Long-term Effects of Lime, Phosphorus, and Iron Amendments on Water-extractable Arsenic, Lead, and Bioaccessible Lead from Contaminated Orchard Soils

Lead arsenate was used on orchard soils as a pesticide from 1900 to 1960. An in vitro method has been used to determine the bioaccessiblity of Pb in soils contaminated by Pb paint, fossil fuel combustion, and metallurgical industries. The Pb fraction that is potentially bioavailable from orchard soils, however, has not been reported. Potential reductions in availability of Pb and As as a result of amendment addition have also not been explored. The objectives of this study were to (i) determine the effects of lime, phosphate (P), and iron (Fe) amendments, on waterextractable As and Pb in 2 orchard soils and (ii) determine the fraction of total Pb that is potentially bioaccessible in the amended soils. Two orchard soils, Thurmont loam and Burch loam, with total Pb of 676 and 482 mg kg and As of 133 and 93 mg kg respectively, were used. Soil pH increased over time with P application but not with Fe. Phosphate and Fe + P increased water-extractable As and Pb in both soils. Bioaccessible Pb was 57 and 42% of the total Pb for Thurmont and Burch soils, respectively, when Fe + P was added. Although the Fe + P treatment—reduced bioaccessible Pb, levels were high enough to be potential health risk if these lands are urbanized. Further studies are warranted to determine the levels of phosphorus that will be effective in reducing water-extractable As and Pb in these orchard soils. (Soil Science 2007;172:811-819)