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

The Environmental Protection Agency set a drinking water dose limit of 4 mrem per year for beta emitters and provides a table of isotope concentrations that will result in the 4 mrem limit.  The table assumes a daily water intake of 2 liters per day and states the concentrations are based on ingestion values from National Bureau of Standards (NBS) Handbook 69. Can anyone reproduce the calculation for this table that results in 900 pCi/L for ⁹⁹Tc?

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 4 mrem. For these emitters the MCLs were calculated for a 2 liter per day 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 3 x 10^-3 µCi/cc – the critical organ being the lower large intestine (LLI). This value was derived assuming a 2.2 liter per day intake rate with the targeted annual dose to the LLI being 15 rem. One can simply "ratio" the NBS 69 value with the targeted value of 4 mrem (0.004 rem) to derive the MCL as

Multiplying by 103 cc/L and 106 pCi/µCi and then rounding to one significant figure, one obtains an MCL of 900 pCi/L.

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, h_LLI, is

Using this dose coefficient, the concentration of ⁹⁹Tc in water that would deliver an annual dose of 4 mrem to the LLI, assuming an intake rate of 2 L/d (2,000 cc/d), 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.