ANALYTICAL SOLUTIONS AS A MEANS TO INVESTIGATE PAST AND FUTURE CLIMATE CHANGE, GROUNDWATER FLOW, AND SUBSURFACE THERMAL REGIMES

Groundwater flow and climate change can strongly influence subsurface temperature-depth profiles, causing them to deviate from the geothermal gradient. Careful interpretation of subsurface temperature profiles can thus yield valuable insight into local hydrogeological and climatic activity. Analytical solutions to the one-dimensional conduction-advection equation are simple, useful tools for investigating the interrelationships among climate change, groundwater flow, and subsurface thermal regimes. In this talk, three new analytical solutions will be presented that differ based on the nature of the boundary and/or initial conditions.

The first solution, which employs a series of step changes in surface temperature as the boundary condition, is applied to simulate observed (1970-2010) groundwater temperatures (GWT) in four shallow wells in rural Germany. The ability of this simple solution to reproduce the observed GWT trends lends credibility to the application of this approach for studying the influence of climate change on GWT. The second solution, which employs exponential initial and boundary conditions, is used to simulate future subsurface warming (up to the year 2100) in deeper wells in the Sendai Plain, Japan by using the present day temperature-depth profile as the initial condition and climate model projections as the boundary condition. The third solution combines elements of the first two solutions and is applied in an inverted form to infer vertical water fluxes in the Sendai Plain. The use of subsurface temperature as a hydrologic tracer is facilitated by programming this more complex analytical solution in Python to create the FAST (Flexible Analytical Solution using Temperature) model. Other applications of these solutions will also be discussed.