Current Results

» View full version

How We Analyze Samples and Report Data

Samples collected by scientists and citizens for Our Radioactive Ocean are analyzed in our labs at WHOI using a method that is capable of detecting extremely low levels of radioactivity produced by cesium isotopes in seawater. We report our data in units of Bequerels per cubic meter of seawater (Bq/m3), where one Bequerel is equal to one decay event per second.

Analysis

  1. Once we receive a sample in the lab, we first weigh it and measure the salinity of the water. Then we add a known quantity of stable cesium (Cs) to the sample (see step 4 below) and slowly pass (1 ml/min) the sample through a 5 ml column of potassium-nickel-hexacyanoferrate composite ion resin beads. This resin is specifically designed to selectively separate cesium (stable or radioactive) from the sample and has been optimized here for use in seawater samples.1
  2. We dry the resin and transferred it to a plastic “counting jar,” which we then place in an expensive ($75,000), high-purity germanium well detector made by Canberra Industries2 for between 24 and 72 hours (see 3:05 here). Every time a cesium atom decays, that event is registered in the instrument’s multichannel analyzer, which has the ability to discern energy given off by decay of two critical isotopes of cesium: 134Cs and 137Cs.3 By counting the decay events associated with each isotope, we calculate the total counts per second (cps) for a given sample.
  3. By periodically analyzing standards samples with known levels of cesium, we can calculate the detector efficiency. With this information, the sample weight, the cps, and the number of cesium gamma events per decay (the so-called “branching ratio”), we can calculate the total activity of 134Cs and 137Cs in each sample.
  4. By adding a known quantity of stable cesium to each sample at step 1, and then measuring how much passes through the resin column, we can determine a chemical yield for the extraction procedure. This stable cesium “yield monitor” is determined using an inductively coupled mass spectrometer. Extraction yields for the resin columns are typically 96 to 99 percent.

analysis

Accuracy

The result for each sample includes the total activity (Bq/m3) and an associated counting error, which is a measure of how precisely we can analyze for Cs isotopes: for example, 2.0 ± 0.2 Bq/m3. This number reflects a combination of uncertainties related to the number of decay events in a sample and are larger for a smaller cps), as well as uncertainties in the processing and calibration steps. To minimize these uncertainties, we regularly participate in proficiency tests with the International Atomic Energy Agency (IAEA) to ensure that our results are not just precise, but extremely accurate when compared to international seawater standards.

Detection limit

Our current detection limit using this method is about 0.1 Bq/m3 for 137Cs and 0.2 Bq/m3 for 134Cs. Values below this are reported as “below detection,” but this detection limit will vary with the sample size, the methods and detector used, and the total time each sample spends on the gamma detector. In general, larger sample sizes (we process a relatively large 20 liter sample), longer counting times (we typically leave a sample on for 48 hours or more), and more efficient detectors (we use some of the world’s most sensitive gamma detectors) lead to the lowest possible detection limits.

Results

We expect samples from the surface waters of the western Pacific that have not been contaminated by the Fukushima source to have 137Cs activity of between 1 and 2 Bq/m3 and for 134Cs to be “below detection.” This is because the only significant source of cesium in the Pacific prior to Fukushima was nuclear weapons testing during the 1950s and 1960s, and with its shorter 2-year half-life, all of the 134Cs from this source would have decayed by now, but because 137Cs has a 30-year half-life, we still see about 25 percent of the amount that was released (50 percent lost in first 30 years, half again of the remaining 50 percent lost in the following 30 years).

By January 2014, about 40 percent of the original Fukushima 134Cs remains in the environment compared to March/April 2011 when the disaster occurred, so we correct our data to account for decay of both cesium isotopes from the time of peak release directly to the ocean from the reactor complex in Fukushima: April 6, 2011. We do this to look for changes in the levels of cesium that result from ocean mixing and dilution, rather than just radioactive decay. For human health concerns, the activity at sampling may be of greater interest, and will be lower than the decay-corrected value we report.

