August 7, 2010 | Posted by Michael "Aquadoc" Campana
A Tweet rolled across my screen the other day. I can’t recall the exact verbiage but it was something like this:
Gulf volume: 643 quadrillion gallons; dispersant volume: 1.5 million gallons. What’s the problem?
Sounds like the Tweeter had a great point: the volume of the Gulf is so huge relative to the volume of dispersant that the dispersant is unlikely to be a threat to Gulf life.
Let’s do some arithmetic. If you divide the dispersant volume by the Gulf’s volume (6.43E+17 gallons) you find the former represents something like 2E-12 (or 0.0000000002 %) of the Gulf’s total volume. Put another way, it is about 2 parts dispersant per one trillion parts of Gulf water, or 2 pptr. I have trouble imagining that small of an amount. Nevertheless, if you are trying to demonstrate that the dspersant is not a problem because its relative volume so small, the reasoning is specious.
The dispersant is not uniformly distributed (‘perfectly mixed’ in chemical engineering jargon) throughout the Gulf’s entire volume. There’s more of it some parts of the Gulf, and less of it in other parts. Recall the Gulf is not like a big fish tank with relatively uniform or regular dimensions. About 38% of the Gulf’s area consists of shallow intertidal zones, followed by 22% for the continental shelf (<200 m or <650 feet depth), 20 % for the continental slope (depths between 200-300m or 650 – 1,000 feet), and 20% for the abyssal regions (depths >3,000m or >10,000 feet). The greatest depth in the Gulf is 4,384m or 14,400 feet. Because of depth variations and currents, the dispersant is not likely to be evenly distributed. So the 2 pptr figure is likely a fiction.
Perhaps more importantly, keep in mind that this analysis does not consider the toxicity of the dispersant, how it degrades, or the effects of the dispersed (smaller particle size) oil. And some compounds can be toxic in extremely small amounts.
You can play the same game with the total volume of oil leaked. It is far greater than the dispersant volume, but it will still be small relative to the Gulf’s volume., So it’s not a problem, right?
Click here and scroll down to my comment for a similar analysis of the total volume of oil released relative to the Gulf’s volume.
As an aside, a fascinating book is How to Lie with Statistics, first published in 1954.
“If your experiment needs statistics, you ought to have done a better experiment.” – Ernest Rutherford