Radioactive isotopes

Unstable, or radioactive, nuclides emit spontaneously one or more particles or quanta to reach stability. This process is random, the emission rate being proportional to the number of radioactive atoms. The equation governing radioactive decay is:

N = N₀ e^-λt (4.4.1)

where N is number of radioactive atoms at time = t; N₀ is the number of radioactive atoms at time t = 0; λ is the decay constant. The function is plotted in Fig. 4.4.1. When only half of the radioactive atoms remain, i.e. N/N₀= 1/2, the time elapsed is called the half-life, t_1/2.

When equation 4.4.1 is written in the differential form:

dN/dt = -λ .N (4.4.2)

it is clear that the rate of decay is proportional to the number of radioactive atoms remaining. When the rate of decay, or particles emitted per unit time, is measured by low level counting techniques, we have a measure of the concentration of the radionuclide.

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Why environmental isotopes?

The term ‘environmental isotopes’ is employed to refer to isotopic ratios of elements making up substances in the environment, such as oxygen and hydrogen in water. Environmental isotopes label ground water on an ongoing basis, through either natural or anthropogenic processes. Isotope ratios are established largely during the recharge process. It is often necessary only to analyse one suite of samples taken from a number of points in the (groundwater) system – an ‘isotope snapshot’ – in order to gain an initial understanding of the hydrology, i.e. set up a tentative conceptual model. This may apply to a small basin or to an entire catchment. The complexity of the system will determine the extent to which more detailed information is required.

Underlying the application of environmental isotopes is the transport approach, which differs from the classical hydraulic approach, can produce an independent assessment of an existing conceptual hydrogeological model, suggest a model where none existed or produce information such as recharge rate and flow continuity, essential information in calibrating a numerical model. Environmental isotopes are particularly useful in assessing groundwater systems intended for emergency supply, especially where these are often deep-seated. Isotopes supply unique information on the dynamics (movement), the origins and environmental conditions of recharge, including palaeo-conditions, and mixing of ground water.

Environmental isotope hydrology is increasingly seen as indispensable in understanding and quantifying ground water systems, their vulnerability and sustainability.

B o x 4.4.1

Table 4.4.1. Some isotopic species used in isotope hydrology

15N/¹⁴N stable 0.37% (air) Isotope ratio

mass spectrometry _ Tracing of pollutants

beta counting ≈1,000 Fills gap between

3H and ³⁹Ar, ¹⁴C

39Ar 269 Cosmic rays

Crustal

Low level

beta counting ≈2,000 Fills gap between

3H and ¹⁴C

beta counting 5 x 10⁵ Old groundwater

36Cl 3.06 x 10⁵ Cosmic rays Nuclear tests

Accelerator mass

spectrometry 2 x 10⁶ Relatively inert ;

‘fossil’ water

222Rn 3.85 days Decay of uranium

Alpha ray counting;

laboratory or field days Inert gas; groundwater/

surface water relations

Tritium ³H

Tritium, or radioactive hydrogen, is produced in the atmosphere by cosmic ray reactions, oxidised to ³H¹HO and becomes a conservative tracer of rain water with a natural ³H/¹H ratio of about 5 x 10^–18 or 5 TU (tritium units) at continental sites. Isolated from the atmospheric source following rain recharge, no new tritium is added to an imaginary ‘parcel’ of groundwater and the concentration of tritium decreases with its characteristic half-life of 12.32 years as the ‘parcel’ moves through the system, giving time-dependent information or ‘ages’ on fairly recently recharged or

‘young’ groundwater. Routine low level counting techniques (see below) allow for the routine detection of tritium down to about 0.2 TU.

Tritium produced in anthropogenic processes and used commercially and in research escapes readily and adds to the atmospheric hydrogen inventory. During the thermonuclear test series since 1952, atmospheric tritium values rose up to 1,000 x in the northern Hemisphere in the early 1960s. Since the moratorium on testing in 1962 atmospheric concentrations have declined (Figure 4.4.2) and are reflected in rain. In the southern Hemisphere the response to these injections was considerably damped, where present-day rainfall values continue to be lower and closer to natural levels. In both Hemispheres there is a degree of ambiguity when ‘dating’ groundwater with environmental tritium on account of the rising and falling trends in atmospheric concentrations and thus in groundwater recharge, since 1960.

3

H/

He dating

Very low ³H abundances, beyond the range of low-level counting, can be measured by thoroughly degassing and storing a water sample gas-tight for a time t, at least half a year. Subsequently, ³He – the decay product of tritium – is collected and measured by mass spectrometry (see below). The tritium con-tent of the sample is given by:

3H = ³He/(1 - e^{- λt}) (4.4.3)

Alternatively, the age of the sample may be determined without knowing the tritium input function (Mook 2004).

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Figure 4.4.1. Exponential decay curve showing the ratio of the number of radioactive atoms remaining (N) to the original number (N₀) plotted against time in multiples of the half-life t_1/2

Radiocarbon

C

Radiocarbon is produced in atmospheric cosmic ray reactions similar to those producing ³H.

Oxidised to ¹⁴CO₂radiocarbon becomes part of atmospheric carbon dioxide, its natural concentration expressed as 100 PMC - per cent of modern carbon. Atmospheric carbon dioxide, assimilated by plants, liberated by humus and roots and dissolved in infiltrating ground water, leads to ¹⁴C-labelled dissolved inorganic carbon (DIC). As for tritium, reduction in concentration due to ¹⁴C decay after recharge provides time-dependent information on groundwater. The initial biogenic ¹⁴C/¹²C ratio can be altered chemically during recharge, and during chemical processes in the aquifer, but subsequently may be taken as conservative when the hydrochemistry has stabilised (Verhagen et al., 1991). With its much longer half-life of 5,730 years radiocarbon is the principal radioactive environmental tracer of older groundwater and makes it particularly relevant to studies of usually deeper-seated groundwater emergency supplies.

During the historic atmospheric thermonuclear test series, atmospheric radiocarbon values rose up to more than 200 PMC in the northern Hemisphere in the early 1960s and have since declined (Figure 4.4.3). In the southern Hemisphere the response to these injections was considerably damped.

Atmospheric values worldwide at present are similar and tending towards the pre-bomb, natural level.

Argon-39

39Ar is produced in the atmosphere where its natural abundance (8.5 x10^-16) is effectively constant. Dissolved in rain water along with other gases, it enters ground water. With its half-life of 269 years, ³⁹Ar is suitable to fill the gap in dating range between ³H and ¹⁴C with the advantage that it is chemically inert. However, the subsurface production of ³⁹Ar through radiation from thorium and uranium of the rock matrix makes it problematic in certain environments (Mook, 2004).