How should I evaluate natural radionuclides in water, and how do I determine doses from exposures?”

There are numerous analytical methods
that apply to the determination of uranium and progeny from uranium decay. As
to which methods are best, this is somewhat subjective and depends, at least in
part, on the levels of radioactivity in the samples and the sensitivity
requirements that you impose.

The simplest analytical methods
involve the use of gamma spectrometry, usually using a high purity germanium
detector and an associated multichannel analyzing system. Samples with
measurable activity may be placed in an appropriate container, such as a
Marinelli beaker, in order to measure gamma radiation from particular progeny
(e.g., ²¹⁴Pb and ²¹⁴Bi from the ²³⁸U decay
series). Such analyses, however, require making some assumptions about the
extent of equilibrium that exists among the progeny and precursors in order to
assess, for instance, the activity of the parent of the chain, such as ²³⁸U.
If gamma analysis is not suitable because of lack of equilibrium, insufficient
activity, etc., more sensitive methods that involve physical/chemical
separations of radionuclides of concern may be necessary.

Isolation of uranium and preparation
by precipitation and alpha counting in a gas flow proportional counting system
or electrodeposition and counting with a solid state alpha spectrometry system
are examples of methods recommended for uranium analysis. You can get an idea
of methods that have been used and recommended, along with indications of their
sensitivities and usual sample sizes by referring to this online document by
the U.S.
Department of Energy that summarizes some U.S. Environmental Protection Agency
(EPA)-approved procedures.

The conversion of activity
concentration to dose (rate) depends on how the exposure occurs, and the
particular physical and chemical characteristics of the radionuclides. If you
are referring to internal dose resulting from drinking contaminated water, you
may want to refer to publications of the International Commission on
Radiological Protection (ICRP). For example ICRP Publication 72, Age-dependent Doses to Members of the Public from Intake of
Radionuclides: Part 5 Compilation of Ingestion and Inhalation Dose Coefficients,
in Table A.1, provides committed effective dose per unit intake (Sv Bq^-1).
For a particular volume of water taken in the activity intake of a specific
radionuclide would be the activity concentration (e.g., Bq L^-1)
multiplied by the volume ingested; when this is multiplied by the dose
conversion factor for the aged individual of interest you would have an
estimate of the effective committed dose (Sv) from that intake. You can also
find committed effective dose data, taken from International Atomic Energy Agency
Safety Series document No. 115 on the  Radiation Dose Assessment Resource site. 

If you are concerned with external
dose from immersion in water or exposure to contaminated soil, you can also
find relative online data. One source is Federal
Guidance Report No. 12, External Exposure to Radionuclides in Air, Water, and Soil,
EPA-402-R-93-081, 1993. (Be aware that in this report effective doses are
based on the earlier set of tissue weighting factors that ICRP had used. This
is discussed in the report.) Another useful tool is the Radiological Toolbox developed by Oak Ridge National Laboratory
for the U.S. Nuclear Regulatory Commission. To evaluate external dose from contaminated air, water,
or soil, first specify the radionuclide(s), Select Dose Coefficients on the
left and Public External Coefficients (FGR 12) at the bottom, and click the
radio button to pick the external exposure pathway, which includes submersion
in air or water, exposure to a surface ground source, or exposure to volume
distributed activity in contaminated soil layers (1 cm, 5 cm, 15 cm, or
infinite thickness). Clicking “Display” shows the individual organ
doses and the effective doses, based on both ICRP 26 recommendations and ICRP
60 recommendations. Data and methods used to obtain the values here are
essentially the same as those used for EPA FGR 12.

For your specific needs you will
likely have to do more research, but I hope the above will get you started in
the right direction.

George Chabot, PhD