Hydrogeochemistry of groundwater in Chihuahua, Mexico: a focus on arsenic, fluoride and uranium

The risk of exposure to chemical parameters with concentrations above drinking water standards has been documented for nearly 50 years in 15 of the 32 states defined in Mexico. Water supply quality problems and associated public health issues have been observed in specific regions of the country (e.g., Sierra Madre Occidental), especially associated with geogenic sources of As, F and U.

The study area in the region of Chihuahua, Mexico, is characterized by the presence of a heterogeneous and anisotropic unconfined aquifer, constituted in the upper portion by lacustrine and alluvial sediments, and polymictic conglomerates. The lower portion is characterized by a sequence of volcanic rocks, which may have secondary permeability due to fracturing. At greater depth, Cretaceous limestones represent the hydrogeological basement of the aquifer.

Hydrogeochemical analysis of 63 surface and groundwater samples and petrographic assessment of 26 rock and 16 sediment samples, identified 3 different water types associated with different geological environments: a) the calcium-bicarbonate (Ca-HCO₃) water type, represented by surface and shallow groundwater (rain, dams, wells and springs), hosted over fill sediments with average temperatures of 21 °C, influenced by anthropogenic activities (agriculture and wastewater), and low As (0.05-13 mg/l), F (0.5-1.30 mg/l) and U (0.05-5.2 mg/l) concentrations; b) the mixed-bicarbonate (Mix-HCO₃) water type, identified in wells with depths of 100 to 350 m, with water circulating through the fill sediments and the sequence of volcanic rocks (andesites, rhyolitic tuffs, rhyolites and ignimbrites), characterized by average temperatures of 26 °C, intermediate concentrations of As (4.0-41.5 mg/l), F (0.47-3.47 mg/l) and U (3.76-16.8 mg/l), which is associated with intermediate groundwater flows in the study area: c) the sodium-bicarbonate (Na-HCO₃) water type, characterized by groundwater from wells with depths of 250 to 500 m, with water circulating mainly through the sequence of volcanic rocks (rhyolitic tuffs, rhyolites and ignimbrites), temperatures that can reach up to 34 °C, and higher concentrations of As (21.06-71.87 mg/l), F (1.3-4.95 mg/l) and U (4.05-15.5 mg/l). Based on the depth of the wells and their location, they can be associated with deep groundwater flows (indicated by high Li: 69.42-198.3 mg/l) where the origin of As, F and U in the water is mainly geogenic.

This hydrogeochemical characterization is necessary to understand the water-rock interaction mechanisms that allow in some cases and limit in others the mobility of As, F and U in groundwater. This is particularly important for sustainable water management, especially in (semi)arid regions of Mexico, where deep groundwater is the main source of water supply for the population.