Geohydrology and hydrochemistry of four tropical watersheds in Central Panama

摘要

The surface water chemistry of the Upper Rio Chagres basin of Central Panama, sampled toward the end of the 2004-05 dry season, has been compared with three smaller adjacent watersheds (Rio Pequini,Rio Cuango, and Rio Pacora) that have similar pristine source regions in steep mountainous tropical rain forest,but are affected by different downstream land-use practices. There is a geospatial trend in CI for the rivers of the region that mirrors the NE-SW precipitation gradient across the region. Dissolved solute contents are low (TDS = 480 ± 18), suggesting a short residence time in the weathering zone for infiltrating precipitation. Mg/Na ratios vary from 0.29 to 1.13 and Ca/Na ratios vary from 0.28 to 1.22, similar to rivers draining basaltic terranes worldwide. Ca/Mg ratios are low (0.7-2.5) but distinct for each basin. Estimated total cations (TZ+) yields are very large, suggesting significant weathering rates in the headwater reaches of these watersheds. The values fall within the mid-range of what has been observed globally for other streams/rivers draining mafic rocks.