No abstract

There is considerable interest in the use of thick argillaceous geologic formations to contain nuclear waste. Here, we show that diffusion can be the controlling transport process in these formations and diffusional time scales for δ18O and δ2H in water, dissolved He, and Cl transport in shale‐dominated aquitards are typically over 106 years, well exceeding the regulatory requirements for isolation in most countries. Our scientific understanding of diffusive solute transport processes through argillaceous formations would benefit from the application of additional isotopic tracers (e.g., using new 4He sampling technology), multidimensional diffusive‐dispersive modeling of groundwater flow and diffusive‐dispersive solute transport over long geologic time scales, and an improved understanding of spatial heterogeneity as well as time‐dependent changes in the subsurface conditions and properties of argillaceous formations in response to events such as glaciation. Based on our current isotopic and geochemical understanding of transport, we argue that argillaceous formations can provide favorable long‐term conditions for isolating nuclear wastes.

We tested two methods for dating groundwaters that cannot be reliably measured by 36 Cl dating alone, one based on groundwater flow velocity plus distance along a flow path and the other based on 4 He accumulation rates calibrated with 36 Cl dates. We sampled groundwaters along six inferred regional groundwater flow paths in the Great Artesian Basin (GAB) of Australia. We selected three groundwater paths where the decrease in 36 Cl was largely controlled by cosmogenic 36 Cl radioactive decay without a significant increase in chloride concentration. The extrapolated groundwater velocities were 0.133 ± 0.018 m/y to 0.433 ± 0.140 m/y. The estimated residence time of 1.06 × 10 6 y at the discharge area around Lake Eyre was comparable to the estimate of (1-2.2) × 10 6 y in previous studies. On the other hand, our estimated 4 He accumulation rates for the selected three groundwater flow paths (1.85 ± 0.31 × 10-11 to 1.51 ± 0.63 × 10-10 ccSTP/cm 3 •y) were approximately 2-15 times lower than previously reported rates for the central GAB. Our estimated rate of 1.51 × 10-10 ccSTP/cm 3 •y-1 in the western GAB is compatible with previous estimates based on 81 Kr ages. The groundwater residence time estimated from the 4 He accumulation rate was approximately 7 × 10 5 y near the discharge area at Lake Eyre. Finally, both estimations were mutually compatible with a 30% error.

No abstract

Fluorescent dyes are frequently used for tracing the flow of groundwater. The quantitative determination (QD) of dyes is affected by some factors during measurement and storage of the samples and that could be a cause of error in measurement. In this research, factors affect to the QD are considered and the effects of each factor are investigated. In addition, the methods to control such effects are proposed. In the measurement and pre-treatment, pH of solutions, temperature of solutions, co-existence of natural organic matter and filtration were found to affect QD significantly. However they could be controlled by buffering the pH with buffer solution, adjustment sample temperature, investigation of the fluorescent/quenching characteristics of natural organic matter and using rinsed PTFE filter, respectively. In the storage of the samples, most important point is light shielding. However sometimes dyes might be changed by microbial activity.