The Mezquital Valley system is the world's oldest and largest example with regard to use of untreated wastewater for agricultural irrigation. Because of the artificial high recharge associated with the Mezquital Valley aquifers, groundwater is extracted for human consumption, and there are plans to use this groundwater as a water resource for Mexico City. Thus, this study analyzed 218 organic micro-contaminants in wastewater, springs, and groundwater from Mezquital Valley. Five volatile organic compounds (VOCs) and nine semi-volatile organic compounds (SVOCs) were detected in the wastewater used for irrigation. Only two SVOCs [bis-2-(ethylhexyl) phthalate and dibutyl phthalate] were detected in all the wastewater canals and groundwater sources, whereas no VOCs were detected in groundwater and springs. Of the 118 pharmaceutically active compounds (PhACs) and 7 reproductive hormones measured, 65 PhACs and 3 hormones were detected in the wastewater. Of these, metformin, caffeine, and acetaminophen account for almost sixty percent of the total PhACs in wastewater. Nevertheless, 23 PhACs were detected in groundwater sources, where the majority of these compounds have low detection frequencies. The PhACs sulfamethoxazole, N,N-diethyl-meta-toluamide, carbamazepine, and benzoylecgonine (primary cocaine metabolite) were frequently detected in groundwater, suggesting that although the soils act as a filter adsorbing and degrading the majority of the organic pollutant content in wastewater, these PhACs still reach the aquifer. Therefore, the presence of these PhACs, together with the high levels of the endocrine disruptor bis-2-(ethylhexyl) phthalate, indicate that water sources derived from the recharge of the studied aquifers may pose a risk to consumer health.

Adsorption kinetics and adsorption/desorption isotherms of atrazine (AT), and its degradation products, deethylatrazine (DEA) and hydroxyatrazine (HyA) on solids from two depths of a gleyic planosol and the underlying sandy aquifer were studied using laboratory batch systems. Adsorption of the three molecules decreased with the sampling depth of the solid. Atrazine and DEA were not adsorbed on the aquifer solids, whereas HyA was. For all three solids, the adsorption increased in the order DEA < AT << HyA. Adsorption equilibrium was reached after 72 h for the upper soil sample (0‐0.33 m) and after 24 h for the deeper soil sample (0.9–1.05 m) for all three molecules. For HyA, equilibrium was reached after 24 h for the aquifer solid (6–7 m). High pressure liquid chromatography (HPLC) analyses showed no significant degradation of the three molecules after 72 h. The adsorption isotherms of the three molecules were described by Freundlich equations. The Kfads values ranged from 3.1 to 0.4 L kg−1 for AT, 1.5 to 0.3 L kg−1 for DEA and 7.9 to 0.9 L kg−1 for HyA. The Koc values suggested that the affinity for the organic matter bound to the solid increased in the order DEA < AT << HyA. The desorption isotherms were also described by Freundlich equations. The Kfdes value was 7.5 L kg−1 for AT in the upper soil sample and ranged from 0.1 to 31.1 L kg−1 for DEA and from 0.7 to 96.8 L kg−1 for HyA, depending on the sample. In the upper soil sample, desorption was hysteretic and DEA and HyA formed nonextractable residues, HyA more so than DEA. For the deeper soil sample for all three compounds and the aquifer solids for HyA, adsorption was completely reversible and did not lead to the formation of nonextractable residues.

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