Answer to Question #9894 Submitted to "Ask the Experts"
The following question was answered by an expert in the appropriate field:
Simultaneous with a planned replacement of our septic system, my wife will be taking 8.25 GBq of 131I as post-surgical treatment for thyroid cancer which required total removal of the thyroid. We had expected treatment to not start for another three to four weeks, but she reached her needed TSH (thyroid-stimulating hormone) level unexpectedly early and is "crashing" quickly, so we may need to start the radiation treatment before the septic-system replacement can start. How long after start of the 131I treatment would you recommend waiting before the construction team works within the existing drain field and with the old extracted tank and pumped waste?
Using the following assumptions, I conservatively estimate you should put off the septic service for about three months!
If your wife has a 1 percent uptake, she will excrete 8.25 GBq into the septic system. The Environmental Protection Agency says the average U.S. household water use is about 400 gallons per day (1.51 x 106 millliters per day). 131I, with a half life of eight days will lose about 8.3 percent of its activity each day. The current Nuclear Regulatory Commission (NRC) concentration limit for unrestricted effluent concentrations in water is 0.037 Bq per milliliter. If you take into account the daily dilution and decay factors, it will take 87 days to get concentrations below the NRC limits for unrestricted access.
Hopefully, your sewer system isn't in an emergency situation that you can't wait that long. If it needs immediate service, you might want to see if the hospital could hold your wife in-house for 48 hours. She should excrete about 98 percent of the 131I within the first two days, if she has normal renal function. That will avoid the large amount of radioiodine being deposited into the septic tank, immediately before servicing.
Radiation Safety Officer