Resumen de Mixing and chemical reaction hotspots in saline-freshwater mixing zones
Seawater intrusion presents one of the most unique and ubiquitous groundwater problems in the field of hydrogeology. Its complex boundary conditions coupled with variable density flow results in non-uniform flow which lead to non-trivial mixing dynamics. The mixing of fresh and saline waters in the subsurface is known to play a key role in an array of geochemical reactions. Such reactions include the dissolution and precipitation of calcite which shape the coastal landscape over millennia, as well as reactions associated to anthropogenic contamination of coastal groundwater such as nitrogen reduction or iron oxide precipitation which can act to limit the fluxes of harmful pollutants to vulnerable near shore marine ecosystems. In addition to reactions, understanding the mixing mechanisms is crucial to predicting the movement of the saltwater wedge and the size of the salt-freshwater interface. This has direct implications on the management of fresh groundwater reserves and its risk to salinization.
In this thesis we focus on understanding the influence variable density flow on mixing and the spatial distribution of enhanced geochemical activity along the saline-freshwater interface. First, we look to see the impact of heterogeneity on mixing and reaction for fast calcite dissolution. We find that heterogeneity permits reactions to occur over a greater area of the mixing zone due to enhanced mixing rates than would otherwise be permitted in homogeneous media. Furthermore, we observe unique patterns of enhanced localized reactivity that strongly deviate from the homogeneous case. Our study suggests that karst topology in coastal aquifers may be strongly linked to non-uniform flow induced by variable density flow and strongly linked to the type of heterogeneity present. Since initial stages of karst development are known to dictate the evolution of cave systems and conduits, we hypothesize that heterogeneity plays an integral role in its propagation.
Second, we study mixing across the salt-freshwater interface and the influence of compression caused by the flow of freshwater towards the saltwater body. In the absence of transient effects and heterogeneity, we find that mixing interface grow via transverse dispersion until some critical distance at which point it begins to recompress due to accelerating flow resulting from a decrease in area between the confining unit and the interface. Using a modified Glover solution to define the mixing interface, we are able to capture the velocity change along the interface from which a stretching rate (acceleration) can be deduced. We find that the behaviour of interface compression can be well-approximated by assuming a local Bachelor scale, which defines the equilibrium between dispersive growth and compression due to accelerating flow. Finally, we establish a new method to study fast mixing-dependent reactions across the salt-freshwater interface. Our findings lead us to the use of luminol chemiluminescence, which allows for the direct visualization of reaction rates along the saline-freshwater interface. Unlike conservative studies of mixing in sand tank, we bypass the need for complex image analysis techniques required to resolve local concentration gradients needed to evaluate mixing metrics. Results from experiments performed under both steady state and transient conditions consistently confirm local reaction hotspots at both the interface toe and at the head of the interface, which we attribute to stagnation point flow and accelerating flow respectively. Evaluation of the interface during the transient advance and retreat were also able to capture unique reactive behaviours that capture a shift in mixing behaviour due to a change in the flow field across the mixing zone.
