Variable density groundwater flow: From current challenges to future possibilities

Excerpt

Over the last decade or two, there has been an explosion in the field of variable density flow because of worldwide concern about the future of energy and water resources and environmental pollution. Recent exhaustive review articles on this topic by Diersch and Kolditz (2002) and Simmons et al. (2001) clearly illustrate the widespread importance, diversity and interest in applications of variable density flow phenomena in groundwater hydrology. These include seawater intrusion, fresh-saline water interfaces and saltwater upconing in coastal aquifers, subterranean groundwater discharge, dense contaminant plume migration, DNAPL studies, density driven transport in the vadose zone, flow through salt formations in high level radioactive waste disposal sites, heat and fluid flow in geothermal systems, palaeohydrogeology of sedimentary basins, sedimentary basin mass and heat transport and diagenesis, processes beneath sabkhas and salt lakes and buoyant plume effects in applied tracer tests. A number of interesting but yet implicit points also emerge from these recent reviews that set the scene for this essay and it is useful to examine the ancestry of this field briefly. Firstly, the field evolved from traditional fluid mechanics and its evolutionary timeline appears to contain four major stages. Earliest work was carried out in the early 1900s in heated fluids only with a move to combined heat and porous media studies in the 1940s, then to solute and porous media studies in the 1950s and 1960s and finally combined thermohaline studies in porous media in the 1960s. Whilst these studies provide the necessary foundations for the study of variable density flow in groundwater, there are numerous implicit and explicit assumptions, simplifications and fundamental differences that make the direct application of traditional fluid mechanics to groundwater hydrology challenging. Secondly, as groundwater hydrologists, our interest in energy and solute transport is relatively recent (post-1950s in general) so that there is only a small amount of overlap between the timelines for the developments made in traditional fluid mechanics and their somewhat immediate applications in groundwater hydrology. These points are of fundamental importance because it is often in the underlying subtle and not so subtle differences between groundwater applications and their fluid mechanics ancestry that currently identified challenges and shorter-term future research needs in the field of variable density flow are found. The number of unresolved issues in this field of research leaves little room for speculation on this point and it appears that in comparison to some other areas of hydrogeology, our understanding of variable density flow in real field scale applications is in its infancy. For the safe of brevity, this essay does not repeat the exhaustive reviews of Diersch and Kolditz (2002) and Simmons et al. (2001) and is therefore not a detailed bibliographic review. The reader is referred to those reviews for extensive reference lists. The intention here is to draw upon those cited reviews as well as a current literature investigation to highlight key emerging research challenges and to conclude by speculating on what appears to be enormous range of future possibilities for this field of hydrogeology. …