Mineral Weathering in a Semiarid Mountain River: Its assessment through PHREEQC inverse modeling KARINA L. LECOMTE * , ANDREA I. PASQUINI and PEDRO J. DEPETRIS Centro de Investigaciones Geoquı´micas y de Procesos de la Superficie (CIGeS), Facultad de Ciencias Exactas, Fı´sicas y Naturales, Universidad Nacional de Co ´rdoba, Avenida Ve ´lez Sarsfield 1611, X5016GCA, CORDOBA, Rep. Argentina (Received 2 March 2004; accepted 20 September 2004) Abstract. By means of PHREEQC inverse modeling, we have simulated the weathering reactions in Los Reartes River, a mountainous (2400–670 m a.s.l.) drainage basin from the Sierras Pampeanas of Co´rdoba, 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 pre- cipitated 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, 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) CO 2 consumption is important all along the Los Reartes drainage basin and, in terms of mmol/kg H 2 O, the lowermost portion of the basin is the most significant supplier; (f) CO 2 accounts for over 50% of all the species involved in the weathering reactions occurring at the Los Reartes drainage basin. Key words: Argentina, hydrochemistry, inverse modeling, mountain rivers, PHREEQC, weath- ering 1. Introduction The international scientific community is placing special emphasis on intensifying the integral study of mountainous regions. The Earth’s moun- tains occupy about one-fourth of its surface and provide goods and services to about one-half of the world population. Recognizing the importance of *Author for correspondence: E-mail: klecomte@com.uncor.edu. Aquatic Geochemistry (2005) 11:173–194 Ó Springer 2005 DOI 10.1007/s10498-004-3523-9