ABSTRACT
From ecological and environmental considerations, one of the most significant karstic aquifer units in the Sierra Madre Oriental of Mexico is located in the CCB in Coahuila (Figure 1). The area is semi-arid and includes a plain with more than 300 dispersed sites showing natural groundwater discharge features (pozas and lagoons) and playa lakes (Aldama et al. 2007). The geologic basement consists of metamorphic Jurassic rocks and the San Marcos Sandstone, overlain by reef-like limestone facies, dolomite with abundant gypsum, and a sequence of limestone and clay (González-Sanchez 2008). A 300-500 m thick heterogeneous karstic aquifer unit has been identified covered in the plain by poorly consolidated young sediments. The lateral physical boundaries of the karstic aquifer unit are beyond the surface periphery of the CCB. The hydraulic conductivity of the aquifer is 3 × 10-5 m s-1 (Aldama et al. 2005). Water-table elevation ranges from 680 m masl in the
1 INTRODUCTION
Understanding the connection between the hydrogeological environment and the chemical and isotopic composition of groundwater can help to evaluate its origin, evolution and flow, as well as the effects of its extraction. In a given area, the hydrogeological features, residence time, conditions for water-rock interaction and the flow rate are often the main factors in controlling the overall composition of groundwater flow systems. In addition, mixing processes between end-members with contrasting origin and chemical composition could also have a significant influence in groundwater evolution. (Back & Hanshaw 1970, Appelo & Postma 1993). The objective of the present study is to establish the processes controlling the chemical and isotopic composition of waters in two different environments, where enrichment in 2H and 18O compared to Craig (1961) GMWL was detected. The studied areas are; i) a karstic carbonate aquifer in Mexico and ii) a sedimentary aquifer system in Hungary.
