GEOGENIC OCCURRENCE OF AS AND F IN GROUNDWATER OF NORTHERN MEXICO

The highest content of As and F in groundwater in Mexico occurs in its northern part. Groundwater As and F concentrations were plotted for the northern states of Chihuahua, Coahuila, and Durango to determine their spatial distribution. Their content was compared to geochemical processes that may cause their enrichment. The results show that As and F concentrations vary among a wide range of values and their distribution pattern is highly disperse. These patterns point to the alluvial fill of the aquifers as the most probable source of origin, particularly to the chemical weathering of the fill material. Rhyolite and ignimbrite are likely important primary sources, and desorption from solid surfaces, such as iron oxyhydroxides, a secondary source. A map of co-occurring As and F (>25 µg/L and 4 mg/L respectively) shows the hotspot locations for these potentially harmful elements. About 67% of the study area is occupied by closed basins. The As-F content of groundwater from closed basins was not found to be significantly different from the non-closed basins. Many aquifers in this area are depleted, regardless if closed or non-closed basins, highlighting the need of a more sustainable management of this important resource.