Diffusion is sometimes a dominant process affecting radiotracers commonly used to quantify water resources, such as 3H, 14C and 36Cl. However, measuring diffusion in heterogeneous environments including fractured rock currently requires many samples and assumptions. Typical diffusion coefficients derived from small rock samples are insufficient to determine diffusive losses in such settings. Larger representative lengths of drill cores are often available, but current diffusion methods cannot utilise large samples. Isolating the diffusive fluxes from other tracer losses, such as adsorption or precipitation, within a representative sample over a reasonable period is necessary to gauge and correct for diffusive tracer losses.
A paired radioactive-stable tracer radial diffusion method using lengths of drill core in a close-fitting acrylic tube diffusion cell has been devised and tested in the laboratory. With frequent injections, the radioactive decay of the short-lived radiotracer (82Br or 131I) maintains a steeper activity gradient and therefore diffusive flux into the core than the accumulating stable isotope of the same ion (Br or I). Therefore, the normalised radiotracer lost from the diffusion cell reservoir (1-At/A0) remains greater than the diminishing stable tracer losses (1-Ct/C0). The range of possible diffusive loss becomes [(0.5 + Ct/C0 – At/A0) ± (0.5 - Ct/2C0)], which converges logarithmically with repeated injections.
Within two weeks, five drill cores yielded daily diffusive reservoir losses for bromide tracers: coarse sandstone 10±2%, medium sandstone 11±1%, massive limestone 2±1%, basalt 2±1% and basaltic breccia 3±1%. The iodide results were very similar once corrected for relative diffusion coefficients and duration, though higher in one basalt core.
Larger, representative heterogeneous cores can now be assessed. The derived rate of tracer loss can be applied to correct for diffusive losses of commonly used environmental radiotracers. This provides a previously unavailable correction for determining flow paths, rates and ultimately water resources in heterogeneous environments.