Hydrogeochemical Evaluation of an Urban Aquifer Under Stress: Rare Earth Elements Fractionation and Human Health Implications

This study evaluates the hydrogeochemical behavior and groundwater quality under significant anthropogenic pressure in a volcanic-carbonate aquifer, using trace elements and Rare Earth Elements (REE) as environmental tracers. A total of 25 samples from wells of different depths and flow rates in the city of Puebla were analyzed by ICP-MS. Positive Ce anomalies (18 samples with an average of 1.4) and negative Eu anomalies (7 samples with an average of 0.5) were directly related to local oxygen conditions, while positive Eu anomalies across all samples (avg. of 3.7) were determined to be influenced by rock weathering. An inverse correlation between REE concentrations and dynamic water level indicates that deeper and older waters contain higher levels of these elements. The Water Quality Index (WQI) classified 96% of samples as good to excellent quality, and the Human Health Risk Assessment revealed that over 70% of samples exceeded the non-carcinogenic threshold for arsenic. Four samples (2, 4, 22, and 24) exhibited distinct geochemical signatures not associated with well depth, yield, or quality indices. These anomalies suggest potential mixing with other water bodies, likely through vertical flow along microfractures, indicating vulnerability in the hydrogeological boundaries between units. The findings highlight both the aquifer system’s current good quality and its underlying risk, using REE as tracers into water-rock interactions and contamination pathways. These findings emphasize the need for a comprehensive integration of geochemical data and groundwater system characterization to inform the development of scientifically grounded extraction and management strategies. Such an approach is essential to safeguard both groundwater quality and quantity, particularly in urbanizing regions facing water security challenges.