HYDROCHEMICAL AND ISOTOPIC CHARACTERIZATION OF GROUNDWATER FLOW SYSTEMS IN NORTH-CENTRAL MEXICO: IMPLICATIONS FOR WATER MANAGEMENT

A combination of chemical and isotopic indicators were used to characterize rainfall, surface and groundwater, to identify recharge processes, water–rock interactions, and groundwater residence time, for the delineation of groundwater flow systems (GWFS) in one of the several closed basins existing in the north-central part of Mexico. GWFS develop through a sequence of fractured volcanic rocks and a clastic sequence of debris flow sediments filling a regional graben structure. Because of volcanic mineral weathering, the natural baseline quality for groundwater flowing through the fractured rock is predominantly HCO₃-Na, whilst groundwater in the basin fill sediments is HCO₃-Ca-Mg type. Contrast in water chemistry and isotopic values with depth, allowed differentiation of local, intermediate and regional GWFS. Results indicate that the variability of the chemical composition of the local GWFS is significant; in contrast, intermediate and regional GWFS have homogeneous composition and lower major ions concentrations. Intermediate and regional GWFS show environmental isotopic (δ¹⁸O, δ²H, δ³⁴S) evidence that they are not affected by evaporation processes, as identified for the local GWFS. In this semiarid area, natural recharge is restricted to specific regions with preferential pathways, local GWFS are modern according to the ³H concentrations and the identification of induced recharge indicators such as NO₃ and Cl. Trace elements such as Li and Ge first estimated the relative groundwater age of regional GWFS. Absolute age determinations for selected samples using ¹⁴C isotope data is between 1000 and 10000 years, revealing that the regional GWFS have a very long trajectory (tens of kilometers). The chemical composition of analyzed samples could also be explained in part by groundwater age and variation in flow patterns and climate over time. The hydrodynamic characteristics and hydrochemical distributions corroborated mixing of differently hierarchical GWFS produced by intensive water extraction for drinking water purposes. Local GWFS were identified as severely impacted by diffuse contamination, whereas regional GWFS are enriched in F and As from geogenic sources; intermediate GWFS have the best drinking water quality. Geological and climatic similarity to numerous volcano-sedimentary basins makes the findings useful for water management purposes and transferable to other semi-arid regions facing challenges of anthopogenically and geogenically impacted drinking water resources.