Malone and Shakya (2024) conducted an integrative literature review of trace metal contamination (Pb, As, Cd, Zn) in US urban community garden soils, synthesizing 52 peer-reviewed articles from Web of Science and PubMed. The central finding is that lead is the primary soil contaminant of concern in urban community gardens, driven by legacy industrial activity, leaded paint chips, and leaded gasoline residues, and that the US EPA soil screening standard (recently reduced from 400 ppm to 200 ppm in January 2024) remains significantly higher than standards in California (80 ppm), WHO (85 ppm), and Finland/Europe (~60 ppm). Arsenic and cadmium are secondary concerns, often co-occurring with lead. Environmental justice is a cross-cutting theme: contamination hotspots disproportionately affect lower-income and minority communities.
Key numbers
US community garden soils — Pb, As, Cd, Zn summary (Table 2, mean and range in ppm, except Boston which reports range only):
- Aspen, CO (former mine dump sites, n=65 gardens, >195 samples): Pb 172 (9.2–808), Cd 2.5 (0.2–14.2), Zn 120 (8.4–484)
- Baltimore, MD (urban, n=104 gardens, 616 samples): Pb 104.5 (7.4–130.4), As 3.7 (0.2–13.5), Cd 1.4, Zn 139.7 (39.7–542)
- Boston, MA (3 gardens, 88 plots): Pb 117–170 (garden a), As 30–39 (CCA-treated wood plots)
- Cleveland, OH (vacant lots, n=65 samples): Pb 224 (14–1241), As 15 (7–58), Cd 1.2 (0.5–2.5), Zn 197 (83–543)
- New York City (urban/residential, n=905 gardens, 1652 samples): Pb 600 (3–8912), As 12 (0.9–7.6), Cd 1.6 (0.1–11), Zn 327 (35–2352)
- New York City (second dataset, n=54 gardens, 564 samples): Pb 102 (11–2455), As 5.7 (<5.3–93.2), Cd <0.4 (<0.4–3.1), Zn 138 (21–2317)
- New Orleans, LA (urban/suburban, n=27 gardens, ~600 samples): Pb 38.4 (1.4–9540), As 3 (0.7–61.7), Cd 0.318 (0.248–8.8), Zn 91.5 (17.8–7330)
- Oakland, CA (urban, 3 gardens): Pb 47–326 ppm
- Philadelphia, PA: Pb 47.6–351.4, As 0.9–9.6, Cd 0.1–1.4 ppm
Regulatory thresholds (Table 1, all in soil, ppm):
- Finland/Europe: ~60 ppm Pb
- California OEHHA: 80 ppm Pb
- WHO: 85 ppm Pb
- Canada: 140 ppm Pb
- US EPA (updated January 2024): 200 ppm Pb (residential), 100 ppm (multiple lead sources)
- Natural background US soil: 22 ppm Pb
Lead uptake by food plants: root vegetables show greater bioaccumulation from soil than leafy vegetables for lead because lead concentrates in the xylem/core of vegetables. Onions: >93% frequency of soil Cd contamination >0.6 mg/kg in reviewed sites; half of sampled onions showed >0.1 mg/kg Cd uptake. Arsenic in soils near CCA-treated wood borders: 68% of plots exceeded 5 mg/kg As.
Methods (brief)
Integrative literature review. Search strategy: Web of Science and PubMed with terms “community garden” and “urban garden” filtered by “contamination” and “USA”; 52 final articles after deduplication. No original measurements. Limitation: soil data reviewed are soil concentrations, not food crop concentrations; the relationship between soil concentration and crop uptake is complex and crop-specific. This paper is primarily a soil-contamination reference, not a food occurrence study.
Implications
Certification: This paper documents the urban garden soil context for US consumers who grow produce in community gardens. For the wiki’s purposes, it anchors the US urban Pb baseline for garden soil and provides comparison points against EPA, WHO, and California standards. The January 2024 EPA update (400→200 ppm) is a timely regulatory development.
Courses: Strong source for environmental justice and urban food safety modules. The comparison table of international Pb soil standards (Table 1) is a ready-made regulatory comparison asset for courses.
App: Supports a “community garden” or “urban-grown” flag for garden produce, particularly root vegetables and leafy greens, especially in older US industrial cities. The 22 ppm natural background vs. 102–600 ppm urban mean gives a 5–25× urban amplification factor for Pb that the app could use as a modifier.