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Gross-beta activity in ground water: natural sources and artifacts of sampling and laboratory analysis
Ist Teil von
Applied geochemistry, 1995, Vol.10 (5), p.491-503
Ort / Verlag
Oxford: Elsevier Ltd
Erscheinungsjahr
1995
Quelle
Elsevier ScienceDirect Journals
Beschreibungen/Notizen
Gross-beta activity has been used as an indicator of beta-emitting isotopes in water since at least the early 1950s. Originally designed for detection of radioactive releases from nuclear facilities and weapons tests, analysis of gross-beta activity is widely used in studies of naturally occurring radioactivity in ground water. Analyses of about 800 samples from 5 ground-water regions of the United States provide a basis for evaluating the utility of this measurement. The data suggest that measured gross-beta activities are due to (1) long-lived radionuclides in ground water, and (2) ingrowth of beta-emitting radionuclides during holding times between collection of samples and laboratory measurements.
Although
40K and
228Ra appear to be the primary sources of beta activity in ground water, the sum of
40K plus
228Ra appears to be less than the measured gross-beta activity in most ground-water samples. The difference between the contribution from these radionuclides and gross-beta activity is most pronounced in ground water with gross-beta activities > 10 pCi/L, where these 2 radionuclides account for less than one-half the measured ross-beta activity. One exception is groundwater from the Coastal Plain of New Jersey, where
40K plus
228Ra generally contribute most of the gross-beta activity. In contrast,
40K and
228Ra generally contribute most of beta activity in ground water with gross-beta activities < 1 pCi/L.
The gross-beta technique does not measure all beta activity in ground water. Although
3H contributes beta activity to some ground water, it is driven from the sample before counting and therefore is not detected by gross-beta measurements. Beta-emitting radionuclides with half-lives shorter than a few days can decay to low values between sampling and counting. Although little is known about concentrations of most short-lived beta-emitting radionuclides in environmental ground water (water unaffected by direct releases from nuclear facilities and weapons tests), their activities are expected to be low.
Ingrowth of beta-emitting radionuclides during sample holding times can contribute to gross-beta activity, particularly in ground water with gross-beta activities > 10 pCi/L. Ingrowth of beta-emitting progeny of
238U, specifically
234Pa and
234Th, contributes much of the measured gross-beta activity in ground water from 4 of the 5 areas studied. Consequently, gross-beta activity measurements commonly overestimate the abundance of beta-emitting radionuclides actually present in ground water. Differing sample holding times before analysis lead to differing amounts of ingrowth of the two progeny. Therefore, holding times can affect observed gross-beta measurements, particularly in ground water with
238U activities that are moderate to high compared with the activity of
40K plus
228Ra. Uncertainties associated with counting efficiencies for beta particles with different energies further complicate the interpretation of gross-beta measurements.