ARSENIC AND FLUORIDE IN GROUNDWATER OF CALERA, ZACATECAS, MEXICO: IMPLICATIONS FOR WATER MANAGEMENT

In semiarid regions of Mexico, groundwater is the main source for drinking water supply. Groundwater from the Calera aquifer in Zacatecas, Mexico has been reported with high arsenic (As) and fluoride (F) concentrations, this investigation evaluates the co-occurrence of As and F in groundwater flow systems and its implications in the future management of water resources. The aquifer is continuous from a fractured media to a porous media with a composite thickness of more than 500 m. Mix-HCO₃ and Na-HCO₃ water types associated with regional groundwater flow were identified with higher concentrations of As (40-75 ug/l), and F (1.5-5.7mg/l). Microscale characterization of volcanic rocks identified fluorapatite and topaz as the main fluoride-bearing minerals. Groundwater is aerobic and oxidizing, facilitating the presence of the stable oxy-anions HAsO₄^-2; arsenic concentrations are most likely controlled by oxi-hydroxide phases identified in the rock and basin-fill sediments. Local flow systems represent the best quality (As: 0.05-10 ug/l and F: 0.1-0.8mg/l). In this semiarid environment with identified geogenic sources of As and F, the long-term sustainability of good groundwater quality for domestic and public use must rely on the local groundwater flow systems. This action-oriented research gave the opportunity for collaboration among government, regulators, industry, and farmers. Actions included the implementation of several nature-based solutions (NBS) (conservation agriculture, integration of native flora along contours, restoration of crucial habitats, reforestation) to increase recharge to local flow systems. A hydrological model was used to evaluate the net effect on the infiltration increase derived from the implementation of NBS, including climate change for intermediate and long-term scenarios (2039-2099). Predictions under RCP 8.5 and RCP 4.5 climate change scenarios suggest that infiltration could increase by 30% to 59% respectively. A groundwater monitoring network was also implemented for the long-term observation of water levels in the aquifer based on the location of NBS and identified natural recharge zones. This case study is a good example of the implementation of NBS for climate change adaptation in semiarid regions of the world with water quality problems.