The volume of the Gulf of Mexico is 2,424,000 cubic kilometers, or 6.43 * 1017 US gallons. The volume of oil spilt is estimated at 20m gallons to 50m US gallons; let's take the max, 5 * 107 gallons. That's one part of oil to 1.29 * 1010 parts of water.The volume of the Thames at mid tide between Teddington and Gravesend is about 2.4 * 107 cubic metres (633 * 107 US gallons, or 127 times the total volume of the BP oil leaked). To replicate the 'environmental disaster' the Septics are claiming, I'll therefore have to empty 1.87 litres of engine oil into the river.
Now it appears that 75% of the oil has evaporated and a further 24% has been eaten by bacteria. The Armageddon merchants have crawled back in their (unpolluted) shells and the talk has switched to 'undetectable long term damage'
Whatever. The fact remains that 1 part of oil to 100,000,000,000 parts of water is nothing to get excited about.