Figure 1.

Characterization of study sites by water chemistry. Upper left panel - conductivity, chloride, and sulfate; upper right panel - conductivity, hardness, and alkalinity; lower left panel - hardness, chloride, and alkalinity; lower right panel - hardness, sulfate, and chloride. Sampling regions: black - San Luis Potosί; dark grey- Cuatro Ciénegas; white - Ojos Altos; light grey - Ojos en de Medio, de la Punta, de la Casa, Caliente (Camargo); cross hatched - Presa Chihuahua, Presa la Boquilla, Presa Francisco I. Madero, Lago Colina; hatched - Méxican spring flowing into Rio Grande downstream of Big Bend National Park (TX, USA).