Chemical tracing of interbasin groundwater transfer in the lowland rainforest of Costa Rica

Abstract

Chemical data from several hundred surface water and groundwater samples collected mainly during baseflow over 4.5 years were used to detect and quantify the natural interbasin transfer of deep groundwater into watersheds at La Selva Biological Station, a research site in the lowland rainforest of Costa Rica. Most of the variability in major ion concentrations at La Selva can be explained by mixing of two chemically and hydrologically distinct waters: high-solute bedrock groundwater, and low-solute local water draining from hillslope soils within the study watersheds. Several lines of evidence indicate that high-solute bedrock groundwater represents subsurface interbasin transfer into the study site.The fraction of water due to interbasin transfer (fwater) ranged from zero to about 0.49 for major streams at La Selva; fwater values were even higher (up to 0.84) for small riparian seeps and shallow groundwater near the Salto stream. The relative contribution of major ions by interbasin transfer was even more significant than of water itself. fwater values of 0.49 and 0.84 correspond to fCl values of 0.92 and 0.99, respectively (fCl, the fraction of dissolved chloride in a water sample that is due to interbasin transfer, is approximately equal to the fraction of all major ions contributed to the sample by interbasin transfer, given the observed linear correlation between Cl and other major ions). fwater and fCl of streams and riparian seeps varied on both long (monthly/seasonal) and short (storm event) time scales, in each case decreasing as conditions at La Selva became wetter. The high fwater values found in riparian groundwater and seeps indicate that local water and bedrock groundwater derived from interbasin transfer mix in the shallow subsurface at La Selva, not just in stream channels. With fwater values up to 0.84, it appears that some areas of riparian wetland may be maintained largely by interbasin transfer.This large interbasin transfer significantly affects both terrestrial (e.g. wetland) and aquatic ecosystems. Results suggest the importance of a regional approach to land use planning in this and similar environments. Complete protection of lowland streams, wetlands, and ecosystems in this hydrogeologic setting requires protection of a deep interbasin groundwater system whose precise volume, boundaries, and recharge areas are presently unknown.