Let's do the math (I have already):
Fukushima is widely accepted to have released into the Pacific Ocean between estimated 3,000,000,000,000,000 - 34,000,000,000,000,000 Becquerels of Cesium 137 (just one isotope, not including airborne).
If the Pacific Ocean is estimated to have 137 million cubic miles of water and we use a 'mid' number of 13PBq 137cs, by my math (done twice) that equals 2.399 Bq/Liter, or 1.6 Bq/cubic meter, if the entirety of all the released radioactivity were 100% diluted in the Pacific.
That's 26 1/2 Million times background pre-Fukushima. Even is my math is egregiously in error, it's still an order of magnitude, not 'diluted'. And the Pacific won't 100% dilute anything in 3 years, let alone 100.
Give the dilution garbage a rest. I urge you to learn more about Pacific currents. /s
The total amounts of 137 Cs directly released into the sea have been estimated to be from 3.5 ∼ 4 PBq (Tsumune et al., 2012; Kawamura et al., 2011) up to 27 ± 15PBq (Bailly du Bois et al., 2012).
(for those not able to do the math this late at night, PBq is Peta Becquerel, or One Quadrillion Becquerels, or 15 zeroes)
Background Cesium 137 in the Pacific in 2010 was 0.00000009 Bq/cubic meter.
Oh, and for the sake of stating it, that "background" radiation people defending keep parroting? That's largely a result of all the nuclear testing, with a little bit added from Windscale & Chernobyl, or 950 PBq spread all over the planet. What will be Background or ALARA next year???
Cesium 137 is an artificial element. Comparing current levels stemming from Fukushima to nearly entirely-decayed levels left over from bomb testing is obviously going to give you a very large scary-sounding number. The real question is what does 1.6, or 160, or even 1,600 Bq/m3 really mean in the real world?
The EPA believes that 7,410 Bq/m3 is the safe limit in drinking water, and presumably they've done the math far more than twice.
Sorry it took so long to get back to you.
I checked your numbers, and there are errors.
137 million cubic miles of ocean converts to 5.7E17 cubic meters of ocean.
If I take 13E+15 Bq and divide it by 5.7E+17 cubic meters of ocean, I get 0.023 Bq/m3. Not 1.6 Bq/m3, as you found.
A Bq equals one disintegration per second. Just one. 0.023 Bq/m3 would be equal to one disintegration per second in one standard size olympic swimming pool. This is waaaay below background levels.
Cesium-137 has an activity of 3.215 TBq per gram. That means that, using your numbers, that 933 grams of Cesium-137 has been released into the Pacific ocean. 933 grams of cesium would fit inside a cube that is 3 inches long on each side.
Now take that cube and disperse it in the entire Pacific ocean.
Dilution.
In regards to Pacific currents, your discussion would need to factor in density. Hydrated compounds of cesium are far more dense than water, and would thus sink to the bottom of the ocean rather than float around.