Geochemical Analysis of Waters in a Tropical Glacial Valley, Cordillera Blanca, Peru

Résumé du livre

Abstract: The Cordillera Blanca, located in the central Andes of Peru, is the most glacierized mountain range in the tropics. Most of the Peruvian population lives in the arid Pacific plains which are heavily dependent on glacial melt and runoff from these mountains, especially during the dry season. Thus understanding the current physical and chemical hydrology of the region is imperative for making future predictions about climate impacts. The objective of this study is to examine a variety of geochemical parameters in a glacial valley and to determine how and why these parameters vary with elevation and between groups (surface water vs. groundwater). During the dry season of July 2009, surface water and groundwater samples were collected from the proglacial zone of Quilcayhuanca valley, a basin that is 20% glacierized and drains an area of 90 km2. The water samples from the valley (n=25) were analyzed for major cations (Ca, Mg, Na, K, Fe(II)), major anions (F, Cl, SO4), nutrients (total N, total P, and Si), and stable isotopes of water ([delta]18O, [delta]2H). Field measurements like pH, conductivity, and temperature were also collected. The valley's acidic waters (pH 3-4) are dominated by the major ions Ca2, Mg2, and SO42−, the last of which is a remnant of pyrite oxidation. Total P and total N show no trend with elevation while Si generally increases with decreasing elevation. Groundwater samples are differentiated from surface water samples by lower pH, specific conductance, and total P and higher Na, K, HCO3−, Si, and [delta]18O. Using a simple, two end-member mixing model the surface waters leaving the valley at the last site in the drainage basin were calculated to be a mixture of approximately two-thirds surface water (mostly glacial melt) and one-third groundwater.