By JOSEPH MARTZ
Los Alamos

Recent new stories regarding detection of plutonium contamination in soils around Los Alamos have received considerable attention. I have a quite personal interest, as I spent nearly 50 years in Los Alamos, and grew up playing in these canyons. The studies report only the levels measured and leave the most important question unanswered: are these levels a hazard? 

As I could find no reporting discussing the actual hazard of this level of contamination, I took the liberty of using the provided data to see if these levels are something I should be concerned about. A bit of research reveals that the EPA has published a detailed set of instructions on how to calculate the risk of radionuclides in the environment. It’s available at this link: https://semspub.epa.gov/work/HQ/175427.pdf

Using the EPA guidance, you can calculate something called the “Soil Screening Level” which is the final number the EPA uses to determine if there’s both an elevated risk and whether a site should be considered for remediation.* Calculation of the SSL is a bit complex. The calculation considers various factors, including the concentration of contaminants, the total exposure an individual might receive, and the risk of cancer from such exposures. The EPA method assumes extremely conservative exposures in these assessments. For example, in calculating the risk from radiation exposure in the soil, the EPA SSL calculation assumes someone will live in that location for 350 days/year for 30 years.

The EPA considers an SSL value above 2.0 to be the threshold at which further study should be done. Levels below 0.5 are such that no action is required and no significant additional risk is imposed upon the public. I emphasize again that these calculations are done assuming exceptionally conservative conditions, such as 30-years residence at these sites for 350 days/year.

With that background, I took the data from Dr. Ketterer’s study, and found the highest plutonium concentrations he measured, a concentration 78 pCi/g. Using this, I calculated SSL levels for both ingestion and radiation exposure to see if these levels were significant. Ultimately, I wanted to know if I should be worried about the quantity of plutonium measured in his samples.

Here’s the results for the highest-level sample measured:

• Soil Screening Level (SSL) for ingestion: 0.036
• Soil Screening Level (SSL) for radiation exposure: 0.246

These levels are below the level that the EPA says warrants any further action. As far as the EPA is concerned, these levels present no additional risk. And these calculations use the worst-case amount of plutonium found in the study coupled with extremely conservative assumptions when calculating cancer risk.

I thank Dr. Ketterer for this information. It confirms what the Department of Energy has said for years that these extremely low levels of contamination present no additional risk to the public.

—–

* the Soil Screening Level is quantified. An SSL of 1.0 means that there’s a 1 in 1,000,000 chance of 1 additional cancer death among the exposed public. Nearly any study would be very hard pressed to distinguish such a small risk from among the background data.