Mineral weathering in a semiarid mountain river: Its assessment through PHREEQC inverse modeling

By means of PHREEQC inverse modeling, we have simulated the weathering reactions in Los Reartes River, a mountainous (similar to 2400-670 m a.s.l.) drainage basin from the Sierras Pampeanas of Cordoba, Argentina, analyzing the effect of lithology, relief, and climate. The steep upper half of the basin (slopes > 20%) is occupied by exposed granite; the remaining area is mostly metamorphic, with cropping out gneisses and progressively decreasing slopes (< 6%). Climate is semihumid to semiarid; rainfall mainly occurs in summer and decreases with decreasing height. PHREEQC inverse models developed using water chemical data showed that (a) oligoclase was the major supplier of solutes, while the main precipitated phase was kaolinite in the granite domain; (b) muscovite is the chief supplier of solutes and illite is the main precipitated phase in the gneissic realm; (c) the steeper portions of the metamorphic reach are less crucial in supplying solutes than the lower ones, thus highlighting the importance of the water residence time in the kinetics of dissolution; (d) in the driest time of the year (winter, similar to 20 mm/month) we registered the highest production of dissolved and precipitated phases; fluxes (mmol/month), however, are higher at the end of the rainy season; (e) CO2 consumption is important all along the Los Reartes drainage basin and, in terms of mmol/kg H2O, the lowermost portion of the basin is the most significant supplier; (f) CO2 accounts for over 50 of all the species involved in the weathering reactions occurring at the Los Reartes drainage basin.