Existing data suggest interbasin groundwater flow (IGF) beneath surface topographic divides is responsible for large inputs of water and solutes to some lowland rainforest watersheds in Costa Rica. In this work age-dating tracers and trace gases will be used together with stable isotopes and major ions to test this conceptual hydrologic model for mixing of old regional groundwater (IGF) and much younger local groundwater in the watersheds. The tracers to be used are CFCs, 3H/3He, SF6, 4He, 14C, 36Cl, 18O, dissolved trace gases (Ne, Ar, Kr, N2, and O2), and major ions. Tracers will be measured in groundwater, spring water, and (for the non-volatile tracers only) streamwater, during both the wet season and dry season. The project will: 1. Improve understanding of an important but understudied hydrologic process, IGF: Wherever IGF exists, quantitative knowledge of it is central to understanding the hydrology and water quality of lowland watersheds, and the status and function of those watersheds as ecosystems. 2. Test the consistency of several tracer methods through their simultaneous application to a well-posed problem on an important hydrologic process in a new setting (rain forest with volcanic geology): Testing our conceptual hydrologic model will also test the general utility of state-of-the-art tracer data in identifying and quantifying mixing between groundwaters of very different age in a complex discharge area. 3. Help illustrate the practical significance of IGF for land/water management: The conceptual hydrologic model to be tested here suggests that full protection of lowland rainforest watersheds and ecosystems requires understanding a regional groundwater system whose volume, boundaries, and recharge areas are currently not known (but will be constrained by the results). Support for the model would underscore the importance of a regional approach to land and water management in the presence of IGF, and motivate further study of IGF at other sites. 4. Advance education and research: The project will have strong educational/training benefits for a graduate student who will master several state-of-the-art tracer techniques. The research partners (NCSU, USGS, Univ. of Utah, and the Organization for Tropical Studies) will build collaborations that enhance their current research and training activities, make this work possible, and have the potential to carry over to future activities.
