Radionuclides atmospheric origin

Table 1. Radionuclides of Atmospheric Origin and of Potential Use in Dating Ground Water.

Several anthropogenic constituents which are present in the atmosphere are potentially useful as an index of water age. Two radioactive gases from nuclear weapons and from power reactors, 3H and 85Kr, have been discussed already. Several other radionuclides of man-made origin are present in the atmosphere and in... 

Three primary problem areas exist in dating groundwater. These are (1) Formulation of realistic geochemical-hydrodynamic models needed to interpret data which are generated by field and laboratory measurements, (2) development of sensitive and accurate analytical methods needed to measure trace amounts of various stable and unstable nuclides, and (3) theoretical and field oriented studies to determine with greater accuracy the extent and distribution of the subsurface production of radionuclides which are commonly assumed to originate only in the atmosphere. 

Exposure from terrestrial radionuclides present at trace levels in all soils are specific and relate to the types of rock from which the soils originate. Higher radiation levels are associated with igneous rocks, such as granite, and lower levels with sedimentary rocks. There are exceptions, however, as some shales and phosphate rocks have relatively high contents of radionuclides. Radon and its short-lived decay products in the atmosphere are the most important contributors to human exposure from natural sources. 

Since the production rates of the cosmic ray radionuclides increase rapidly with increasing altitude in the lower atmosphere, the atmospheric concentrations and ratios of short lived cosmic ray radionuclides can be used to study rapid vertical air motions if the equilibrium concentrations of the radionuclides are known. For example, the concentrations of the short lived cosmic ray radionuclides in air which has moved upward recently from a lower altitude will be less than the equilibrium concentrations. The concentrations of the radionuclides will therefore increase with time until equilibrium is reached. However, the concentration of the shorter lived of two short lived radionuclides will increase more rapidly initially, causing the ratio of the two radionuclides of different half-lives to change with time until equilibrium is reached. Therefore, the time since the air moved from a lower altitude, the speed of the upward motion, and the altitude from which the air originated could be calculated from the concentrations and concentration ratios of cosmic ray radionuclides of different half-lives. Vertical motions of different speeds could be studied since several cosmic ray radionuclides of different half-lives are present in the atmosphere (Table I). Many other radionuclides are produced by cosmic rays in the atmosphere, but they have not yet been detected. Some of these with half-lives of a few minutes could serve as tracers of very short term processes such as post-nucleation scavenging. 

FIGURE 3. Steric exclusion chromatography of water from waste trenches and inert atmosphere wells at Maxey Flats, sampled July, 1982. Water samples from waste trenches 27 and 19S (440 ml) and wells WIN (585 ml) and W2NA (238.5 ml) were concentrated to 10 ml, pH-adjusted to their original values and chromatographed on a Sephadex G-15 column at flow rates ranging from 28-33 ml/hr. Individual column fractions were collected every 10 min and analyzed for specific organic compounds and radionuclides. 

The radionuclides encountered in the atmosphere are of either natural or artificial origin — produced by human activity during controlled or uncontrolled nuclear reactions. On the basis of this, the topic may be considered in two parts, concerning natural and artificial radionuclides. 

More confident attribution usually is possible when radionuclides in a sample can have only limited origin from, say, atmospheric fallout and one or two nuclear facilities. For tritium, I, or uranium in the environment, for example, the specific activity (radioisotope/stable isotope ratio) can indicate the origin. Certain activation products can be attributed to specific nuclear medicine or reactor facilities. 

Radionuclides in soil are a source of the contamination of forage and food with radionuclides, of which strontium, cesium, and radium isotopes are the most significant. Radionuclides penetrate into plants either from the atmosphere as deposits on soil surface or through roots from the soil. Plants radio-actively contaminated are a significant hazard to man, either directly (food of plant origin) or indirectly (the milk of animals which receive contaminated forage). 

Natural radionuclides formed in the atmosphere by gas-to-particle conversion reactions include the relatively short-lived radionuclides with half-lives of a number of days, of cosmo-genic origin, such as Be T i2 = 53.3 d), (14.3 d), (25.3 d) and (87.4 d) and... 

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