The suite of summer 2015 oceanic data are now ready and they show quite a change from 2014. Comparing these data side by side, it is plain to see that the concentrations of 137Cs have increased considerably in the central NE Pacific. It appears that the plume has spread throughout this vast area from Alaska to California. While natural processes of radioactive decay are slowly decreasing concentrations of 134Cs (with a 2 year half-life roughly 25% of the original concentration was present in April 2015), the signal for 137Cs is getting smeared by the currents of the NE Pacific and as they paint the path of the highest flows. For the sampling details and to see the values for 134Cs, see the interactive map.
Using the leading edge of the 3 Bq m–3 contour (the lightest of the green shades) as a reference point and measuring the difference in distance along the Line P stations, we estimate that the plume advanced roughly 650 km over the year; a rate of 1.75 km day-1. It’s important to note that that calculation is very rough and assumes many things, including that the ocean moved in a straight line along the sampling path and that the same mass of water was measured in both instances. While neither of these assumptions are likely correct, the calculation does serve as rough guide that the plume is approaching the coast at about twice the speed of a garden snail.
It is good to point out that these data, courtesy of our partners at Our Radioactive Ocean and Fisheries and Oceans Canada, do not yet indicate a back side of the plume where the concentrations are beginnning to decrease. While this could mean that concentrations will continue to increase, numerical model simulations suggest that the smearing effect will continue to disperse the plume and that we are approaching the maximum. Data from samples collected this summer will show if that hypothesis is correct.
Samples collected this summer are now currently going through processing back at the Smith Lab at the Bedford Institute for Oceanography. Two undergrads had the opportunity to head to sea for their first research cruises and both seemed to be yearning for next chance to get on the water. While Saskia‘s journey was filled with water, sea ice, walrus, hikes, and more water, Sara got to see the green flash, water, whales, viperfish, and yes, more water. You can read about their respective experiences in their posts written on board (Saskia’s posts, Sara’s posts).
Data from June samples keep creeping up the steady trend of increasing 137Cs concentation that indicates the leading edge of the Fukushima plume. Tracing that trendline back in time, it would appear that the plume arrived in BC’s coastal waters around the same time as InFORM monitoring began. This is evident in the regional graph as the data from each area of the BC coast was reasonably tightly clustered for the first few months of sampling. Since then the data have spread out indicating that the plume has arrived on the coast of Vancouver Island and the inland waters of the North Coast. However, slower estuarine circulation throughout the Salish Sea results in a delayed increase.
The steady rise allows us to forecast that the average sample will have double the initial background concentrations of 137Cs during the summer of 2016. While still far below the 10,000 Bq m-3 level of concern for cesium radionuclides in drinking water, these more contaminated samples should also more regularly contain the Fukushima fingerprint isotope, 134Cs.
As we know from the oceanic sampling, which is seawater that is still approaching the coast, the increasing trend in the nearshore waters will increase for some time to come. We can expect that concentrations of 137Cs will likely triple current levels before declining, but continued monitoring will be necessary to validate this hypothesis.
Salmon are starting to roll into the Pacific Biological Station in Nanaimo where they are processed and quickly sent off to Health Canada in Ottawa for radionuclide analyses. Due to the short (138 days) half-life of 210-Polonium, and expected low concentrations, we prioritize these analyses over the gamma spectrometer detection of cesium isotopes (with half-lives of 2 and 30 years for 134Cs and 137Cs, respectively). We’re currently expecting over 100 samples to be collected and analyzed with full results available mid-winter (roughly 100 days + quality control days from final collection).