THE GEOCHEMISTRY OF HEADWATER STREAMS, CENTRAL PANAMA: INFLUENCE OF WATERSHED ROCK TYPE

Waters from eleven low-order streams in central Panama, collected toward the end of the dry season in February 2005, have been analyzed for their major ion concentrations. The headwater streams of the Rio Pacora (n=6) drain coase-grained gabbro lithologies, while the other three rivers (Chagres, Pequini, Cuango; n=5) drain hydrothermally altered andesites and basalts. Compared to the main river channels, all of these lower order streams have higher TDS and H₄SiO₄ concentrations, indicating that the most intense chemical weathering within the watersheds occurs in the headwater regions, and dilution of these solute-rich waters occurs at lower elevations downstream. The mean concentrations of weathering derived HCO₃^-, H₄SiO₄, Ca²⁺, Mg²⁺ and Na⁺ are 2.1, 1.3, 2.2, 1.5 and 1.6 times higher, respectively, in the gabbro lithology steams. The K⁺ concentrations are uniformly low and similar in both terranes. The gabbro terrane streams are also slightly more enriched in cations (TZ⁺) and HCO₃^-, relative to H₄SiO₄, than the other streams. Ca/Mg ratios from both the gabbro and the greenstone watersheds are lower than the world river average, indicating the importance of the weathering of Mg-rich minerals in both watersheds. The Ca/Na, HCO₃/Na and Mg/Na ratios for the Rio Pacora headwaters fall within the mid-range of what has been observed globally for other streams/rivers draining mafic rocks. Our roughly estimated TZ⁺ yields are very large, suggesting significant weathering rates in these headwater watersheds.