The Mechanisms for the Differential Mobility of Lead and Arsenic Derived From Lead Arsenate Pesticide.

Lead and arsenic sourced from lead arsenate pesticide persist in high concentrations in former orchard soils even though application ceased over 50 years ago. This contamination has become an area of concern as suburbanization leads to development of agricultural lands where lead arsenate was applied. Development has increased erosion and thereby the potential mobility of As and Pb in suspended sediment in overland flow during intense (>1 cm hr^-1) rainstorms. Surprisingly, the solid phase As and Pb have different transport distances during a given storm event. Arsenic is consistently transported farther. We analyzed suspended sediments collected during overland flow events using x-ray spectroscopy, scanning electron microscopy, and Pb isotopes to identify the mechanism of differential transport of Pb and As. Arsenic is transported in suspended sediments on colloidal Fe oxides on which it strongly adsorbs. Lead, however, is much less mobile due to differences in the solid-phase speciation of Pb in the soil. While a portion of Pb is adsorbed to Fe and Mn oxides, up to 40% is bound in silt-sized pyromorphite grains (~3-8 μm). These dense (7.04 g cm^-3) pyromorphite grains are preferentially removed from overland flow by faster settling velocities. The differential transport of these colloidal Fe oxide phases and pyromorphite is effectively modeled using a simple model for settling particles of multiple densities and sizes in a turbulent flow. The model is consistent with known particle densities and observed particle size distributions. The difference in particle settling velocities explains the longer distance transport of As from contaminated orchard soils relative to Pb.