1Kamenik, J., Dulaiova, H., Sebesta, F., Stastna, K., "Fast concentration of dissolved forms of cesium radioisotopes from large seawater samples," Journal of Radioanalytical and Nuclear Chemistry 296(2012): 841-846.

2http://www.canberra.com/products/detectors/germanium-detectors.asp

3We look at following energies: 661 keV for 137Cs; 604 and 795 keV for 134Cs.

4 cf., M. Aoyama, M., et al., "Cross equator transport of 137CS from the North Pacific Ocean to South Pacific Ocean (BEAGLE2003 cruises)," Progress in Oceanography 89(2011): 7-16.

scale

updates

November 10, 2014

Fukushima Radioactivity Detected Off West Coast

Monitoring efforts along the Pacific Coast of the U.S. and Canada have detected the presence of small amounts of radioactivity from the 2011 Fukushima Dai-ichi Nuclear Power Plant accident 100 miles (150 km) due west of Eureka, California. Scientists at the Woods Hole Oceanographic Institution (WHOI) found the trace amounts of telltale radioactive compounds as part of their ongoing monitoring of natural and human sources of radioactivity in the ocean.

In the aftermath of the 2011 tsunami off Japan, the Fukushima Dai-ichi Nuclear Power Plant released cesium-134 and other radioactive elements into the ocean at unprecedented levels. Since then, the radioactive plume has traveled west across the Pacific, propelled largely by ocean currents and being diluted along the way. At their highest near the damaged nuclear power plant in 2011, radioactivity levels peaked at more than 10 million times the levels recently detected near North America.

“We detected cesium-134, a contaminant from Fukushima, off the northern California coast.  The levels are only detectable by sophisticated equipment able to discern minute quantities of radioactivity,” said Ken Buesseler, a WHOI marine chemist, who is leading the monitoring effort. “Most people don’t realize that there was already cesium in Pacific waters prior to Fukushima, but only the cesium-137 isotope.  Cesium-137 undergoes radioactive decay with a 30-year half-life and was introduced to the environment during atmospheric weapons testing in the 1950s and ‘60s.  Along with cesium-137, we detected cesium-134 – which also does not occur naturally in the environment and has a half-life of just two years. Therefore the only source of this cesium-134 in the Pacific today is from Fukushima.”

The amount of cesium-134 reported in these new offshore data is less than 2 Becquerels per cubic meter (the number of decay events per second per 260 gallons of water). This Fukushima-derived cesium is far below where one might expect any measurable risk to human health or marine life, according to international health agencies.  And it is more than 1000 times lower than acceptable limits in drinking water set by US EPA.

Scientists have used models to predict when and how much cesium-134 from Fukushima would appear off shore of Alaska and the coast of Canada. They forecast that detectable amounts will move south along the coast of North America and eventually back towards Hawaii, but models differ greatly on when and how much would be found.

“We don’t know exactly when the Fukushima isotopes will be detectable closer to shore because the mixing of offshore surface waters and coastal waters is hard to predict. Mixing is hindered by coastal currents and near-shore upwelling of colder deep water,” said Buesseler.  “We stand to learn more from samples taken this winter when there is generally less upwelling, and exchange between coastal and offshore waters maybe enhanced.  ”

Because no U.S. federal agency is currently funding monitoring of ocean radioactivity in coastal waters, Buesseler launched a crowd-funded, citizen-science program to engage the public in gathering samples and to provide up-to-date scientific data on the levels of cesium isotopes along the west coast of North America and Hawaii. Since January 2014, when Buesseler launched the program, individuals and groups have collected more than 50 seawater samples and raised funds to have them analyzed. The results of samples collected from Alaska to San Diego and on the North Shore of Hawaii are posted on the website. To date, all of the coastal samples tested in Buesseler’s lab have shown no sign of cesium-134 from Fukushima (all are less than their detection limit of 0.2 Becquerel per cubic meter).

