Concentration Units – In Other Words

By John Brereton, PhD, Vice-President of NJB Soft

Here’s an interesting point of view I put together from some old lecture notes I have…

Most pollutants or contaminants in the water matrix (e.g., wastewater, drinking water, stormwater) are measured in units of milligrams per litre (mg/L), micrograms per litre (ug/L), nanograms per litre (ng/L), or sometimes even picograms per litre (pg/L).   For example, the drinking water Maximum Contaminant Level (MCL) for fluoride is 4.0 mg/L; for arsenic, 10 ug/L; and for dioxin, the MCL is 0.00000003 mg/L, or 30 pg/L.  Have you ever pondered just how tiny these concentration units are?  Let’s take a look from several different perspectives.

Since 1 liter of water (pure water at 4ºC and 1 standard atmosphere pressure) has a mass of 1 kilogram, 1 milligram in 1 liter of water under most conditions is approximately equal to one part per million parts (i.e., 1 mg/(1000 mg/g x 1000 g/kg)).  Similarly, 1 ug/L is analogous to 1 part per billion, 1 ng/L is 1 part per trillion, and 1 pg/L is 1 part per quadrillion.

We can perhaps imagine one dollar in one million dollars, but for most of us, it is difficult to visualize how much money $1 million dollars really is.  Would one million single dollar bills fit in a suitcase?  a closet?  a room?  a house?

Let’s think in terms we may be able to better relate.  In terms of time, one second in a million seconds  is equal to one second in 11.6 days (or one minute in 2 years).  One second in a billion seconds is the same as one second in 31.7 years.  One part per trillion?  That’s the equivalent of 1 second in 31,710 years, and one part per quadrillion is 1 second in 31 million years.

Alternatively, in terms of distance, 1 ppm = 1 millimeter in 1 kilometer (or one inch in 16 miles).  One ppt is equivalent to 1 mm in 1,000,000 km  (or 3 millimeters in the distance between the earth and the moon).

Next time you measure a contaminant concentration of 1 mg/L, that’s analogous to one car in bumper-to bumper traffic from Cleveland to San Francisco.  One ppb or 1 ug/L is like one silver dollar in a roll stretching from Detroit to Salt Lake City, or one sheet in a roll of toilet paper stretching from New York to London.

Gives you some idea of how small these quantities can be.  I don’t mean to say that these measurements are too small to be concerned about.    Just gives you a different perspective on the quantities we deal with every day.

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