ORIGINAL ARTICLE Hydrogeological and mixing process of waters in aquifers in arid regions: a case study in San Luis Potosi Valley, Mexico J. A. Ramos-Leal V. J. Martı´nez-Ruiz J. R. Rangel-Mendez M. C. Alfaro de la Torre Received: 1 August 2006 / Accepted: 15 January 2007 / Published online: 14 February 2007 Ó Springer-Verlag 2007 Abstract The climatic conditions of arid regions are characterized by high temperatures, low precipitation and high evapotranspiration rates that can explain the reduced recharge of aquifers. Thus, in these regions, there are some problems related to the groundwater quality and recharge that makes worse the problem of groundwater supply. A model, taking into account ternary mixtures, is presented and applied to a case study: the aquifer of San Luis Potosi valley located in the highlands of the central part of Mexico. In this valley, four hydrochemical facies were identified that correspond to the Ca–Na + K–HCO 3 , Na + K–Ca– HCO 3 , Ca–HCO 3 and Ca–SO 4 types. From this char- acterization, it was found out that the recharge area (known as Bledos Graben) is located at the SE of the valley; the deep water flow comes from there (Villa de Reyes and Alvarez Range) to the center of the valley. Mixture fractions were obtained by using chlorides and fluorides as conservative elements, from which it was possible to quantify the contribution of each member to the groundwater quality. According to these results, the contributions to the water extracted from this aquifer are as follows: shallow flows 50%, deep flows from Villa de Reyes 27%, and flows coming from the Alvarez Ranges about 15%. Keywords Hydrochemistry  Mixing process  End-member  Groundwater Introduction The groundwater can be a mixture of waters of dif- ferent chemical composition due to natural or anthro- pogenic processes (Harpaz and Bear 1964). To quantify the contributions of water mixtures to the underground flows one can make use of traditional hydrochemistry tools, particularly the conservative elements frequently considered as tracers. The mix- tures can be binary or ternary; the first ones have been explained thoroughly in numerous publications and have been utilized to quantify the mixture of fresh and marine water (Apello and Postma 1996; Genereux et al. 2002; Ravenscroft and McArthur 2004; Wallick 1981; Lee and Krothe 2001; Abu-Jaber 2001; Skalbeck et al. 2002; Valentino and Stanzione 2002; Moise et al. 2000). However, models of ternary mixtures have been less identified and studied. For instance, Rice and Hornberger (1998) evaluated the contribution of peak flow to the groundwater in USA, and Genereux et al. (2002) evaluated the interbasin transfer in Costa Rica. Also, Laaksoharju et al. (1999) and Douglas et al. (2000) have reported that the mixture of three types of water with different chemical conditions can promote chemical reactions in an aquifer. This manuscript describes the case of a ternary mixture in an arid zone in the central part of Mexico, J. A. Ramos-Leal (&)  V. J. Martı´nez-Ruiz  J. R. Rangel-Mendez Instituto Potosino de Investigacio ´n Cientı´fica y Tecnolo ´ gica, A.C. (IPICYT), C. Presa San Jose # 2055, Lomas 4ª. Seccio ´n, San Luı´s Potosı´, SLP 78216, Me ´ xico e-mail: jalfredo@ipicyt.edu.mx V. J. Martı´nez-Ruiz  M. C. Alfaro de la Torre Universidad Auto ´noma de San Luis Potosı´, Av. Dr. Manuel Nava 6, San Luis Potosı´, SLP 78210, Me ´ xico 123 Environ Geol (2007) 53:325–337 DOI 10.1007/s00254-007-0648-3