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In the Mezquital valley, untreated wastewater (45 m s) from Mexico City is used for the irrigation of around 900 km of agricultural soil. High concentrations of metals including methylmercury (3.8 ± 2.5 ng l) and lead (0.16 ± 0.05 mg l) were measured in anoxic wastewater canals. Downstream, dissolved, and particulate polymetallic (Hg, Pb, Cr...) concentrations decreased by factors 10 to 1,000 in the Tula River (which received a mix of fresh and wastewater) due to the dilution and oxidation of surface water, and to the decrease of contaminants concentration in wastewater downstream irrigated soils. However, dissolved and particulate methylmercury concentrations (0.06 to 0.33 ng l and 1.6 to 4.5 μg kg, respectively) remained elevated in comparison to other natural hydrosystems. The monitoring of an irrigation event and the distribution of metals in a soil profile irrigated for more than 80 years showed that metals were retained in the draining tilled layer. The oxic conditions and slightly acidic pH (~6.5) in this layer were found favorable for metal adsorption and co-precipitation with redox-sensitive elements (Fe, Mn) and suggestively for mercury demethylation. In the downstream Tula River and groundwater, almost all metallic concentrations remained below guideline thresholds. Only, dissolved As and Pb concentrations remained two to five times above thresholds for drinking water, highlighting a potential health risk for approximately 500,000 people who use groundwater as water supply. [ABSTRACT FROM AUTHOR]