Chemical mixing model of streamflow generation at La Selva Biological Station, Costa Rica

La Selva Biological Station occupies an area of lowland tropical rainforest in central Costa Rica. Sodium and chloride data were used to quantify the mixing proportions of local runoff and geothermal groundwater at several sites throughout the stream channel system in late April, at the end of the dry season. The fraction of streamflow accounted for by geothermal groundwater varied spatially between 0 and 0.85, indicating a significant contribution to streamflow and to stream solute loads from geothermal groundwater at some sites. In general, higher inputs of geothermal groundwater were found at lower elevations. Over half the flow from one basin (the Salto) was due to geothermal groundwater, suggesting a minimum annual runoff of about 0.7 m of geothermal groundwater from this basin, A plot of Na/Cl vs. fraction of geothermal groundwater revealed watershed-scale chemical differences between the two major drainage systems (the Sura and the Salto), differences that were not apparent from a traditional two-solute plot of Cl vs. Na concentration. A small (21 mm) storm produced relatively little change in mixing proportions, as most throughfall was apparently retained in the relatively dry soils. (C) 1997 Elsevier Science B.V.