LITHOLOGIC CONTROL ON RIVER CHEMISTRY IN CENTRAL PANAMA

Chemical weathering of geological materials is a primary process affecting landscape development. Hydrochemical examination of 97 rivers and streams along a ~400 km transect across Panama from the Bayano region in the east to the Costa Rica border in the west documents large variations, e.g. SPC = 38-2924 mS/cm, Si = 0.005-3.02 mM & Ca/Mg = 0.68-5.32 for large streams and rivers compared to SPC = 32-1736 mS/cm, Si = 0.011-1.601 mM, & Ca/Mg = 0.66-33.7 for soil seeps and small streams. Watershed lithology exerts the major control on riverine chemistry, as illustrated by two adjacent watersheds in central Panama. The Rio Pacora watershed (374 km²) is predominantly composed of mafic bedrocks consisting of Late Cretaceous gabbro and diorite, as compared to the Rio Chagres watershed (580 km²) which is developed on more a compositionally and lithologically varied bedrock that ranges from hydrothermally altered Late Cretaceous igneous lithologies (gabbro to granite, basalt to dacite) and Mid-Tertiary volcanic rocks. In order to better determine the rocks/minerals undergoing weathering in each watershed, 20 samples from soil seeps and low-order headwater streams to the main stem rivers were analyzed for their ⁸⁷Sr/⁸⁶Sr signatures and compared to bedrock compositions. Data for the two watersheds form a linear trend on a ⁸⁷Sr/⁸⁶Sr versus 1/Sr plot, with no difference observed between the soil seeps, headwater streams, and main channel river waters. Rio Chagres waters are enriched in ⁸⁷Sr and depleted in Sr and also have lower TDS levels and Ca/Mg ratios compared to Rio Pacora waters, clearly documenting lithologic control on water chemistry throughout the two watersheds. Less-radiogenic Sr-isotope water ratios are equivalent to values measured for the least-altered rocks in the two watersheds, whereas the more radiogenic values suggest an input from weathering of minerals produced via early post-deposition alteration of volcanic lithologies by seawater.