Incubation and Batch desorption experiments were conducted on ultisol and a mollisol to assess the effect of aging temperature and contact time on Pb sorption in soils A and B horizon ultisol and Mollisol from Humboldt and San Luis Obispo County, California were incubated at room temperature, 50 and 70 ⁰C for 3 hours, 4 weeks and 2 months in the laboratory. After each aging period, samples were extracted with 10-4 M H2SO4 solution, equilibrated for 24 hours, centrifuged and filtered through a 0.45 µm filter. A separate subsample of the B-Horizon supernatant was passed through 0.22 µm filter. Increase in aging temperature and fixation time was observed to enhance Pb solubility and bioavailability in the A horizon soils. Mobility of Pb in the B horizon was greatly retarded. XRD analysis showed no phase change upon heating; while Kd computations showed partitioning of Pb in the solid phase of the mollisol was greater. This allows for assessing the risk associated with mobility of Pb in tropical soils, and shows that ground water in areas overlain by ultisols are probably more liable to Pb contamination
