IMPACT OF LAND USE ON THE MOBILIZATION OF LEAD ARSENATE PESTICIDES APPLIED TO HISTORICAL ORCHARD LANDS

Lead arsenate pesticides were widely applied to fruit orchards throughout New England and elsewhere the United States until the late 1940's. While previous work has demonstrated that elevated levels of As are present in stream channel sediments draining historical orchards, much uncertainty remains regarding the source, mechanisms of transport, and fate of this As and associated Pb. We obtained stream channel sediment cores from two field sites with documented histories of lead arsenate pesticide application; 1) McQuade Brook in New Hampshire and 2) perennial tributaries of the Yakima River in Washington. The vertical distribution of Pb and As in the cores was determined via total digestions of 2-cm increments. The cores in McQuade Brook contained abundant Pb and As in the top 20-30 cm, while cores from the Yakima River tributaries contained little measurable metals. To determine the timing of As and Pb deposition, in, the vertical distribution of Cs-137 activity within the McQuade Brook cores was measured by gamma counting using a Canberra Broad-Energy Intrinsic-Ge Detector. Cs-137 peaks in sedimentary cores correspond with the peak deposition of fallout Cs-137 in the early 1960's prior to the ban of atmospheric nuclear weapons testing. Initial dating results indicate that deposition of both As and Pb in the New Hampshire occurred after the peak in Cs-137 deposition, well after the application of lead arsenate pesticides. Thus it appears that Pb and As were mobilized following residential land development (~1960 based on historical records for the conversion of orchard lands to residential housing tracts), and continue to be mobilized. Sediment collected downstream of the orchard lands contained a higher ratio of As: Pb than in the orchard soils, suggesting the preferential transport of As. Chemical speciation using X-ray absorption spectroscopy and other methods suggests that this preferential transport may result from the association of Pb and As with different mineral colloids.