Geochemistry of four tropical montane watersheds, Central Panama

Abstract

Abstract The major element chemistry was determined for surface waters from four watersheds in Central Panama during the 2005 dry season to ascertain geochemical patterns resulting from differing geology and human influences as well to estimate chemical denudation rates for this montane region of tropical rain forest. The Upper Rio Chagres (580 km2), Rio Pequini (281 km2) and Rio Cuango (175 km2) watersheds are formed on a geologically mixed terrain that consists of strongly hydrothermally altered andesite and volumetrically subordinate mafic-intermediate volcanic rocks and felsic intrusive lithologies, whereas the Rio Pacora watershed (374 km2) is developed largely on gabbroic and dioritic lithologies. The headwater areas of all four river basins lie in pristine tropical rainforest, with the Rio Cuango, Rio Pequini and Rio Pacora subject to varying degrees of different land uses in their middle to lower reaches. Values of pH for the four watersheds are near neutral to slightly alkaline (7.0–8.5), DO saturation is high (typically >90%) and dissolved solute contents of the rivers and tributary streams are low (SPC = 130 ± 31 μS/cm), documenting the overall pristine quality of the waters in all four basins. Cluster analysis, supported by a comparison of elemental variations, indicates a broad geochemical similarity of rivers and streams in the four watersheds, but also reveals subtle differences that can be attributed to lithologic control rather than anthropogenic influences. Low-order streams in the Pacora watershed have distinctly higher TDS values plus silica and Ca2+ concentrations than those forming in the mixed lithology terrain. Streams and rivers developed on mafic terrain are also slightly more enriched in total dissolved cations (TZ+) and HCO 3 - , relative to silica, than streams and rivers developed in the mixed lithology terrain. Potassium concentrations are uniformly low, and like Mg2+ and Na+, are similar in both terrains. Calcium/Mg ratios for all watersheds are lower than the world river average, indicating the importance of the weathering of Mg-rich minerals. The Ca/Na, HCO3/Na and Mg/Na ratios for the Rio Pacora streams and rivers fall within the mid-range of what has been observed globally for other streams/rivers draining mafic rocks. The chemical weathering rate calculated is 108 tons/km2/a, which is about 40% of the physical denudation rate for the Upper Rio Chagres watershed of 275–289 tons/km2/a. The results of this study document that both chemical and physical erosion rates within tropical montane watersheds in central Panama are significant in a global context.