QUANTIFYING THE ROLE OF OLD GROUNDWATER IN THE MODERN HYDROLOGIC CYCLE OF A COSTA RICAN RAINFOREST

La Selva Biological Station in the lowland rainforest of Costa Rica has become a kind of natural laboratory for the study of interbasin groundwater flow beneath watershed topographic divides. Discharge of regional groundwater into lowland watersheds is promoted by La Selva’s geomorphic and hydrogeological setting (at the downslope end of a 3000 m regional elevation gradient, in an area with permeable volcanic bedrock and annual rainfall of 4-8 m). Data from a variety of natural and anthropogenic tracers (major ions, isotopes, dissolved gases) together show that water and elemental fluxes at La Selva are controlled by complex mixing between regional bedrock groundwater roughly a few thousand years old and local groundwater on the order of several years old. Up to half the water and well over 90% of some solutes discharging from watersheds is due to regional bedrock groundwater. Dissolved helium and inorganic carbon in regional bedrock groundwater have isotopic signatures consistent with magmatic outgassing in a subduction zone. Results collected over 12 years, and simulations with a watershed hydrologic model (TOPMODEL), suggest the discharge of regional groundwater has been essentially constant on monthly to multi-year time scales. Current work focuses on understanding how the carbon discharge from regional groundwater may affect fluxes of carbon from the lowland rainforest watersheds (export via streamflow, degassing from stream water, land surface – atmosphere exchange) and watershed-scale carbon budgets.