How much petroleum is there in the stormwater pollution that enters Puget Sound? That question has lately generated a good deal of confusion and obfuscation. So let’s take a moment to get the facts straight.

You can think of stormwater pollution as a set of Russian dolls. The biggest doll is total toxic pollution. Inside that doll, and nearly as big, is oil & grease. And inside that doll is petroleum hydrocarbons. Here’s how they stack up, size-wise:

Low-end estimate (annual average):

  • Total toxic pollution: 14 million pounds
  • Oil & grease pollution: 13 million pounds
  • Petroleum pollution: 7.9 million pounds
  • Petroleum is 58 percent of all toxic stormwater pollution.

High-end estimate (annual average):

  • Total toxic pollution: 94 million pounds
  • Oil & grease pollution: 92 million pounds
  • Petroleum pollution: 55 million pounds
  • Petroleum is 58 percent of all toxic stormwater pollution.

You might think that there are pretty big differences between the high and low ends of the ranges, and there are. But let’s be clear about where the uncertainty lies: it’s about the total volume of stormwater pollution entering Puget Sound. What’s not uncertain—what we actually do know with some degree of precision—is that virtually all of the pollution is oil & grease and that the lion’s share of oil & grease is petroleum.

In other words, petroleum represents more than half of all toxic pollution entering the Sound in stormwater.(It’s likely about 58 percent, to be precise). By volume, petroleum is the single largest pollutant entering the Sound.


Notes and sources: The numbers I’ve presented here for total toxics and oil & grease are the latest—the new January 8, 2010 numbers—released by Washington’s Department of Ecology as a recalculation of their “Phase 2 Study”. The figures for petroleum pollution are conservative calculations based on the best available scientific understanding of the problem. They assume that 60 percent of oil & grease is petroleum, a rough-and-ready assumption that is based on scientific studies in Washington and other places in North America. (Previous estimates in the Phase 2 study put the numbers for petroleum pollution at much higher levels.) “Low-end estimates” refer to a 75 percent probability of exceedance, which is basically a fancy way of saying 25th percentile. High-end estimates refer to a 25 percent probability of exceedance, or the 75th percentile. Mostly likely, the actual numbers fall somewhere between the two estimates.

To write this post I consulted with the lead researchers at the Department of Ecology. You can find more information at the Department of Ecology’s summary sheet, Focus on Toxic Chemicals in Puget Sound. Also, Table 5 of the technical memorandum Addendum 2: Phase 1 and Phase 2 Toxics Loadings Reports (where figures are given in million metric tons rather than pounds). Of further interest is Addendum 1: Comparison of Loading Estimates to Puget Sound for Oil and Petroleum Products.

Update 1/19/10: Lisa Stiffler dug into the technical documentation and offered some minor modifications to the original numbers that I posted. (New numbers in the tables above may not appear consistent due to rounding.)