The offshore radioactivity reported this week came from water samples collected and sent to Buesseler’s lab for analysis in August by a group of volunteers on the research vessel Point Sur sailing betweenDutch Harbor, Alaska, and Eureka, California. These results confirm prior data described at a scientific meeting in Honolulu in Feb. 2014 by John Smith, a scientist at Fisheries and Oceans Canada in Dartmouth, Nova Scotia, who found similar levels on earlier research cruises off shore of Canada. Buesseler and Smith are now working together on a new project, led by Jay Cullen at the University of Victoria, Canada, called InFORM that involves Canadian academic, government and NGO partners to determine and communicate the environmental risks posed by Fukushima for Canada’s Pacific and Arctic coasts and their inhabitants.

Buesseler believes the spread of radioactivity across the Pacific is an evolving situation that demands careful, consistent monitoring of the sort conducted from the Point Sur.

“Crowd-sourced funding continues to be an important way to engage the public and reveal what is going on near the coast. But ocean scientists need to do more work offshore to understand how ocean currents will be transporting cesium on shore.  The models predict cesium levels to increase over the next two to three years, but do a poor job describing how much more dilution will take place and where those waters will reach the shore line first,” said Buesseler. “So we need both citizen scientists to keep up the coastal monitoring network, but also research vessels and comprehensive studies offshore like this one, that are too expensive for the average citizen to support,” said Buesseler.

Buesseler will be presenting his results on Nov. 13, 2014, at the SETAC conference in Vancouver. He is also responding to questions from the public on the “Ask Me Anything” forum on Reddit at 1 p.m. EST on Nov. 10. [ MORE ]

August 14, 2014
Using the most sensitive methods to measure your water samples, we have detected only cesium-137, the “legacy” cesium that remains from 1960s atmospheric weapons testing. This isotope is still in all ocean basins because of its relatively long 30-year half-life, which means it takes a long time to decay away. Levels of cesium-137 in all 43 samples analyzed thus far average 1.5 Bequerels per cubic meter of water, which is equivalent to one-and-a-half decay events per second per metric ton of water. This is a very small number if we compare it to the 7,400 Bq/m3 used by US EPA as the drinking water limit, and the millions of Bq/m3 of cesium detected in the ocean off Japan in 2011 at the peak of the accident, which at that level are of considerable concern for direct negative impacts on marine biota and human health.

The Fukushima reactors also released cesium-134 into the ocean and because it has a shorter half-life (2 years) any cesium-134 detected in the ocean today must have come from Fukushima. Though we do detect this isotope in abundance off Japan, cesium-134 is not YET present in any of the sample collected by citizen scientists along the North American west coast and Hawaii. Our instruments are capable of detecting as little as 0.2 Bq/m3 so the concentration of cesium-134 is below this level.

We emphasize that cesium-134 has not been detected “YET” as it has been detected offshore of North America by Canadian oceanographers. It’s difficult to predict when these radionuclides will arrive onshore because the mixing of offshore and onshore waters is complicated, and not represented in the simple models that predicted the arrival onshore of Fukushima radionuclides this year. The uncertainty in the predictions by these ocean models only emphasizes the importance of collecting samples from along the shores. Remember too that while those models predict increasing levels of both cesium isotopes for the next 2-3 years, the highest published prediction is for 20-30 Bq/m3, or well below what is thought to be of human health or fisheries concern. But it’s important to continue making observations with real data!

Your continued support will help us monitor how, when, and where offshore water reaches the beaches and what levels of cesium isotopes are transported by that water.

June 2, 2014
So far, none of the seawater samples taken from the Pacific Coast have contained any trace of radiation from Fukushima. They have contained the same levels of radiation that were evident in the Pacific Ocean before the Fukushima accident. These levels of cesium-137 measured at all sites are between 1 and 2 Bequerels per cubic meter, and are from the 1960s atmospheric nuclear weapons testing programs.

The lack of cesium-134, which only has a two-year half life for radioactive decay, indicates that none of the Fukushima contaminants have reached the West Coast sampling sites. Therefore, continued support for monitoring is needed as the cesium isotopes are expected to reach the coast in 2014 and levels are predicted to increase over the coming 2-3 years.

