Impacts of long-term wastewater irrigation on soil-aquifer interactions: evidences from the Critical Zone Observatory Valle del Mezquital, Central Mexico.

In semi arid regions the reuse of wastewater is a crucial asset, optimizing water resources and recycling nutrients to agricultural fields. We have installed a Critical Zone Observatory in the Mezquital Valley, Central Mexico, where wastewater from the metropolitan area of Mexico City is used for irrigation for more than a century. We work at the regional scale by sampling repeatedly fields which have been irrigated for different lengths of time (0 to 100 years), and at the local scale, by monitoring observation wells installed at different depths along a piedmont. Recently, we evaluate soil process at the pedon scale in 1 m³ lysimeters equipped with sensors to measure water tension, volumetric water contents and redox potential, and collecting regularly the infiltrating water, the soil solution at different depths and the deep drainage. We aim to understand soil physical, chemical and biological processes occurring at different spatial and temporal scales under scenarios of changing water quality, namely the continuation of irrigation with untreated wastewater and the transition to irrigation with treated wastewater. The experimental set-up has allowed us to understand the water dynamics as well as the fate of nutrients and pollutants in this agroecosystem. Among the nutrients we have put special effort in understanding the N dynamics, and among the pollutants we have studied the behavior of heavy metals as well as pharmaceuticals and resistance genes. Our main findings concerning the system using untreated wastewater irrigation are that the supply of water and nutrients has increased agricultural productivity by more than 5 fold in this semi-arid region, but the incidence of gastro-intestinal infections in children is 10% larger in comparison with well water irrigated areas. Overflow irrigation has risen groundwater levels, and excess nitrogen is mainly leached as nitrate polluting the groundwater, and emitted as nitrous oxide into the atmosphere. Pollutants as heavy metals and pharmaceuticals accumulate in the upper centimeters of the soil. Irrigation with treated wastewater started two years ago occasionally; it is expected to lower gastro-intestinal infections as well as the loads of nutrients and some pollutants. However pharmaceuticals are expected to remain and resistance genes might even increase.