Surface Water Geochemistry and Chemical Weathering Across Panama

A geochemical study of rivers and streams was undertaken across Panama during 2005-09, from the Lago Bayano area in the east to the Costa Rican border in the west. Low overall dissolved solute contents (TDS = 145±160 mg/L) suggest a short residence time for infiltrating precipitation in the weathering zone. Watershed lithology exerts the main control on riverine chemistry, with streams on marine sedimentary rocks having higher dissolved solids loads than those on igneous rocks, with the latter exhibiting the highest silica contents and increasing trends of total cations with increasing dissolved silica. This feature and the large degree of compositional overlap between large rivers and small tributary streams implies that chemical weathering of silicate materials in the soil zone is the predominant process determining the geochemistry of streams and rivers in this tropical environment. Silicate weathering rates (Casil+Mgsil+Na+K) range over more than an order in magnitude from 2.5 to 28.4 tons/km2/y, whilst H4SiO4 yields range from 7.1 to 65 tons/km2/y. Basin-wide CO2 consumption yields by silicate weathering, calculated from total cation content (corrected for sea salt contribution), basin area and discharge, are high on a global basis.