Heavy metal content in urban residential and park soils: A case study in Spokane, Washington, USA

Urban soils may be contaminated with heavy metals due to historical and current use of fertilizers, pesticides, wood preservatives, and construction materials. To examine the amount of trace metals from natural and anthropogenic sources, we focused on a middle-to-upper income, residential area in Spokane, Washington which was developed in the late 1800s but has not experienced heavy industrial activity. Surface soil was collected from thirty sites (including residential lawns, gardens, and city parks), and digested using EPA Method 3050B. The average As, Pb, and Zn concentrations in residential soils (11 mg kg−1, 59 mg kg−1, and 107 mg kg−1, respectively) exceeded those in below surface rural soils (9 mg kg−1, 15 mg kg−1, and 66 mg kg−1) indicating sources of contamination in this urban environment. Four sites had As or Pb concentrations above Washington State Department of Ecology levels for unrestricted land use (20 mg kg−1 and 250 mg kg−1, respectively). At one of these sites, Pb exceeded 1200 mg kg−1, the USEPA threshold for residential soil with unrestricted use, and an additional site had Pb concentrations above the 400 mg kg−1, the limit for children's play areas. The Pb:As ratio in soils with elevated As concentrations suggest that the likely source is arsenical pesticide or an As-bearing fertilizer. In soils with elevated Pb concentrations, Ba, Pb, and Zn were correlated which suggests ─ along with the soils' proximity to structures ─ that lead-based paint is likely the source. Although vehicle emissions also add these metals to soils near roadways, traffic density near our study sites was low.