Abstract. The Cordillera Blanca, located in the central Peruvian Andes, is the most glacierized mountain range in the tropics. The study objective is to determine the spatial and topographic controls on geochemical and isotopic parameters in the Quilcayhuanca drainage basin. During the dry season of July 2009, surface water and groundwater samples were collected from the proglacial zone of the 90 km² Quilcayhuanca basin which is 20% glacierized. The basin water samples (n = 25) were analyzed for pH, conductivity, 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 (δ¹⁸O, δ²H). The valley's surface water is acidic (pH 3–4) and is dominated by Ca²⁺, Mg²⁺, and SO₄²⁻, the last of which is likely due to pyrite oxidation. Total P and total N show no trend with elevation down valley, 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 δ¹⁸O. A two-component mixing model indicates that discharge from the watershed is approximately two-thirds surface water (mostly glacier melt) and one-third groundwater. The results were compared to data from the Rio Santa and indicate that this trend may persist at the regional scale.