Groundwater salinization processes in the coastal area of the Netherlands due to transgressions during the Holocene

Vries, J.J. de [Promotor]Kooi, H. [Copromotor

Free convective controls on sequestration of salts into low-permeability strata: insights from sand tank laboratory experiments and numerical modelling

Using sand tank experiments and numerical models, local-scale solute-transport processes associated with free convection in both the region surrounding as well as within discrete low-permeability strata are explored. Different permeability geometries and contrasts between high- and low-permeability regions are tested. Results show that two free convective processes occur at different spatial and temporal scales. In the high-permeability region, salinisation was rapid and occurred predominantly by free convective flow around the low-permeability blocks (interlayer convection). A free convection flow field also became concurrently established within the low-permeability lenses (intralayer convection). It was found that upward vertical flow created by the large-scale interlayer free-convective flow field in the high-permeability region retards salinisation of the lenses as buoyant freshwater displacements oppose the downward penetration of dissolved salts. Salinisation of the low-permeability structures eventually takes place from below as saltwater is dragged upwards. This bottom up convective salinisation process of low permeability strata has not been reported in previous literature. These results demonstrate that variable-density sequestration of solutes driven by a source resident above the low-permeability layer is a complicated function of the geometry of the permeability distribution and the permeability contrast between low- and high-permeability regions. © 2009 Springer-Verlag

Comparison of parameter sensitivities between a laboratory and field scale model of uranium transport in a dual domain, distributed rate reactive system

A laboratory-derived conceptual and numerical model for U(VI) transport at the Hanford 300A site, Washington, USA, was applied to a range of field-scale scenarios of different geochemical complexity to identify the importance of individual processes in controlling the fate of U(VI), as well as to elucidate the characteristic differences between well-defined laboratory and the more complex field-scale conditions. Therefore, a rigorous sensitivity analysis was carried out for the various simulation scenarios. The underlying conceptual and numerical model, originally developed from column experiment data, includes distributed rate surface complexation kinetics of U(VI), aqueous speciation, and physical nonequilibrium transport processes. The field scenarios accounted additionally for highly transient groundwater flow and variable geochemical conditions driven by frequent water level changes of the nearby Columbia River. The results of the sensitivity analysis showed not only similarities but also important differences in parameter sensitivities between the laboratory and field-scale models. It was found that the actual degree of sorption disequilibrium, actual concentration of sorbed U(VI), and the sorption extent (i.e., theoretical concentration of sorbed U(VI) at equilibrium) are the major controls for the magnitude of the calculated parameter sensitivities. These internal model variables depended mainly on (1) the groundwater flow conditions, i.e., the relatively long phases of limited groundwater movement in the field scale (intercepted by short peak flow events) and the long sustained flow phases in the column experiment (intercepted by relatively short stop flow events), and (2) the sampling location in the field-scale model, i.e., plume fringe versus plume center. Copyright 2010 by the American Geophysical Union

Rates of salinization by free convection in high-permeablility sediments: insights from numerical modeling and application tot the Dutch coastal area.

Numerical modeling and dimensional analysis is used to study the salinization of thick, high-permeability aquifers by free convection from a salt source at the surface. Current understanding of this process mainly concerns the initial stages of salinization only (boundary-layer development, break-up into fingers and initial phase of finger descent). In the modeling, special attention is paid to the role of two processes in the long-term salinization rate: (1) the progressive loss of salt from fingers by lateral diffusion, and (2) the coalescence of fingers during their descent. From the numerical simulations a relationship is derived that describes the development of the horizontally averaged salinity with depth and time as a function of permeability and initial-density contrast for aquifer Rayleigh numbers up to Ra =6,000. This relationship is consistent with and provides an extension to previous generalized relationships of the rate of finger descent. Its applicability to real-world aquifers (Ra >1