Anthropogenic Contamination of Urban Soils: Sources, Risks, Remediation

Research has shown that center city soils are highly contaminated by Pb, Zn, Cd, As, and some other elements as well as PCBs, PAHs, etc. Older cities have more severe contamination both due to the length of exposure, and multiple contamination sources. Paint rich in Pb, etc., was commonly used until the 1950s; gasoline contained Pb until the 1990s. Burning or demolition of urban structures often caused high local contamination with Pb, etc. Contamination of urban soils was hardly appreciated until 1983 when Mielke et al. showed that average garden soil-Pb in center city (0-3 mi) of Baltimore, MD was 964 mg/kg (range 26-10900 mg/kg). Soil-Zn was highly correlated with soil-Pb levels, and soil-Cd with soil-Zn. Soil-Cu and -Ni were seldom higher than background levels. Paint has been the dominant source of high Pb contamination, but automotive exhaust Pb and other stack emission of Pb, Zn, Cd and As (e.g., incinerators) contribute to urban soil contamination. Expanding cities overtake old orchard soils rich in Pb and As. Widespread use of CCA-treated wood in housing and playgrounds has distributed As. Risks are recognized from ingestion of soils (Pb, As), and growing of garden crops. Exterior soils become housedust thru human actions, exposing younger children; bare soils promote movement of exterior soils indoors. Soil Pb is partially bioavailable depending on soil management. The most sensitive risk from soil Pb is to children who ingest soil. EPA and HUD guidance: bare soil should not exceed 400 mg Pb/kg, and vegetated soils should not exceed 1200 mg Pb/kg. Treatment of high Pb soils with phosphate or biosolids composts can strongly reduce soil Pb bioavailability and risk. Avoiding growing low-growing leafy and root vegetables in urban gardens greatly reduces Pb potential exposure thru garden crops.