PHOSPHATE-ENHANCED MOVEMENT OF ARSENIC OUT OF LEAD ARSENATE-CONTAMINATED TOPSOIL AND THROUGH UNCONTAMINATED SUBSOIL

Past use of lead arsenate insecticides has resulted in elevated concen trations of lead (Pb) and arsenic (As) in topsoils of many existing an d former deciduous tree fruit orchard sites throughout the world. Appl ication of phosphate (PO4)-containing fertilizers to these soils can i ncrease soil As solubility, phytoavailability and downward mobility. A laboratory soil column experiment was conducted to determine if As re leased by phosphate additions to a topsoil artificially contaminated w ith lead arsenate (1.65 mmol total Pb/kg; 1.10 mmol total As/kg) would be appreciably resorbed by the underlying uncontaminated subsoil. Tre atments were a factorial combination of topsoil amendment with monoamm onium phosphate (MAP, 0 or 16.67 mmol PO4/kg), and amount of leaching (1, 5 and 10 pore volume displacements (PVD) with distilled water unde r saturated flow conditions). Soil As decreased in the topsoil with in creasing amount of leaching and increased in the subsoil. Addition of MAP substantially increased loss of topsoil As, promoted As transport into and through the subsoil, and increased dissolved As concentration s in the column leachates. After 10 PVDs, 95% of the initial soil As r emained in the -MAP columns, while 56% of the initial soil As remained the +MAP columns. Dissolved Pb concentrations were <0.05 mu mol/L in all column leachates. The data are consistent with a mechanism of PO4- enhanced release of As in the topsoil and subsequent promotion of As m ovement through the subsoil by continuing competition of dissolved As and PO4 for ion adsorption sites. The experimental results indicate th at use of PO4-containing fertilizers on lead arsenate-contaminated soi ls has the potential to greatly enhance downward movement of soil As.