GEOHYDROLOGY AND HYDROCHEMISTRY OF FOUR TROPICAL WATERSHEDS IN CENTRAL PANAMA

The Upper Rio Chagres watershed provides approximately half of the water utilized to operate the Panama Canal so that an understanding of the hydrometeorological and hydrochemical character of the watershed is required as a foundation for sound water resources management. A study was initiated in 2005 to compare the surface water chemistry of the Upper Rio Chagres basin (580 km²) with three smaller adjacent watersheds (Rio Pequini – 281 km², Rio Cuango – 175 km², and Rio Pacora – 374 km²) that have similar pristine source regions in steep mountainous tropical rainforest, but are affected by different downstream land-use practices. The bedrock geology of the region consists of Upper Cretaceous to Tertiary age basalts, basaltic andesites, gabbros, and diorites as well as less-abundant granodiorites, tonalities, and plagiogranites. IFASRE 90-m resolution elevation data was used in a GIS to provide the geospatial and hydrologic framework for the initial field study, conducted in February 2005, toward the end of the 2004-05 dry season, in which 11 major rivers and 13 low-order tributary streams in the 4 watersheds were sampled. Temperature, TDS, pH, and DO were measured in the field and subsequent determinations of 6 cation (Si, Ca, Mg, Na, K, & Sr), and 3 anion (Cl, SO₄, and NO₃), species made in the laboratory, with bicarbonate content estimated from mass balance considerations. Overall, river and stream water dD and d¹⁸O values fall on the Global Meteoric Line without systematic d-excess change, suggesting that the H and O isotope variations are governed by air mass trajectories rather than altitude or topographic effects, and there is a geospatial trend in Cl that mirrors the NE-SW precipitation gradient across the region. Dissolved solute contents are low (TDS=80±18), suggesting a short residence time in the weathering zone for infiltrating precipitation. Mg/Na ratios vary from 0.29-1.13 for Ca/Na ratios of 0.28-1.22, similar to rivers draining basaltic terranes worldwide. Ca/Mg ratios are low (0.7-2.5) but distinct for each basin. Alkali/alkaline earth relationships suggest that erosion is weathering rather than transport limited and distinct linear trends of positive slope on Si-TZ⁺ diagrams, suggest slightly different lithologic controls on the chemical evolution of river waters in each watershed.