Click on a point on the map to see the data for that location.

January 28, 2014
The first results from seawater samples come from La Jolla and Point Reyes, Calif., and Grayland and Squium, Wash. Four samples from these three locations show no detectable Fukushima cesium. We know this because Fukushima released equal amounts of two isotopes of cesium: the shorter-lived cesium-134 isotope (half-life of 2 years) and the longer-lived cesium-137 (half-life of 30 years). Cesium-137 was found at levels of 1.5 Bq per cubic meter (Bq/m3), but this was already detectable prior to releases at Fukushima and came primarily from nuclear weapons testing in the Pacific during the 1950s and 1960s.

This so-called "negative" result has two immediate implications. First there should be no health concerns associated with swimming in the ocean as a result of Fukushima contaminants by themselves or as a result of any additional, low-level radioactive dose received from existing human and natural sources of radiation in the ocean (existing levels of cesium-137 are hundreds of times less than the dose provided by naturally occurring potassium-40 in seawater).

Secondly, and just as important from a scientific perspective, the results provide a key baseline from the West Coast prior to the arrival of the Fukushima plume. Models of ocean currents and cesium transport predict that the plume will arrive along the northern sections of the North American Pacific Coast (Alaska and northern British Columbia) sometime in the spring of 2014 and will arrive along the Washington, Oregon, and California coastline over the coming one to two years. The timing and pattern of dispersal underscores the need for samples further to the north, and for additional samples to be collected every few months at sites up and down the coast.

For this reason, we are also pleased to report that funds are already in hand to continue sampling at both the La Jolla and Pt. Reyes locations thanks to the foresight and generous donations of the groups who volunteered to adopt these sites. We expect levels of cesium-134 to become detectable in coming months, but the behavior of coastal currents will likely produce complex results (changing levels over time, arrival in some areas but not others) that cannot be accurately predicted by models. That is why ongoing support for long-term monitoring is so critical, now and in the future.[ LESS ]

scale

updates

November 10, 2014

Fukushima Radioactivity Detected Off West Coast

Monitoring efforts along the Pacific Coast of the U.S. and Canada have detected the presence of small amounts of radioactivity from the 2011 Fukushima Dai-ichi Nuclear Power Plant accident 100 miles (150 km) due west of Eureka, California. Scientists at the Woods Hole Oceanographic Institution (WHOI) found the trace amounts of telltale radioactive compounds as part of their ongoing monitoring of natural and human sources of radioactivity in the ocean.

In the aftermath of the 2011 tsunami off Japan, the Fukushima Dai-ichi Nuclear Power Plant released cesium-134 and other radioactive elements into the ocean at unprecedented levels. Since then, the radioactive plume has traveled west across the Pacific, propelled largely by ocean currents and being diluted along the way. At their highest near the damaged nuclear power plant in 2011, radioactivity levels peaked at more than 10 million times the levels recently detected near North America.

“We detected cesium-134, a contaminant from Fukushima, off the northern California coast.  The levels are only detectable by sophisticated equipment able to discern minute quantities of radioactivity,” said Ken Buesseler, a WHOI marine chemist, who is leading the monitoring effort. “Most people don’t realize that there was already cesium in Pacific waters prior to Fukushima, but only the cesium-137 isotope.  Cesium-137 undergoes radioactive decay with a 30-year half-life and was introduced to the environment during atmospheric weapons testing in the 1950s and ‘60s.  Along with cesium-137, we detected cesium-134 – which also does not occur naturally in the environment and has a half-life of just two years. Therefore the only source of this cesium-134 in the Pacific today is from Fukushima.”

The amount of cesium-134 reported in these new offshore data is less than 2 Becquerels per cubic meter (the number of decay events per second per 260 gallons of water). This Fukushima-derived cesium is far below where one might expect any measurable risk to human health or marine life, according to international health agencies.  And it is more than 1000 times lower than acceptable limits in drinking water set by US EPA.

