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 b e a remedial strategy for in sib immobilization of soil Pb, Treatment homog eneity with three application methods was assessed by determining microscal e pH variability through both conventional and spatial variance analyses. S oil 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 s oil 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 vertica l variations of soil pH were estimated by variance analyses, and the spatia l 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 signif icantly, but that in the horizontal did not. Higher total variability cause d by the treatment methods resulted from increased spatially dependent vari ability, which may be attributed to strong trends of pH with soil depth. Ro totilling resulted in the lowest variation and smallest spatial variances, suggesting the most effective mixing of soil with added H3PO4 among the met hods tested. This study demonstrated that microscale pH measurements and an alyses of total and spatial variances may aid in assessing the efficacy of H3PO4 treatment in Pb-contaminated soil.