5.1  Non‑Conservative Tracer Behavior

Peter Cook

5.1 Non‑Conservative Tracer Behavior

To this point, this discussion of groundwater age tracers has mostly assumed that they are neither produced nor consumed in the subsurface, other than by radioactive decay processes. This is not always the case. Carbon-14 activities can be altered by chemical reactions with geologic materials containing mineral or organic carbon. In some cases, these reactions can be identified, and a correction can be made using geochemical modelling. Numerous authors have shown that CFC-11 is subject to microbial degradation in anaerobic environments. This will lead to CFC-11 ages that are significantly older than true groundwater ages. There is some evidence for sorption of CFC-113 (sorption is delayed migration because a dissolved constituent is either temporarily or permanently attached to solids), particularly in aquifers with a high organic carbon content, and this will also lead to apparent ages that are greater than true water ages. In contrast, CFC-12 appears to behave conservatively in most groundwater systems.

Some tracers can also be produced in groundwater systems. SF₆ production has been observed in some groundwaters, particularly those in igneous rocks. Also, subsurface production of ³⁶Cl can occur. This does not affect use of this isotope as an indicator of recent recharge because ³⁶Cl concentrations in rainfall were very high during the 1960s, but it can affect groundwater dating on timescales of hundreds of thousands to millions of years. ³He and ⁴He can be contributed from other sources, including excess air. Excess air can also lead to elevated concentrations of SF₆ in groundwater, and this needs to be accounted for correctly in order to make accurate age determinations. Corrections for excess air are sometimes required for CFC-11, CFC-12 and CFC-113. All dissolved gases can be contaminated during drilling and well development using compressed air. This is most significant in low permeability environments, and where sampling occurs shortly after well installation, and is most significant for the low solubility gases (e.g., SF₆).

Artificially elevated tracer concentrations can occur in contaminated environments, and this can preclude the use of such tracers as groundwater dating tools. Elevated CFC concentrations are widely reported in groundwater in urban and industrial areas, and groundwater contamination is possible for other tracers with industrial sources, such as SF₆. Some of the radioactive tracers (e.g., ¹⁴C, ³H, ⁸⁵Kr) may be contaminated at sites with a history of nuclear activities.

When more than one tracer is measured, it is often instructive to prepare tracer – tracer plots that compare the relationship between different tracers in atmospheric input with the measured values (Figure 48). Because the different tracers are affected by the above processes to different extents, it is often possible to identify non-conservative behavior by comparing concentrations or apparent ages obtained from different tracers. Thus, for example, degradation of CFC-11 may be apparent from CFC-11 ages that are significantly older than ages obtained with other tracers such as CFC-12 or SF₆, for example. Contamination of one or more of the CFCs can similarly often be identified from discrepancies in ages.

**Figure** 48 – Plot showing expected CFC-11 and CFC-12 concentrations in groundwater, against which field data can be compared. The solid lines depict the relationship between concentrations of the tracers for rainfall in equilibrium with atmospheric gas concentrations between 1950 and 2018, at recharge temperatures between 10 and 30 °C. Concentrations of these tracers in the atmosphere (and hence in groundwater recharge) increased until the mid to late 1990s, but have decreased by 5 – 15 percent since then. This recent decrease in concentration causes the apparent ‘hooks’ at the end of each line. Samples that fall significantly below these equilibrium lines probably indicate degradation of CFC-11, although contamination of CFC-12 cannot be ruled out. Samples that fall above the lines probably indicate CFC-11 contamination (Cook, 2020).

Groundwater mixing can also be identified with tracer – tracer plots. The solid black line in Figure 49 depicts the relationship between CFC-12 and ¹⁴C concentration that should be observed in groundwater if these tracers are non-reactive and there is no significant mixing within the aquifer. Note that there is not a unique relationship between CFC-12 concentration and ¹⁴C activity because concentrations of these tracers have risen and fallen over time (Figure 6). The shaded area denotes possible concentrations that might occur if both tracers are non-reactive, but mixing occurs within the aquifer or during sampling, with the red circles denoting two mixing possibilities as explained in the caption of Figure 49. Water samples with concentrations outside of the shaded area cannot occur by mixing alone and would imply non-conservative behavior of at least one of the tracers and/or contamination.

Example of a mixing plot between CFC-12 and <sup>14</sup>C — **Figure** 49 – Example of a mixing plot between CFC-12 and ¹⁴C. The solid black line depicts the relationship between concentrations of the tracers for Australian rainfall in equilibrium with atmospheric gas concentrations between 1950 and 2014, at a temperature of 24 °C. This is assumed to be the concentration in recharge between these times. Prior to 1950, recharge would have had zero CFC-12, and so the line would extent along the x-axis to the origin. Points 1 to 5 therefore represent water with progressively younger ages (5 being the youngest). Two-component mixing lines can be drawn to connect any two points on the solid black line. Mixed samples can then occur anywhere on these lines, with the location on the line depending on the mixing proportion. For example, Sample A could be produced by mixing between end members at point 2 and point 5 (with a larger proportion from the end member at point 2). Alternatively, it could be produced by mixing between end members at point 1 and point 4. However, many other mixing combinations are also possible. Sample B could be produced by mixing between points 3 and 5. The shaded region indicates all possible concentrations that could arise due to mixing of water of different ages. Groundwater samples falling outside of this mixing envelope indicate that processes other than mixing have affected either ¹⁴C or CFC-12 ages. This could include chemical reactions affecting ¹⁴C or, degradation or contamination of CFC-12 (Cook, 2020).

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