Answer to Question #482 Submitted to "Ask the Experts"

Q

What would be the maximum allowable concentration of ⁴⁰K (potassium-40) in drinking water (under U.S. regulations and in pCi/L units)? Where can one look up this kind of information? If it cannot be looked up directly, how does one locate the necessary information and do the calculation?

A

There is no U.S. drinking water standard for ⁴⁰K. This is no doubt because ⁴⁰K is a constituent of natural potassium, the body does a pretty good job of regulating potassium, and thus there isn't much of a chance of getting a buildup of ⁴⁰K. Merril Eisenbud (Environmental Radioactivity, 3rd Edition, 1987, Academic Press, page 149) says, "It is important to recognize that the potassium content of the body is under strict homeostatic control and is not influenced by variations in environmental levels." The preceding not withstanding, one can run through the exercise of calculating a ⁴⁰K limit. ⁴⁰K is a beta emitter. The new drinking water regulation being proposed by the Environmental Protection Agency does not include a listing for ⁴⁰K but specifies that the Maximum Contamination Limit for beta and photon emitters should correspond to a committed effective dose equivalent of 4 millirem/yr from an annual intake at the rate of two liters of drinking water per day. (A committed effective dose equivalent of 4 mrem/yr means that the effective dose equivalent expected over the next 50 years following one year's intake should not exceed 4 mrem). The ingestion dose conversion factor for ⁴⁰K is 2.29×10⁵ mrem/pCi. Thus, the concentration corresponding to the dose limit would then be: Concentration = 4 mrem/yr / (2 L/d×365.25 d/yr×2.29×10⁵ mrem/pCi) = 239 pCi/L (i.e. 240 pCi/L). In terms of natural potassium, this calculates to be 0.28 g/L (or 280 mg/L), or, for 2 L/day drinking water consumption, a daily intake of 0.56 g of potassium. The typical daily potassium intake for a reference person is 3.3 g/day (ranging from 1.43 to 6.54 g/day), so it is debatable whether limiting the ⁴⁰K content of drinking water to the calculated value would have any impact on radiation dose.

C. E. Roessler, PhD, CHP