Scientists have used models to predict when and how much cesium-134 from Fukushima would appear off shore of Alaska and the coast of Canada. They forecast that detectable amounts will move south along the coast of North America and eventually back towards Hawaii, but models differ greatly on when and how much would be found.

“We don’t know exactly when the Fukushima isotopes will be detectable closer to shore because the mixing of offshore surface waters and coastal waters is hard to predict. Mixing is hindered by coastal currents and near-shore upwelling of colder deep water,” said Buesseler.  “We stand to learn more from samples taken this winter when there is generally less upwelling, and exchange between coastal and offshore waters maybe enhanced.  ”

Because no U.S. federal agency is currently funding monitoring of ocean radioactivity in coastal waters, Buesseler launched a crowd-funded, citizen-science program to engage the public in gathering samples and to provide up-to-date scientific data on the levels of cesium isotopes along the west coast of North America and Hawaii. Since January 2014, when Buesseler launched the program, individuals and groups have collected more than 50 seawater samples and raised funds to have them analyzed. The results of samples collected from Alaska to San Diego and on the North Shore of Hawaii are posted on the website. To date, all of the coastal samples tested in Buesseler’s lab have shown no sign of cesium-134 from Fukushima (all are less than their detection limit of 0.2 Becquerel per cubic meter).

The offshore radioactivity reported this week came from water samples collected and sent to Buesseler’s lab for analysis in August by a group of volunteers on the research vessel Point Sur sailing betweenDutch Harbor, Alaska, and Eureka, California. These results confirm prior data described at a scientific meeting in Honolulu in Feb. 2014 by John Smith, a scientist at Fisheries and Oceans Canada in Dartmouth, Nova Scotia, who found similar levels on earlier research cruises off shore of Canada. Buesseler and Smith are now working together on a new project, led by Jay Cullen at the University of Victoria, Canada, called InFORM that involves Canadian academic, government and NGO partners to determine and communicate the environmental risks posed by Fukushima for Canada’s Pacific and Arctic coasts and their inhabitants.

Buesseler believes the spread of radioactivity across the Pacific is an evolving situation that demands careful, consistent monitoring of the sort conducted from the Point Sur.

“Crowd-sourced funding continues to be an important way to engage the public and reveal what is going on near the coast. But ocean scientists need to do more work offshore to understand how ocean currents will be transporting cesium on shore.  The models predict cesium levels to increase over the next two to three years, but do a poor job describing how much more dilution will take place and where those waters will reach the shore line first,” said Buesseler. “So we need both citizen scientists to keep up the coastal monitoring network, but also research vessels and comprehensive studies offshore like this one, that are too expensive for the average citizen to support,” said Buesseler.

Buesseler will be presenting his results on Nov. 13, 2014, at the SETAC conference in Vancouver. He is also responding to questions from the public on the “Ask Me Anything” forum on Reddit at 1 p.m. EST on Nov. 10. [ MORE ]

August 14, 2014
Using the most sensitive methods to measure your water samples, we have detected only cesium-137, the “legacy” cesium that remains from 1960s atmospheric weapons testing. This isotope is still in all ocean basins because of its relatively long 30-year half-life, which means it takes a long time to decay away. Levels of cesium-137 in all 43 samples analyzed thus far average 1.5 Bequerels per cubic meter of water, which is equivalent to one-and-a-half decay events per second per metric ton of water. This is a very small number if we compare it to the 7,400 Bq/m3 used by US EPA as the drinking water limit, and the millions of Bq/m3 of cesium detected in the ocean off Japan in 2011 at the peak of the accident, which at that level are of considerable concern for direct negative impacts on marine biota and human health.

The Fukushima reactors also released cesium-134 into the ocean and because it has a shorter half-life (2 years) any cesium-134 detected in the ocean today must have come from Fukushima. Though we do detect this isotope in abundance off Japan, cesium-134 is not YET present in any of the sample collected by citizen scientists along the North American west coast and Hawaii. Our instruments are capable of detecting as little as 0.2 Bq/m3 so the concentration of cesium-134 is below this level.

