UNDERSTANDING TRANSPORT IN DIFFUSION-LIMITED SYSTEMS

Within hydrogeologic systems, transport of heat or a chemical species is retarded as heat/chemicals diffuse away from the region of groundwater flow. New analytical solutions that estimate temperature/concentration both along the aquifer flow path and within the diffusive medium adjacent to the groundwater flow path were developed and applied to two systems: (1) chemical transport in a fracture-matrix system, and (2) heat transport within a volcanic aquifer system. For chemical transport in a fracture-matrix system, we demonstrate the effects of both first-order reactions and matrix diffusion, estimating the resulting decrease in concentration as a function of distance along the fracture. For our second example (heat transport), we develop estimates of thermal response to climate change at groundwater-dependent ecosystems in the volcanic Medicine Lake highlands, California, USA. The thermal response of springs at high elevations is dominated by advection and will mostly occur within 20 years of a shift in recharge temperatures. Low elevation springs will respond more slowly, with significant response to conduction from the land surface within the first 20 years and lagged response to changes in recharge conditions at 20-60 years.