UC Riverside

Urban and residential areas are covered by impervious surfaces such as concrete. During rainfalls and irrigations, these surfaces transfer runoff directly to surface water through urban sewage. Impervious surfaces are highly possible to be contaminated with pesticides, through direct treatments or through indirect deposits from surrounding areas (e.g., lawns) via wind- or water-aided movements. Although pesticides have been widely found in urban watersheds, their persistence and runoff potential from impervious surfaces have been seldom studied.

This dissertation includes a series of lab and field scale experiments to assess pesticide contamination on concrete surfaces, as well as different environmental and application parameters that may affect their runoff potential.

The results showed pesticide wash-off potential from concrete rapidly decreased after treatments, but low levels were constantly detected in the water from concrete with extended outdoor exposure. Immediately after treatments with pesticides in liquid formulations, concrete surfaces were equilibrated with 30 mL CaCl₂ solution for 10 min to mimic runoff, and <25% of applied pesticides were recovered. The wash-off percentage further decreased to <5% after 7-d. Higher wash-off potential was observed for solid formulated pesticides. The initial rapid loss of wash-off transferability was followed by a stage of sustained pesticide wash-off, which continued for over 112-d. The persistent wash-off potential was also observed for concrete repeatedly equilibrated with water once every 3-d. Most pesticides were still detectable after 10 times of washing-drying cycles.

To explain the observed wash-off patterns, sorption/desorption and degradation of permethrin on concrete were further studied using ¹⁴C-labeled permethrin. Although the sorption equilibrium was quickly achieved within 24-h, permethrin desorption continued for over 300-h, and around 20 % of applied ¹⁴C still remained on concrete after 300-h. If the desorption started 7-d after permethrin treatments, the amounts of desorbed permethrin decreased by 58 % compared the freshly spiked concrete.

Finally, field study was conducted to assess pesticide runoff following simulated and natural rainfalls. 80 % of runoff pyrethroids were found on suspended particles. Besides, a surface wiping method was developed to collect transferable particles from concrete and its potential to predict pesticide runoff prior to rainfall starts was demonstrated.