Impact of long-term wastewater irrigation on sorption and transport of atrazine in Mexican agricultural soils

Abstract

In the Mezquital Valley, Mexico, crops have been irrigated with untreated municipal wastewater for more than a century. Atrazine has been applied to maize and alfalfa grown in the area for weed control for 15 years. Our objectives were to analyse (i) how wastewater irrigation affects the filtering of atrazine, and (ii) if the length of irrigation has a significant impact. We compared atrazine sorption to Phaeozems that have been irrigated with raw wastewater for 35 (P35) and 85 (P85) years with sorption to a non-irrigated (P0) Phaeozem soil under rainfed agriculture. The use of bromide as an inert water tracer in column experiments and the subsequent analysis of the tracers’ breakthrough curves allowed the calibration of the hydrodynamic parameters of a two-site non equilibrium convection-dispersion model. The quality of the irrigation water significantly altered the soils’ hydrodynamic properties (hydraulic conductivity, dispersivity and the size of pores that are hydraulically active). The impacts on soil chemical properties (total organic carbon content and pH) were not significant, while the sodium adsorption ratio was significantly increased. Sorption and desorption isotherms, determined in batch and column experiments, showed enhanced atrazine sorption and reduced and slower desorption in wastewater-irrigated soils. These effects increased with the length of irrigation. The intensified sorption-desorption hysteresis in wastewater-irrigated soils indicated that the soil organic matter developed in these soils had fewer high-energy, easily accessible sorption sites available, leading to lower and slower atrazine desorption rates. This study leads to the conclusion that wastewater irrigation decreases atrazine mobility in the Mezquital valley Phaeozems by decreasing the hydraulic conductivity and increasing the soil's sorption capacity.

Keywords:

• Pesticide
• pesticide mobility
• filtering
• soil organic matter
• normalized sorption coefficient
• non-equilibrium sorption
• dissolved organic matter

Acknowledgments

A major part of this study was conducted during the first author's stay at the Colegio de Postgraduados in Montecillo, Mexico, which was made possible through the International Science and Technology (ISAT) Linkages fund. The authors acknowledge financial support through the New Zealand Foundation for Science, Research and Technology Fund in the Sustainable Land Use Research Initiative and the Waste to Resource Programme (Contracts C02X813 and C04X0301). We also thank Patrice Delmas for assisting with the soil sampling.

Notes

^a: cation exchange capacity

^b: sodium adsorption ratio calculated as [Na⁺]/surd[Ca²⁺+Mg²⁺]

^a: total organic carbon

^b: dissolved organic carbon

^a: soil-dependent constant in the Gardner equation ^{[ 20 ]}