Answer to Question #314 Submitted to "Ask the Experts"

Category: Doses and Dose Calculations — Internal dose calculations

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

Given a level of 5.5 pCi/Ll of combined radium, how do I determine the radiation exposure (in mrem/hr?/yr?) if the radium is found in well water that is used to cook.

A recently published Environmental Protection Agency (EPA) report (Estimated Per Capita Water Ingestion in the United States, Based on Data Collected by the United States Department of Agriculture's 1994-96 Continuing Survey of Food Intakes by Individuals, EPA-822-00-008, U.S. EPA, Washington, DC, 20460, April 2000) includes data on indirect water intake. Indirect water is defined as water added to foods and beverages during final preparation at home or in food service establishments. It does not include biological water (for example, water in the juice of grapes) or water added to foods by the manufacturer. Its source is, therefore, water "used to cook."

Table 1. Mean individual indirect water intake rate.
Age interval (years)	Intake rate (L/d)
<1	0.284
1-10	0.137
11-19	0.206
20+	0.524
all ages	0.421

Age-specific effective dose coefficients for ingestion of radium isotopes are listed in Table A.1 of International Commission on Radiological Protection (ICRP) Publication 72. The ingestion dose coefficients (converted from Sv/Bq to mrem/pCi) for ²²⁶Ra and ²²⁸Ra are tabulated below.

Table 2. Effective dose coefficient, e (mrem/pCi), for ingestion.
Nuclide	Ingestion age (years)
Nuclide	Infant	1	5	10	15	Adult
²²⁶Ra	1.7x10^-3	3.6x10^-3	2.3x10^-3	3.0x10^-3	5.6x10^-3	1.0x10^-3
²²⁸Ra	1.1x10^-3	2.1x10^-3	1.3x10^-3	1.4x10^-3	2.0x10^-3	2.6x10^-3

Using Table 1, the radium activity intake rate at five years of age would be 0.137 (L/d) × 5.5 (pCi/L) = 0.754 pCi/d. If the radium is all ²²⁶Ra, the corresponding effective dose rate, using Table 2, would be 0.754 (pCi/d) × 2.3 ×10^-3 (mrem/pCi) × 365 (d/y) = 0.63 mrem/y. Similarly, for ²²⁸Ra, the dose rate would be 3.6 mrem/y. The dose rate for a 5.5 pCi L^-1 combination of the two radionuclides would, of course, be between these two values.

Neal Nelson, Ph.D.
Christopher Nelson, Ph.D.

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