in today's laboratories is capable of measuring even smaller quantities, such as one part per trillion.

Detection Limits
When a result is listed as "less than one milligram per liter," what does it mean? "Less than" relates to the lower limit of detection for the analysis. Detection limits are basically the smallest distinguishable signal that can be detected above the "noise" in a procedure. The term "noise" can be though of like ripples in the ocean. The always present small waves can be though of as noise. A larger wave or swell that is noticeably larger than the ripple can be thought of as a signal. The smallest noticeable "signal" that can be detected consistently is the detection limit.

Reporting a result as "zero" can be confusing. There is usually more than one method for any analysis. Each method has its own detection limit. One analytical method may detect nothing, while another with a lower detection limit may detect a measureable amount. Reporting a result as "less than 0.01 milligram per liter" means that the analyte was not detected, and the lowest concentration that could be detected is 0.01 milligrams per liter. The analyte is either not present in any amount, or is present at a concentration less than 0.01 milligrams per liter.

Maximum Contaminant Levels
Each analyte regulated by the Environmental Protection Agency (EPA) has a MCL. This number is a limit that must not be exceeded for water to meet Federal standards. For example, the maximum contaminant level for mercury is 0.002 milligrams per liter. both health effects and detection limits are considered when establishing a MCL.

As technology improves, it enables scientists to detect lower and lower concentrations of analytes. Careful and unbiased scientific studies are critical in determining what substances consumers should be concerned about and at what level they could conceivably pose a health risk.

If you have any questions or want additional information on water quality in general, please call 628-6000 and ask for the Water Quality Laboratory.