Mechanisms for Differential Mobility of Arsenical-Pesticide Derived Lead and Arsenic

Concern over the long-term consequences of exposure to low levels of arsenic and lead has spurred interest in the fate of arsenical pesticides applied to orchard lands during the early 1900's. To investigate the mobility of As and Pb ~50 years after application we collected contaminated orchard soil, channel surface sediment, and suspended sediment from the adjacent streams from a former orchard in Southern New Hampshire and a commercial orchard in Wenatchee, Washington. Colloids of different sizes were isolated from contaminated soil and sediments from neighboring catchments and characterized for their bulk composition by ICP-OES. Flow field-flow fractionation interfaced with ICP-MS was also used to identify the composition of Pb- and As-bearing particles. Sequential extractions and X-ray fluorescence spectroscopy also were used to identify the composition and speciation of As and Pb contaminated grains. Pb isotopes were used to fingerprint the source of the contaminated stream sediment.

Preliminary analyses show As in the stream sediment in high concentrations several kilometers below the contaminated orchard. However, in the same area Pb concentrations never rise above background. We investigate three possible mechanisms for the observed differential mobility of As and Pb. (1) As and Pb preferentially adsorb onto different sized colloids allowing preferential erosion of those particles smaller in size. (2) Pb-bearing Mn oxides may aggregate more than Fe oxides onto which As preferentially adsorbs. (3) Local, transient reducing conditions in shallow soils preferentially release As into solution, thus making it more mobile. Of these, it appears the association of As and Pb with different soil particles of different mineralogies is likely key to the preferential transport of As from these soils.