MICROSCALE pH VARIABILITY FOR ASSESSING EFFICACY OF PHOSPHORIC ACID TREATMENT IN LEAD-CONTAMINATED SOIL

Application of phosphoric acid (H3PO4) to lead (Pb)-contaminated soil may be a remedial strategy for in situ immobilization of soil Pb. Treatment homogeneity with three application methods was assessed by determining microscale pH variability through both conventional and spatial variance analyses. Soil containing an average 2570 mg Pb kg−1 near a smelter in Jasper County, Missouri, was treated with H3PO4 (10 g P kg−1) utilizing three methods: rototilling, pressure injection, and surface application. An undisturbed soil core (10 cm deep, 10 cm wide, 2.5 cm thick) was taken from each plot 30 days after treatment. Soil pH was measured at 1-cm horizontal and vertical intervals with a pH-sensitive glass microelectrode. Horizontal and vertical variations of soil pH were estimated by variance analyses, and the spatial variability was assessed by semivariogram function. Addition of H3PO4 to the soil decreased pH significantly and increased total variability in the measured zone. The pH variability in the vertical direction differed significantly, but that in the horizontal did not. Higher total variability caused by the treatment methods resulted from increased spatially dependent variability, which may be attributed to strong trends of pH with soil depth. Rototilling resulted in the lowest variation and smallest spatial variances, suggesting the most effective mixing of soil with added H3PO4 among the methods tested. This study demonstrated that microscale pH measurements and analyses of total and spatial variances may aid in assessing the efficacy of H3PO4 treatment in Pb-contaminated soil.