Widespread Pb contamination in urban backyard soils for & gt;100 years identified in soil cores constrained by Pb-210 and Cs-137

Contaminated soil in urban residential areas is often overlooked as a source of childhood exposure to toxic levels of lead (Pb). We document mean Pb concentrations of 1200 +/- 1000 mg/kg, three times the now outdated EPA soil hazard standard of 400 mg/kg, for 370 surface soils collected from 76 homes in the boroughs of Brooklyn and Manhattan of New York City. The mean Pb content of 250 +/- 290 mg/kg Pb for 571 surface soils collected from tree pits and public parks was much lower. A subset of 22 surface samples analyzed by EPA Method 1340 extracted 86 +/- 21 % (+/- 1SD) of total soil Pb, indicating that it the Pb was highly bioavailable. To investigate the origin of backyard contamination, 49 cores were collected to an average depth of 30 cm from a subset of 27 homes. Twelve soil cores were analyzed for Pb-210 and Cs-137 to constrain processes that impact contaminant distribution and inventories (particle focusing, soil accumulation, loss, and mixing). Concentrations of Pb declined with depth in 60 % of the cores but usually did not reach background. Mean uncorrected Pb inventories of 340 +/- 210 g/m(2) Pb (mean +/- 1SD, n = 12) were more than five times higher than the radionuclide corrected inventory of 57 g/m(2) from Central Park soil cores. Average inventories of 210Pbxs (3.5 +/- 0.9 kBq/m(2)) and 137Cs (0.9 +/- 0.6 kBq/m(2)) corresponded to 71 +/- 19 % and 50 +/- 30 % of the predicted atmospheric inventories. Elevated Pb concentrations were found both in the fine (<1 mm) and coarse (>1 mm) fractions, the latter suggesting a local non-atmospheric source. This was confirmed by individual grains containing up to 6 % Pb and visible pieces of coal, bricks, and ash. Regardless of the source of contamination in backyard soils, systematic testing is needed to identify contaminated areas and reduce child exposure.