It is misleading not to distinguish between; i)bad (natural) quality water and ii)polluted water (anthropogenic effects). The concept of BACKGROUND LEVEL is proposed for the concentration anywhere in a flow system due to natural chemical reactions between the host rock and the groundwater. It is a direct function of rock mineralogy, hydraulic conductivity, effective porosity and flow system. In the case of some trace elements such as Zn, Li, F, B and Fe, their natural concentration in thermal waters is proportional to the salinity, especially when it is considered equivalent to the square of molar sodium concentration. This behavior was studied in the water supply of San Luis Potosí which has an important thermal component (minimum 70% in volume). As expected, the results show that the background level for boron is between 0.07 and 0.27 mg/1 and 0.004 to 0.04 mg/1 for zinc, which falls within drinking water standards; however, some of the analyzed samples are polluted. On the other hand, lithium is present from 0.01 to 0.22 mg/1 and fluoride is between 0.3 and 3.6 mg/1, both as the result of natural mineral solution. Many samples indicate the concentrations for the latter are not on excess of drinking water standards (1.5 mg/1).Es una práctica relativamente común en nuestro país, el definir como agua contaminada aquella cuya concentración de una o varias especies químicas es mayor que el límite máximo recomendado por los estándares nacionales e internacionales. Esta consideración es errónea pues no distingue entre dos conceptos fundamentales: i) agua de mala calidad (natural) y ii) agua contaminada (efectos antropogénicos). Se propone el concepto VALOR de FONDO para una especie disuelta en agua subterránea, como la concentración en un determinado punto del sistema de flujo, debida única y exclusivamente a procesos y reacciones químicas naturales entre el agua subterránea y los minerales del acuífero; es función de la mineralogía de la roca, conductividad hidráulica, porosidad efectiva y patrón de flujo subterráneo. En el caso de ciertos elementos traza como Zn, Li, F, B y Fe,