MEASUREMENT OF DIVALENT LEAD ACTIVITY IN LEAD ARSENATE CONTAMINATED SOILS

Many soils used for tree fruit production before 1968 contain elevated Pb concentrations resulting from use of lead arsenate pesticides. The chemical Form of Pb in these soils may influence its bioavailability and the need for and type of remediation actions. The competitive chelation method was used to determine the single-ion activity of Pb2+ in aqueous suspensions of 11 lead arsenate contaminated soils and, for comparison, three Ph-enriched roadside soils. The activity of Pb2+ ranged from 10(-9.37) to 10(-7.21), consistent with values published elsewhere, and was inversely related to pH. The soils appeared to fall into two solubility groups, one where Pb2+ activity was consistent with control by a Pb-phosphate mineral, possibly Pb-5(PO4)(3)OH or Pb-3(PO4)(2), and a second group where Pb2+ activity was consistent with control by Pb(OH)(2). Both groups contained calcareous soils. Adding monocalcium phosphate to two of the soils from the higher Ph solubility group shifted one soil to the lower solubility group but had no effect on the second soil. The experimental results suggest that Ph solubility in lead arsenate contaminated soils is not appreciably higher than in soils contaminated by Pb from gasoline and that phosphate amendment may not always reduce soil Pb solubility.