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Gross-beta activity in ground water: natural sources and artifacts of sampling and laboratory analysis

Alan H. Welch·Zoltan Szabo·David L. Parkhurst·Peter C. Van Metre·Ann H. Mullin
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Abstract

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 40 K and 228 Ra appear to be the primary sources of beta activity in ground water, the sum of 40 K plus 228 Ra 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 40 K plus 228 Ra generally contribute most of the gross-beta activity. In contrast, 40 K and 228 Ra 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 3 H 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 238 U, specifically 234 Pa and 234 Th, 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 238 U activities that are moderate to high compared with the activity of 40 K plus 228 Ra. Uncertainties associated with counting efficiencies for beta particles with different energies further complicate the interpretation of gross-beta measurements.

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Alan H. Welch, Zoltan Szabo, David L. Parkhurst, Peter C. Van Metre, Ann H. Mullin. 1995. Gross-beta activity in ground water: natural sources and artifacts of sampling and laboratory analysis. https://doi.org/10.1016/0883-2927(95)00020-8

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