Resumen
The natural vulnerability of the phreatic aquifer of the hydrographic basin of the Laja river was evaluated, by means of the determination of the vertical hydraulic resistance, using the 'Aquifer Vulnerability Index' (AVI) that allows to determine the infiltration speed from the pollutants when flowing through different soils horizons. The analysis of the tendencies of behavior of the contents of nitrates, phosphates, chlorides, sulfates and total dissolved solids, in the groundwater allows to establish a correlation between AVI and source-aquifer contamination in the areas of different soil uses. The behavior of the different solutes in the groundwater is similar, with a tendency to increase from the Precordillera Andina towards the Cordillera de la Costa. The highest concentrations are found at the contact of the Depresión Central with the Cordillera de la Costa, where the risk of contamination, according to AVI is moderate to high and very high. This indicates that the sandy soils of medium-fine texture, with low content of clay material provide a low degree of protection for the aquifer, since they have lower reserves of organic matter and sulfur available for sorption of phosphates and denitrification. Also, a high water table reduces the attenuation capacity of these soils and it facilitates the transport of the pollutants of agricultural origin towards the groundwater. On the contrary, the agricultural pollutants are less concentrated in the groundwater linked to sandy soils of coarse and medium texture, with higher contents of clay, where the capacity to attenuate pollutants is related to larger contents of sulfur available and organic matter sorptive/labile, as well as by the greater depth of the groundwater level.