Urban gardens provide valuable opportunities for community engagement and enhance the overall well-being of those they serve. Despite these attributes, soils in metropolitan areas often carry a legacy of heavy metal contamination due to industrial activity and urban waste. This study examined the soil chemistry of 118 samples from four community gardens, referred to as EE, PH, TS, and WT, to assess the concentrations of five heavy metals: Arsenic (As), Copper (Cu), Lead (Pb), Mercury (Hg), and Zinc (Zn). Ohio’s maximum allowable concentration (MAC) is 68 ppm for As, 50 ppm for Cu, 100 ppm for Pb, 0.045 ppm for Hg, and 100 ppm for Zn. Through the use of X-ray fluorescence, we determined that As and Hg are below Ohio EPA MACs or are below detection limits. Conversely, elevated concentrations of Zn, Cu, and Pb in soils were found that exceed Ohio’s recommended levels, which are based on ecological and human health screening guidelines. In total, seven (6% of total) samples from Gardens PH, EE, and WT exceeded the MAC for copper, and six (5% of total) samples in Garden EE were found to exceed the MAC for lead. Most noticeably, 76 samples (64% of the total) from all four gardens exceeded the recommended levels for Zinc. It should be noted that the natural average abundance of Zn in southwest Ohio is ~70 ppm, which is close to regulatory threshold values and background concentrations reported in regional soil studies. The area with the overall highest levels of Zn is Garden PH, and the area with the highest levels of Pb is Garden EE.
Urban gardens are becoming increasingly popular in metropolitan areas as citizens and community organizations revitalize abandoned spaces. However, this change in land use carries the risk of heavy metal contamination of soil from past industrial activities (1)(2). Therefore, it is necessary to test for heavy metal contamination in urban gardens as exposure to these metals can be toxic to humans (3)(4). In this study, we analyzed the soil chemistry of four urban gardens in Cincinnati to assess the heavy metal contamination of the sites.
The urban gardens analyzed are situated on the outskirts of Cincinnati (Figure 1). Each of the four gardens has a site history that impacts its current soil chemistry. Garden WT sits atop what used to be a metal playground that hosted illicit activities, so the citizens of this area decided to replace this plot of land with an urban garden. Garden EE was allotted for public use by the City of Cincinnati due to its location on the Ohio River floodplain, making it unusable for new building development. The plot of land that hosts Garden TS was sold when the City of Cincinnati removed various derelict buildings. Lastly, Garden PH was constructed on top of an infilled swimming pool. These unique site histories helped inform our analysis of the gardens’ soil chemistry.
To gather samples at all four farms within the time restraints of the study, the research team split into two groups of two. Each group created a grid to gather the appropriate amount of samples for each garden, which was proportional to each garden’s size. 23 samples were collected from Garden EE and WT. Site EE is composed of three 50 x 23 ft fields with the addition of a few raised beds. Site WT is composed of both raised beds and a grove of trees. The beds are approximately 3 x 7 ft, and the trees are located 5 ft apart. Site TS consists of two plots. Plot A is 32 x 34 ft, and Plot B is 40 x 27 ft. Finally, site PH is composed of four plots. Plot A measures out to be approximately 20 x 9 ft, plot B measures about 12 x 9 ft, and plot C and D measure to be 30 x 10 ft. Additionally, the team gathered 64 samples from Garden PH and 10 from Garden TS. After the samples were collected and brought back to the lab, the team used a fumigation hood to fully dry the soils. After drying, the team homogenized the samples using a mortar and pestle and loaded the samples into XRF capsules. Samples were then analyzed on Thermo Scientific Niton XL3t 955 Ultra with a run time of three minutes per sample. NIST standard 2709a (San Joaquin Soil) was analyzed every 10-15 samples to ensure accuracy and precision.
Arsenic concentrations at all four urban gardens fall below Ohio’s maximum allowable concentration (68 ppm) (3)(6)(7). Due to the XRF machine’s low sensitivity for Mercury, the results for this element are uncertain. 76 of the analyzed samples (64%) had Zn concentrations that exceeded recommended levels (1)(3)(4)(5). Zn was the most common contaminant across all gardens, with the highest concentrations of the heavy metal found in Garden PH. It is important to note that the natural background for Zn in southwest Ohio is 69 ppm, which helps explain why Zn exceedance is common in our samples. However, this natural background level of Zn does not provide a full answer as to why more than half of the samples yielded results far beyond what is considered normal for this area (4). Cu was found in 7 of our samples (6%), exceeding the limits found in PH, EE, and WT gardens (3). Pb was found in 6 of the samples (5%), all of which exceeded limits and were found in Garden EE. Zn had the most consistently high concentration across all sites. Pb was the most localized contaminant, being especially high in Garden EE. Overall, the data show how some heavy metals exceed Ohio's safety levels. However, due to geologic history and other possible outstanding factors, more research is necessary to increase confidence in these results. It is important to note that this study focuses on soil chemistry alone, not plant systems, meaning the direct risk to human health through consumption of produce is not something that can be remarked upon (5). Overall, the results emphasize the importance of continued monitoring, soil management practices, and further research to better understand contaminants and ensure the long-term safety and use of urban gardens in Cincinnati.