The objectives of this study were to identify potential sources of As and F within an alluvial aquifer system in central Chihuahua, to evaluate their change in concentrations with time, and to determine their degree of co-occurrence. 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 was measured. From 34 wells sampled, seven had As levels above the Mexican maximum contaminant level of 0.025 mgL(-1), and 13 wells surpassed the F value of 1.5 mgL(-1), all of these near the town of Aldama. The surrounding rocks contained enough As and F to support a geogenic origin of these contaminants, especially evident in rhyolites. A correlation between As and F suggests a co-occurrence of these two contaminants (r=0.720; p<0.001). The minor variations of As and F in the measured events indicate a stable evolution for As and F. Yearly monitoring the content of As and F is recommended, in order to observe the behavior of these elements within the aquifer, especially as land use is changing and the demand for groundwater (drinking water, irrigation) is increasing.
