INFLUENCE OF MINERAL WEATHERING ON STREAM WATER CHEMISTRY OF THE QUEBRADA MAQUINA, RIO GUACIMAL WATERSHED, MONTEVERDE, COSTA RICA

Non-point source pollution has altered the quality of water resources in Monteverde, Costa Rica. A comparison of the chemistry of undeveloped subcatchments shows that the ratio of calcium to sodium is higher in catchments underlain by the Monteverde Formation than in those underlain by the Aguacate Group. This suggests that bedrock chemistry is a significant control on stream chemistry. This study establishes the baseline chemistry of stream water in one of the undeveloped subcatchments, the Quebrada Maquina, in order to define the nature and extent of pollution. Existing literature describes this region of the Cordillera de Tilarán to be underlain by andesitic lavas and tuffs of the Monteverde Formation (Pleistocene, approx. 600,000 ybp). More detailed bedrock mapping identified a variety of pyroclastic breccias and layered, massive, and boulder andesite units. The primary minerals identified in fresh bedrock samples by petrography and SEM/EDS are plagioclase, pyroxene, ilmenite, apatite, and amphibole. X-ray diffraction, SEM secondary electron images of morphology, and petrography identified kaolinite and vermiculite as the weathering products of plagioclase and pyroxene, respectively. The contribution of source minerals was reconstructed using a mass balance method to show that plagioclase feldspar has more influence on streamwater chemistry than pyroxene. Unidentified mineral sources of potassium, sodium, and silica are likely significant contributors to the stream water chemistry and future work should focus on identifying additional source minerals. In addition, better characterization of mineral weathering products, chemical input from precipitation, and effects of cation exchange reactions on stream chemistry will fine-tune the application of a mass balance method to the Quebrada Maquina subcatchment.