We emphasize that cesium-134 has not been detected “YET” as it has been detected offshore of North America by Canadian oceanographers. It’s difficult to predict when these radionuclides will arrive onshore because the mixing of offshore and onshore waters is complicated, and not represented in the simple models that predicted the arrival onshore of Fukushima radionuclides this year. The uncertainty in the predictions by these ocean models only emphasizes the importance of collecting samples from along the shores. Remember too that while those models predict increasing levels of both cesium isotopes for the next 2-3 years, the highest published prediction is for 20-30 Bq/m3, or well below what is thought to be of human health or fisheries concern. But it’s important to continue making observations with real data!

Your continued support will help us monitor how, when, and where offshore water reaches the beaches and what levels of cesium isotopes are transported by that water.

June 2, 2014
So far, none of the seawater samples taken from the Pacific Coast have contained any trace of radiation from Fukushima. They have contained the same levels of radiation that were evident in the Pacific Ocean before the Fukushima accident. These levels of cesium-137 measured at all sites are between 1 and 2 Bequerels per cubic meter, and are from the 1960s atmospheric nuclear weapons testing programs.

The lack of cesium-134, which only has a two-year half life for radioactive decay, indicates that none of the Fukushima contaminants have reached the West Coast sampling sites. Therefore, continued support for monitoring is needed as the cesium isotopes are expected to reach the coast in 2014 and levels are predicted to increase over the coming 2-3 years.

Click on a point on the map to see the data for that location.

January 28, 2014
The first results from seawater samples come from La Jolla and Point Reyes, Calif., and Grayland and Squium, Wash. Four samples from these three locations show no detectable Fukushima cesium. We know this because Fukushima released equal amounts of two isotopes of cesium: the shorter-lived cesium-134 isotope (half-life of 2 years) and the longer-lived cesium-137 (half-life of 30 years). Cesium-137 was found at levels of 1.5 Bq per cubic meter (Bq/m3), but this was already detectable prior to releases at Fukushima and came primarily from nuclear weapons testing in the Pacific during the 1950s and 1960s.

This so-called "negative" result has two immediate implications. First there should be no health concerns associated with swimming in the ocean as a result of Fukushima contaminants by themselves or as a result of any additional, low-level radioactive dose received from existing human and natural sources of radiation in the ocean (existing levels of cesium-137 are hundreds of times less than the dose provided by naturally occurring potassium-40 in seawater).

Secondly, and just as important from a scientific perspective, the results provide a key baseline from the West Coast prior to the arrival of the Fukushima plume. Models of ocean currents and cesium transport predict that the plume will arrive along the northern sections of the North American Pacific Coast (Alaska and northern British Columbia) sometime in the spring of 2014 and will arrive along the Washington, Oregon, and California coastline over the coming one to two years. The timing and pattern of dispersal underscores the need for samples further to the north, and for additional samples to be collected every few months at sites up and down the coast.

For this reason, we are also pleased to report that funds are already in hand to continue sampling at both the La Jolla and Pt. Reyes locations thanks to the foresight and generous donations of the groups who volunteered to adopt these sites. We expect levels of cesium-134 to become detectable in coming months, but the behavior of coastal currents will likely produce complex results (changing levels over time, arrival in some areas but not others) that cannot be accurately predicted by models. That is why ongoing support for long-term monitoring is so critical, now and in the future.[ LESS ]

How We Analyze Samples and Report Data

Samples collected by scientists and citizens for Our Radioactive Ocean are analyzed in our labs at WHOI using a method that is capable of detecting extremely low levels of radioactivity produced by cesium isotopes in seawater. We report our data in units of Bequerels per cubic meter of seawater (Bq/m3), where one Bequerel is equal to one decay event per second.

