Glyphosate use in urban landscape soils: Fate, distribution, and potential human and environmental health risks

This novel study investigated the fate and distribution in soils, and potential exposure risk of glyphosate, an extensively used herbicide in urban landscapes. The rate-determining step of glyphosate sorption in urban soils involved chemisorption processes through exchange or sharing of electrons that followed the pseudo-second-order kinetics model. As evidenced by the Freundlich isotherm model, glyphosate gets partitioned into heterogeneous surfaces of soil organic matter (OM) and clay minerals, and then diffused into soil micropores. The principal component analysis revealed that soil OM (R² = 0.873), oxides of Al (R² = 0.361) and Fe (R² = 0.126), and contents of clay (R² = 0.061) and silt (R² = 0.432) were positively correlated with the distribution coefficient (K_d) of glyphosate, while alkaline pH (R² = -0.389) and sand content (R² = -0.343) negatively correlated with the K_d values. Well-decomposed soil OM, consisting of C-H and CO functional groups, enhanced glyphosate sorption, whereas partially decomposed/undecomposed OM facilitated desorption process. Desorption of glyphosate was favoured in seven of nine selected soils due to adverse hysteresis effects (HI = 0.74-1.0). The higher values of leachability index (0.31-1.0) and groundwater ubiquity score (1.60-3.44) calculated for the urban soils indicated the great leaching potential of glyphosate from soil surface to waterbodies. Use of glyphosate on impermeable surfaces might directly contaminate water sources and affect potability of water, non-target biota, and food safety. The calculated values of cancer risk (10^-8‒10^-12) and hazard quotient (1.47 × 10^-6‒4.12 × 10^-6) suggested that the human exposure to glyphosate-contaminated soils through dermal, ingestion and inhalation pathways might cause negligible or no carcinogenic and non-carcinogenic risks to humans. Therefore, glyphosate should be applied judiciously at recommended concentrations in the urban landscapes, mainly on impervious surfaces, to minimize its health impacts in humans and environment.