We've extensively documented the fact that ocean currents bring Japanese radiation to the West Coast of North America, and that - rather than adequate ocean dilution - there could be “pockets” and “streams” of highly-concentrated radiation.

Joke F Lübbecke of the National Oceanic and Atmospheric Administration's (NOAA) Pacific Marine Environmental Laboratory and 3 scientists from the GEOMAR Research Center for Marine Geosciences poured tracer dye into coastal waters off of Fukushima, and monitored its progress as it traveled to the West Coast of North America, to find out what might really happen.

They have revealed their results in a new paper published by journal Environmental Research Letters.

The paper shows that the West Coast of North American could end up with 10 times more radioactive cesium 137 than the coastal waters off of Japan itself.

How could radiation levels be lower closer to the source of contamination: Fukushima?

Because the currents are swift off of the Eastern coast of Japan, and quickly move the contaminated water away.

The paper explains:

In the following years, the tracer cloud continuously expands laterally, with maximum concentrations in its central part heading east. While the northern portion is gradually invading the Bering Sea, the main tracer patch reaches the coastal waters of North America after 5–6 years, with maximum relative concentrations ( > 1 × 10−4) covering a broad swath of the eastern North Pacific between Vancouver Island and Baja California. Simultaneously some fraction of the southern rim of the tracer cloud becomes entrained in the North Equatorial Current (NEC), resulting in a westward extending wedge around 20°N that skirts the northern shores of the Hawaiian Archipelago. After 10 years the concentrations become nearly homogeneous over the whole Pacific, with higher values in the east, extending along the North American coast with a maximum (~1 × 10−4) off Baja California. The southern portion of the tracer cloud is carried westward by the NEC across the subtropical Pacific, leading to increasing concentrations in the Kuroshio regime again. *** With caution given to the various idealizations (unknown actual oceanic state during release, unknown release area, no biological effects included, see section 3.4), the following conclusions may be drawn. (i) Dilution due to swift horizontal and vertical dispersion in the vicinity of the energetic Kuroshio regime leads to a rapid decrease of radioactivity levels during the first 2 years, with a decline of near-surface peak concentrations to values around 10 Bq m−3 (based on a total input of 10 PBq). The strong lateral dispersion, related to the vigorous eddy fields in the mid-latitude western Pacific, appears significantly under-estimated in the non-eddying (0.5°) model version. (ii) The subsequent pace of dilution is strongly reduced, owing to the eastward advection of the main tracer cloud towards the much less energetic areas of the central and eastern North Pacific. (iii) The magnitude of additional peak radioactivity should drop to values comparable to the pre-Fukushima levels after 6–9 years (i.e. total peak concentrations would then have declined below twice pre-Fukushima levels). (iv) By then the tracer cloud will span almost the entire North Pacific, with peak concentrations off the North American coast an order-of-magnitude higher than in the western Pacific.

"Order-of-magnitude" is a scientific term which means 10 times higher. The "Western Pacific" means Japan's East Coast.

Here are the important graphics from the paper:

Figure 4. Decadal evolution of relative surface tracer concentration in the 0.1°-model simulation; boxes in (d) indicate regions for which the temporal evolution is computed in figure 7; contour lines mark power of 10 intervals.

Postscript: Prussian Blue may be used to treat cesium poisoning. But don't take any Prussian Blue before consulting with a qualified healthcare professional. Antioxidants may also help reduce damage from low-level radiation.