Analysis

  1. Once we receive a sample in the lab, we first weigh it and measure the salinity of the water. Then we add a known quantity of stable cesium (Cs) to the sample (see step 4 below) and slowly pass (1 ml/min) the sample through a 5 ml column of potassium-nickel-hexacyanoferrate composite ion resin beads. This resin is specifically designed to selectively separate cesium (stable or radioactive) from the sample and has been optimized here for use in seawater samples.1
  2. We dry the resin and transferred it to a plastic “counting jar,” which we then place in an expensive ($75,000), high-purity germanium well detector made by Canberra Industries2 for between 24 and 72 hours (see 3:05 here). Every time a cesium atom decays, that event is registered in the instrument’s multichannel analyzer, which has the ability to discern energy given off by decay of two critical isotopes of cesium: 134Cs and 137Cs.3 By counting the decay events associated with each isotope, we calculate the total counts per second (cps) for a given sample.
  3. By periodically analyzing standards samples with known levels of cesium, we can calculate the detector efficiency. With this information, the sample weight, the cps, and the number of cesium gamma events per decay (the so-called “branching ratio”), we can calculate the total activity of 134Cs and 137Cs in each sample.
  4. By adding a known quantity of stable cesium to each sample at step 1, and then measuring how much passes through the resin column, we can determine a chemical yield for the extraction procedure. This stable cesium “yield monitor” is determined using an inductively coupled mass spectrometer. Extraction yields for the resin columns are typically 96 to 99 percent.

analysis

Accuracy

The result for each sample includes the total activity (Bq/m3) and an associated counting error, which is a measure of how precisely we can analyze for Cs isotopes: for example, 2.0 ± 0.2 Bq/m3. This number reflects a combination of uncertainties related to the number of decay events in a sample and are larger for a smaller cps), as well as uncertainties in the processing and calibration steps. To minimize these uncertainties, we regularly participate in proficiency tests with the International Atomic Energy Agency (IAEA) to ensure that our results are not just precise, but extremely accurate when compared to international seawater standards.

Detection limit

Our current detection limit using this method is about 0.1 Bq/m3 for 137Cs and 0.2 Bq/m3 for 134Cs. Values below this are reported as “below detection,” but this detection limit will vary with the sample size, the methods and detector used, and the total time each sample spends on the gamma detector. In general, larger sample sizes (we process a relatively large 20 liter sample), longer counting times (we typically leave a sample on for 48 hours or more), and more efficient detectors (we use some of the world’s most sensitive gamma detectors) lead to the lowest possible detection limits.

Results

We expect samples from the surface waters of the western Pacific that have not been contaminated by the Fukushima source to have 137Cs activity of between 1 and 2 Bq/m3 and for 134Cs to be “below detection.” This is because the only significant source of cesium in the Pacific prior to Fukushima was nuclear weapons testing during the 1950s and 1960s, and with its shorter 2-year half-life, all of the 134Cs from this source would have decayed by now, but because 137Cs has a 30-year half-life, we still see about 25 percent of the amount that was released (50 percent lost in first 30 years, half again of the remaining 50 percent lost in the following 30 years).

By January 2014, about 40 percent of the original Fukushima 134Cs remains in the environment compared to March/April 2011 when the disaster occurred, so we correct our data to account for decay of both cesium isotopes from the time of peak release directly to the ocean from the reactor complex in Fukushima: April 6, 2011. We do this to look for changes in the levels of cesium that result from ocean mixing and dilution, rather than just radioactive decay. For human health concerns, the activity at sampling may be of greater interest, and will be lower than the decay-corrected value we report.

1Kamenik, J., Dulaiova, H., Sebesta, F., Stastna, K., "Fast concentration of dissolved forms of cesium radioisotopes from large seawater samples," Journal of Radioanalytical and Nuclear Chemistry 296(2012): 841-846.

2http://www.canberra.com/products/detectors/germanium-detectors.asp

3We look at following energies: 661 keV for 137Cs; 604 and 795 keV for 134Cs.

4 cf., M. Aoyama, M., et al., "Cross equator transport of 137CS from the North Pacific Ocean to South Pacific Ocean (BEAGLE2003 cruises)," Progress in Oceanography 89(2011): 7-16.