Lead is a toxin that affects cognitive abilities in children. While people often think of air and water as pathways for environmental contaminants, soil is a major sink for environmental pollutants, including lead. Children can be exposed to lead through contaminated soil.
Former CAP Ph.D. student Xiaoding Zhuo investigated lead in Phoenix metropolitan area soils, using CAP’s Survey 200 soil samples. Survey 200 is a long-term monitoring initiative that investigates changes in soils, plants, insects, and land use at 204 points located randomly across the metropolitan area and surrounding desert.
Together with CAP scientists Christopher Boone and Everett Shock, Zhuo found elevated lead concentrations in the central core of the Phoenix metropolitan area as well as in southeastern parts of the region. While lead levels in the samples did not exceed the 400mg/kg-1 level of safety standard proposed by the EPA, urban samples had lead concentrations as much as 10 times higher than the general desert background values (Figure 1).
Given these levels of lead, the research team sought to understand their source. When Zhuo analyzed roadside soil samples, she did not find a relationship between the historical use of leaded gasoline and lead deposition at the urban sites under study. The team examined Toxic Release Inventory (TRI) data and industrial records but did not find evidence for industrial emissions as the source of lead. When they compared lead concentrations with the building year of houses, they found that areas with housing built before 1978 had soil samples with high lead concentrations (greater than 50 mg kg-1). Samples from areas of housing built before the 1940s had the highest concentrations. This is consistent with what researchers have found in other US cities since lead paint was banned in 1978. Through isotopic analysis of the soil samples, the research team concluded that the source of elevated levels of lead in their urban samples was from lead-bases paint rather than other sources.
Their next challenge was to understand who in the Phoenix metropolitan area was most likely to be exposed to lead. Using US Census data, they focused on wealth (as represented by median household and median family income, housing tenure, and car ownership), race (African American, Hispanic and white), and vulnerability to lead exposure (children under the age of five). Statistical techniques of regression analysis and spatial autocorrelation revealed that areas with relatively high soil lead concentrations are more likely to be Hispanic neighborhoods with a large percentage of renters. Median family income and median household income turned out not to be good predictors of lead exposure, although households without a car (an indicator of low wealth in this automobile dependent region) correlated with high lead exposure.
These results reveal an environmental justice issue in the Phoenix area where there is an inequitable distribution of risk associated with lead exposure. The Hispanic population at risk includes many immigrants, some of whom have limited English skills and immigrated illegally to the US. Landlords may not perceive economic advantages in maintaining properties in low-income neighborhoods, allowing paint to peel on properties, and low-income renters seldom have much leverage over their housing conditions. Hispanic renter households, those in the population with limited ability to mitigate lead exposure and the least social and political power, are the most exposed to this toxin. These results support findings in other CAP research that found an inequitable distribution of exposure to air pollution and other hazards across the Phoenix metropolitan area with a disproportionate burden falling on Hispanic neighborhoods in South Phoenix.
Zhuo, X., C. Boone, and E. Shock. 2012. Soil lead distribution and environmental justice in the Phoenix metropolitan region. Environmental Justice. 5 (4): 206-213.