Lithological and hydrological controls on water composition: evaporite dissolution and glacial weathering in the south central Andes of Argentina (33-34° S)

Abstract

Lithological and hydrological influence on fluvial physical and chemical erosion was studied in a glacierized sedimentary basin with high evaporite presence. Suspended particulate matter (SPM), total dissolved solids (TDS) and major ion concentrations were analyzed for two years of different hydrologic condition: i) 2009-2010, Q = 100% average; and ii) 2010-2011, Q = 60% average. Annual hydrograph was simple regime ? type with one peak in summer related to snow melting. The intra-annual SPM and TDS variations were directly and inversely associated to Q, respectively. Snow chemistry showed continental influence (Na+/Ca2+=0.17) and atmospheric input of TDS was <1% of the total exported flux. River water was highly concentrated in Ca2+ and SO42- (~4 mmol L-1) and in Na+ and Cl- (~3 mmol L-1). Ca2+/SO42- and Na+/Cl-¬ molar ratios were ~1 and related to Q; directly and inversely, respectively. Major ion relationships suggest that river chemistry is controlled by evaporite (gypsum and halite) dissolution having a summer input from sulphide oxidation and carbonate dissolution, and a winter input from subsurface flow loaded with silicate weathering products. This variation pattern resulted in nearly chemostatic behavior for Ca+, Mg2+ and SO42- while Na+, Cl- and SiO2 concentrations showed to be controlled by dilution/concentration processes. During 2009-2010 hydrological year the fluxes of water, SPM and TDS registered in the snow melting ? high Q season were, respectively, 71%, 92% and 67% of the annual total; while for equal period in 2010-2011, 56% of water, 86% of SPM and 54% of TDS annual fluxes were registered. The SPM fluxes for 2009-2010 and 2010-2011 were 1.19 106 t y-1 and 0.79 106 t y-1; while TDS fluxes were 0.68 106 t y-1 and 0.55 106 t y-1, respectively. Export rates for 2009-2010 were 484 t km2 y-1 for SPM and 275 t km2 y-1 for TDS. These rates are higher than those observed in glacierized granite basins and in non-glacierized evaporite basins, suggesting a synergistic effect of lithology and glaciers on physical and chemical erosion.