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The controlling hydrogeochemical processes of an intermontane aquifer in central Mexico were identified through multivariate statistical analysis. Hierarchical cluster (HCA) and k-means clustering analyses were applied to Na+, K+, Ca2+, Mg2+, F−, Cl−, SO42−, NO3−, HCO3−, As, pH and electrical conductivity in 40 groundwater samples collected from shallow and deep wells, where As and F− are contaminants of concern. The effectiveness of each hierarchical and k-means clustering method in explaining solute concentrations within the aquifer and the co-occurrence of arsenic and fluoride was tested by comparing two datasets containing samples from 40 and 36 wells, the former including ionic balance outliers (>10%). When tested without outliers, cluster quality improved by about 5.4% for k-means and 7.3% for HCA, suggesting that HCA is more sensitive to ionic balance outliers. Both algorithms yielded similar clustering solutions in the outlier-free dataset, aligning with the k-means solution for all 40 samples, indicating that k-means was the more robust of the two methods. k-means clustering resolved fluoride and arsenic concentrations into four clusters (K1 to K4) based on variations in Na+, Ca2+, As, and F−. Cluster K2 was a Na-HCO3 water type with high concentrations of As and F. Clusters K1, K3, and K4 exhibited a Ca-HCO3, Na-Ca-HCO3, and Ca-Na-HCO3 water types, respectively, with decreasing As and F concentrations following the order K2 > K3 > K1 > K4. The weathering of evaporites and silicates and Na-Ca ion exchange with clays were the main processes controlling groundwater geochemistry. The dissolution of felsic rocks present in the aquifer fill is a likely source of As and F−, with evaporation acting as an important concentration factor. [ABSTRACT FROM AUTHOR]

Emerging contaminants are chemical compounds present in the environment, particularly in aquatic systems, which are currently not subjected to regulatory measures. This problem occurs when pollutants are introduced into aquatic systems through multiple sources, including wastewater discharge, agricultural runoff, and atmospheric deposition. Products that contain emerging contaminants are classified into 3 categories: pharmaceuticals, personal care products, and endocrine-disrupting chemicals. Several organisms are affected by the accumulation of the chemicals in water bodies. These emerging contaminants have been found to alter the behaviour and development of fish and organisms that form the base of the aquatic food chain. Additionally, they interfere with the ability of organisms to reproduce and grow appropriately. In the present study, a series of experiments were performed using Stigeoclonium nanum microalgal biomass to determine the removal efficiency of one of these contaminants, that is, caffeine. Experiments were performed in duplicate using 20 mL tubes. A 500 mg L-1 caffeine solution was prepared and applied at different doses at the same time as a known amount of dry biomass. The tubes were maintained at a constant temperature of 24 ± 2 °C with a stirring speed of 400 rpm. Optical absorbance readings were recorded, and calculations were performed to determine the removal percentage for each caffeine dose. At concentrations of 50, 100, and 125 mg L-1, 100% removal was reached, while at 250 and 500 mg L-1, removal was reduced to 79.76 % and 49.66%, respectively. These data indicated a continual decrease in caffeine up to 125 mg L-1, suggesting a possible saturation point for the dry biomass used over this concentration range. Furthermore, these results emphasise the necessity of conducting adsorption isotherms to understand the relationship between caffeine concentration and its removal efficiency using microalgal biomass. This could lead to the optimisation of contaminant removal, especially at higher concentrations. Although the microalgal biomass of S. nanum appears to be effective in removing caffeine at moderate concentrations, more comprehensive experiments surrounding adsorption isotherms are required to enhance efficiency at higher concentrations. [ABSTRACT FROM AUTHOR]

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