Answer to Question #8498 Submitted to "Ask the Experts"

Category: Environmental and Background Radiation — Water

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

Q

The Environmental Protection Agency (EPA) set a drinking water dose limit of 0.04 mSv y-1 for beta emitters and provides a table of isotope concentrations that will result in the 0.04 mSv limit. The table assumes a daily water intake of 2 L d-1 and states the concentrations are based on ingestion values from the National Bureau of Standards (NBS) Handbook 69. Can anyone reproduce the calculation for this table that results in 33 Bq L-1 for technetium-99 (99Tc)?

A

EPA (1976) based the maximum contaminant level (MCL) for man-made beta and photon emitters in drinking water on a target annual dose to the total body or any organ of 0.04 mSv. For these emitters the MCLs were calculated for a 2 L d-1 water intake using the 168-hour maximum permissible concentration (MPC) listed in NBS Handbook 69 (NBS 1963). This was the "critical organ" philosophy of radiation protection in that period.

For 99Tc, the limiting (bold print) value of the soluble 168-hour (MPC)w in NBS 69 is 111 Bq cm-3 – the critical organ being the lower large intestine (LLI). This value was derived assuming a 2.2 L d-1 intake rate with the targeted annual dose to the LLI being 150 mSv. One can simply "ratio" the NBS 69 value with the targeted value of 0.04 mSv to derive the MCL as

Multiplying by 103 cm3 L-1 and then rounding to two significant figures, one obtains an MCL of 33 Bq L-1.

In an alternative approach, one would first derive the value of the dose per unit activity intake (in contemporary terminology, a dose coefficient) underlying the (MPC)w. Based on the NBS 69 data, the 99Tc dose coefficient for the LLI, hLLI, is

Using this dose coefficient, the concentration of 99Tc in water that would deliver an annual dose of 0.04 mSv to the LLI, assuming an intake rate of 2 L d-1, is

This value agrees with the corresponding value obtained via the ratio approach above.

Keith F. Eckerman

References

  • Environmental Protection Agency. National interim primary drinking water regulations. Office of Water Supply, EPA-570/9-76-003; 1976.
     
  • National Bureau of Standards. Maximum permissible body burdens and maximum permissible concentrations of radionuclides in air and in water for occupational exposure. National Bureau of Standards Handbook 69 as amended August 1963, U.S. Department of Commerce; 1963.
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