(Methyl)Mercury, Arsenic, and Lead Contamination of the World’ s Largest Wastewater Irrigation System: the Mezquital Valley (Hidalgo State—Mexico) Stéphane Guédron & Céline Duwig & Blanca Lucia Prado & David Point & Marizol Giovana Flores & Christina Siebe Received: 21 February 2014 /Accepted: 18 June 2014 /Published online: 26 July 2014 # Springer International Publishing Switzerland 2014 Abstract In the Mezquital valley, untreated wastewater (45 m 3 s -1 ) from Mexico City is used for the irrigation of around 900 km 2 of agricultural soil. High concentrations of metals including methylmercury (3.8±2.5 ng l -1 ) and lead (0.16±0.05 mg l -1 ) were measured in anoxic wastewater canals. Downstream, dissolved, and partic- ulate polymetallic (Hg, Pb, Cr…) concentrations de- creased 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 -1 and 1.6 to 4.5 μg kg -1 , 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 favor- able 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 po- tential health risk for approximately 500,000 people who use groundwater as water supply. Keywords Wastewater irrigation . Metal fractions . Bioavailability . Soil . Groundwater 1 Introduction High costs of water treatment force many countries in the developing world to reuse untreated or only partially treated effluents (Qadir and Scott 2010). It is therefore common that each region, decides on the affordable treatment in combination with water management op- tions to allow minimizing the health risks in each Water Air Soil Pollut (2014) 225:2045 DOI 10.1007/s11270-014-2045-3 Electronic supplementary material The online version of this article (doi:10.1007/s11270-014-2045-3) contains supplementary material, which is available to authorized users. S. Guédron (*) ISTerre, Université Grenoble 1, IRD—UMR 5275 (IRD/UJF/ CNRS), BP 53, 38041 Grenoble, France e-mail: stephane.guedron@ird.fr C. Duwig LTHE, IRD—Université Grenoble 1, UMR 5564 (IRD/UJF/ CNRS), BP 53, 38041 Grenoble, France B. L. Prado : C. Siebe Instituto de Geología, Universidad Nacional Autónoma de México, Cd. Universitaria, D.F., CP 04510 Mexico, Mexico City, Mexico D. Point Géoscience Environement Toulouse—Université Paul Sabbatier, UMR GET, 14 Av. Edouard Belin, 31400 Toulouse, France S. Guédron : D. Point : M. G. Flores Laboratorio de Calidad Ambiental, Universidad Mayor de San Andrés, Cota Cota, La Paz, Bolivia