This document presents the develop of a methodology to asses the vulnerability of a shallow unconfined aquifer located beneath the lower part of the Chillán River Watershed. The methodology is based on the use of mathematical indexes created using a lineal combination of variables relates to the processes of pollutant recharge and transport that had been evaluated in the area of the study. The results are presented in thematic charts using a geographic information system (GIS), allowing the identification of areas with lower or high vulnerability to pollution and areas with different levels of pollution risk associated to agricultural activities. A contrast between estimated values of the vulnerability index and values of nitrate concentration measured in shallow wells showed that exist a significant relationship between the index and the measured nitrate concentration