Mobilization of Arsenic and Heavy Metals in Polluted Soil Due to Changes in the Soil-Water Management System

Abstract

Soils act as sinks for heavy metals and metalloids, nevertheless, changes in the soil system conditions, such as an increase in acidity, salinity or decrease of soil organic matter due to agricultural practices could mobilize these potentially toxic elements and pollute the groundwater or enter the food chain. Mexico City’s wastewater has been applied to agricultural soils for more than 100 years, increasing the soil acidity and salinity and the contents of heavy metals. Since 2018 part of the wastewater is treated and chlorinated before being used for irrigation. Here we analyzed the potential mobility of heavy metals and As by sequential extractions of soil profile samples, and evaluated the effect of a water quality change by batch experiments that simulated an increase in the acidity, salinity and chlorine concentrations in the irrigation water. Less than 20% of the concentration of the elements in all the analyzed samples was present in mobile fractions, being Cd and As the most mobile ones. In the subsoil horizons, most elements were mainly present in the residual fraction, while in surface horizons, Cu and Pb were more frequently associated with the organic fraction and iron oxides of the soil. In the batch experiments, As was desorbed the most as acidity increased, while Cd and Zn were readily mobilized with increasing salinity. Cd, Pb and Zn desorbed from the soil when water chlorination was simulated. Geochemical speciation models showed that Cu and Pb were mobilized as organic complexes in the chlorination scenario, Cd as organic complexes and free ion and zinc mainly as a free ion in all the evaluated scenarios. The potential mobilization of Cd and As poses particular risk to crop and groundwater quality and should be monitored carefully as changes to the land use system, such as water treatment, are implemented.

Keywords: Wastewater, Potentially toxic elements, Acidification, Chlorination, Salinity, Desorption