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. Understanding the behavior of shallow groundwater flow
over long time scales is also crucial in determining the origins
and future of fresh groundwater resources, which is an important
task for the next century when groundwater depletion is likely to
become a major environmental issue.

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. The
recent development of SECOFL3D, which uses a dynamically deforming
numerical grid for free surface problems to model fluctuations in
the groundwater table over long time scales, has made the
investigation of these problems tractable (Knupp, 1996). SECOFL3D
was modified to account for solute transport, variable-density
flow and groundwater age dating to compare modeled results to
modern geochemical measurements. The modified code (MWT3D_VDF),
was applied to two case studies. In the first I analyzed the
paleohydrology (50 ka) of the semi-arid Murray Basin of
southeastern Australia. The focus of this investigation was the
impact of water table fluctuations on groundwater/lake
interactions, and on the origin of anomalous zones of freshwater
in the southeastern part of the basin. Fossil groundwater is one
of the primary sources of irrigation water the southeastern Murray
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. Small islands are likely to be disproportionately
affected by global warming. Rising sea-levels and climate change
make the thin lens of fresh groundwater highly susceptible to
saltwater intrusion and contamination. These results highlight the
vulnerability of global groundwater resources to the effects of
global climate change.