The objectives of this study were, to evaluate the change in concentrations of arsenic (As) and fluoride (F) in three aquifers located in the central semi-arid region of Chihuahua, at three different times within a seven year period. The As content in rock, sediment, and groundwater samples was first determined in 2004; later on, in 2007 and 2010, the content of As and F in groundwater were measured. In all three sampling episodes some wells had consistently As concentrations above 0.025 mg L-1. The F concentration surpassed the Mexican limit values of 1.5 mg L-1 in 77% of the measured wells. The surrounding rocks contained enough As and F to support a geogenic origin of these contaminants and their higher concentrations within the rhyolites indicates a possible chemical association to these volcanic rocks. A correlation between As and F suggests a co-occurrence of these two contaminants (r2