A decade of environmental isotope research in a low permeability aquitard system

A decade of multi-isotope and hydrogeological research on a 160 m thick aquitard system has resulted in detailed, high-resolution profiles of stable and radiogenic isotopes (Tritium, δD and δ18O, 14C-DOC and 14C-DIC, 36Cl, δ37Cl, 4He). The interpretations of these independent isotopic tracers reveal that late Pleistocene age porewater remains preserved in the aquitard between 35-55 m below ground. Transport modeling of isotopic profiles indicates this water was emplaced with the till upon deposition between 10ka - 20ka BP, and that the late Holocene glacial-interglacial climatic transition occurred in this area between 7ka - 12ka BP. Interpretation of the isotope profiles further shows transport of solutes in this aquitard is by molecular diffusion. These findings clearly demonstrate solute transport in homogeneous clay-rich aquitards is highly predictable over 20 ka and greater time scales. Long-term stability, plasticity, ease of access, and a high degree of confidence in transport predictability suggest thick aquitards could act as long-term repositories for the isolation and storage of hazardous and nuclear waste. (author)