Parts per billion

In a story I was reading about a water contaminent found in local wells, the statement was made that 4 parts per billion “is exactly equivalent to four drops in an olympic sized swimming pool.”

How accurate is this statement? How is ‘parts per billion’ of a chemical contaminent in water calculated? I’d take it as ‘the number of water molecules to the number of molecules of contaminent’. Correct?

OK, so how many molecules of water are in an olympic swimming pool? Or a ‘drop’?

Here is some information that should help.

Q.E.D., thanks for the info.

Hmmm, those sites list these factoids. Somebody check me here:

1 ppm is 1 drop in 16 gallons of water, so 1 ppb would be 1 drop in 16,000 gallons.

An estimate of the size of an Olympic pool is 50 x 25 x 1.5 meters.

I get 137,585.25 gallons of water in that volume if I’m following the formula correctly: US 50251.5 (1.09*3)3 * 7.48 gallons/cubic foot

So 4bpp would be 8.6 (137,585.25 / 16,000) drops in our theoretical pool, right?

No, wait, it’d 4x that amount: 34 drops. Hmmm.

Well, as the site suggests, the quantities of “drop” and “olympic-size pool” are pretty vague. To be precise, you really need to use standard units of measurements. One ppb is exactly one cc of contaminant per one billion cc of water, for example. Drops and pools are fine for helping to visualize the concept, but are terrible for providing exact measurements.

Basically, a ppb is a microgram of whatever your contaminant is in one liter of water. I’m pretty sure I’ve got that right, as ppm is a milligram per liter of water, so just decrease by a factor of a thousand to get ppb.

It’s an absolutely awful measurement from a chemist’s standpoint–there is no good way to easily compare values, as the whole thing is based on weight and 5 ppb of lithium is a far different amount than 5 ppb of cesium.