| This thesis investigates the past and future impacts of climatic changes on shallow groundwater systems and lake records of climatic change using numerical modeling. Due to its relative inaccessibility, groundwater systems are one of the least understood parts of the hydrologic cycle. Yet understanding groundwater hydrodynamics is essential for quantifying the behavior of the other parts of the cycle, such as surface water bodies. The intimate connection between ground and surface water, particularly in arid environments, makes the long-term transient behavior of the groundwater system essential for interpreting the paleo-environmental information recorded in lacustrine sedimentary deposits and in determining the origins and future of fresh groundwater resources.; Long time scales coupled with the non-linearity of groundwater/landsurface interactions have made analysis of the effects of climate on shallow groundwater flow difficult. In this study SECOFL3D, a model recently developed to investigate fluctuations in the groundwater table over long time periods, was modified to account for solute transport, the effects of variable-density fluids and groundwater ageing. The modified code (MWT3D_VDF, Moving Water Table in 3 Dimensions with Variable Density Flow) can be applied to coastal systems and semi-arid regions where saltwater/freshwater interactions play a substantial role in controlling regional groundwater flow patterns. In the first of two case studies, I analyzed the paleohydrology (50 ka) of the semi-arid Murray Basin of southeastern Australia to investigate the impact of water table fluctuations on groundwater/lake interactions and determine the origin of anomalous zones of freshwater in the southeastern part of the basin.; The second case study shifts time scales, environments, and hemispheres to look at the impact of future climatic change and sea-level rise on the groundwater resources of the small island aquifer system of Nantucket off the northeastern coast of North America. Rising sea-levels and climate change make the thin lens of fresh groundwater beneath small islands highly susceptible to saltwater intrusion and contamination. These results highlight the vulnerability of global groundwater resources to the effects of global climate change. |
