Fallout levels in Berkeley CA in bq/m2

Posted by BC 8/23

I have finally found the time to finish up a little project I have been working on. No doubt those of you who have paid a lot of attention to this Fukushima train wreck have seen maps that show levels of cesium deposition in the areas around the plant (and of course there are maps like that for Chernobyl as well). Well, I wanted to know how much “rain-out” cesium deposition Berkeley CA got in March-April, and so I did the math. Before the actual numbers, let me explain my methods and some of the caveats therein.

First, all activity levels are from the Berkeley lab. Rainfall numbers are from www.wunderground.com, and as there was no data for Berkeley, so I used the nearest location, Oakland.

Second, the rainfall data is only listed as per day, not per hour, and often the BRAWM team took multiple samples in a given day. So what I did was assign a percentage of each given day's rainfall to each sampling period, based purely on the number of hours sampled. So for example if on a given day sample A was taken from 01:00 to 7:00 and sample B was taken from 08:00 to 14:00, I would add the 6 hrs from A and the 8 hrs from B and end up with a total 14 hours of sampling time. If on that same day, there was .50" inch of rainfall, I assigned 43% (0.22") of the rainfall to sample A and 57% (0.29") to sample B. I realize that there is some innaccuracy in this (ie, it might've rained cats and dogs in sample A and very little in sample B), but it's the best way I could think of to do it (and this method does guarantee that all rainfall is accounted for). I also did arbitrarily add .10" of rain for the 4/6-4/7 sample, where there was no precip listed for Oakland but obviously BRAWM was able to take a sample.

Once the amounts of precipitation were assigned to the sampling periods measured by the team, I converted the inches of rainfall into litres/m2 (simply by multiplying the amount in inches by 25.4 to get litres per m2). Then I took the litres/m2 and multiplied it by the activity level for each of the cesium isotopes (found in the respective sample), which then shows me how much of each cesium isotope was deposited during each sampling period. Adding all the results from March and April together gave me rough numbers for the "wet deposition."

There is some inaccuracy inherent in my method, most of which is related to the rainfall data, but I'd guesstimate that the final numbers are within ~25%.

Cs-134 ~ 30bq/m2 Cs-137 ~ 34bq/m2 for Berkeley CA March-April 2011.

Bear in mind that wet deposition is by far the largest contributor to fallout levels, so this gives us a pretty good idea of how much came down. Several subsequent tests came back non-detect, so while there may have been extremely small amounts deposited since then, it wouldn’t add much to the total.