Answer to Question #11601 Submitted to "Ask the Experts"

Category: Environmental and Background Radiation — Water

The following question was answered by an expert in the appropriate field:


If we have a leak in a storage tank that transfers iodine-131 (131I) in water to the soil, what are the permissible limits for 131I in water and soil?


That is an interesting question and one that does not have a direct answer. It is not clear if this is an actual incident or a hypothetical one, but I am going to assume that it is an actual incident.

First, depending on the total activity that was leaked, the incident may have to be reported to the U.S. Nuclear Regulatory Commission or your state agency (if you are in an agreement state). The incident is reportable based on the activity that was lost, not on the concentration in the soil or water.

Regarding the allowable concentration of 131I in water, there are two approaches to this determination:

  1. Under the regulations associated with the U.S. Safe Drinking Water Act, one can define the maximum contaminant level (MCL). The MCL for beta and gamma-ray radiation is 4 millirem per year (mrem y-1) or in International System (SI) units, 40 microsieverts per year (µSv y-1).a Based on an ingestion dose conversion factor of 2.2 × 10-8 sievert per becquerel (Sv Bq-1) (see and ingestion of two liters per day (L d-1) for 365 d y-1, the MCL for drinking water is 2.5 Bq L-1.

  2. Under certain approved and regulated releases to the sewer, the allowable monthly average concentration for 131I is 1 × 10-5 microcuries per milliliter (µCi mL-1) or in SI units, 370 Bq L-1 (10 CFR 20, Appendix B, Table 3).a

Regarding the allowable concentration of 131I in soil, the calculation is not as simple as above. Not knowing anything about your particular site, I am not able to calculate an "allowable" soil concentration. Basically the calculation is to determine the risk of cancer (or a deterministic effect) associated with the exposure of a human in the accessible environment to the 131I in the soil. Details of the steps to follow are available at the U.S. Environmental Protection Agency website. There is a Preliminary Remediation Goals Calculator available at The calculator has a couple of generic scenarios and associated default settings to "calculate a number." The calculator also accounts for the pathway of exposure of the contaminant in the soil: inhalation of the contaminated soil, external radiation exposure, ingestion of food grown in the contaminated soil, and ingestion of the contaminated soil. Typically the milk pathway is an important pathway for iodine, and I assume that the milk pathway is covered under the food ingestion pathway in this calculator. Using the "resident" scenario, the external exposure is projected to be the most significant pathway. The external exposure slope factor is 4.32 × 10-5 (risk y-1)/(Bq g-1), so for a projected 26-year exposure and a lifetime risk of 10-6, the soil concentration is determined to be approximately 1 mBq g-1.

Tim DeVol, CHP, PhD

a Here units are given first in µSv y-1 and µCi mL-1 (called traditional units) because these are the units used by U.S. regulatory agencies. However, the Health Physics Society has adopted the use of SI units and these are given following the traditional units.

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