Trace Metal Contamination in Community Gardens in Pittsburgh, Pennsylvania

Abstract

High levels of trace metals in urban community garden soils pose human health risk due to the potential exposure through the ingestion of crops grown in contaminated soil and other exposures. This study assesses eight trace metal and metalloids (As, Cd, Cr, Cu, Ni, Pb, V, and Zn) in a total of 54 soil samples collected from nine community gardens across Pittsburgh, Pennsylvania, in 2022 using X-ray fluorescence (XRF) spectrometer and inductively coupled plasma mass spectrometry (ICP-MS). There was a strong correlation between XRF and ICP-MS measurement (R² > 0.8) for all elements except V. When the mean concentration of trace elements at each of the gardens was compared against the most stringent standard, none of the gardens had exceedances for Pb, Cd, and V. One specific garden had exceedances for Cr, Ni, Zn, and As. About 15% of soil samples had Pb concentrations exceeding 100 mg/kg. Mean Pb concentration measured by ICP-MS was 53.7 ± 40.2 mg/kg and 72.7 ± 53.7 mg/kg in raised bed and ground soil, respectively. It is important to conduct regular soil testing at community gardens in the areas with industrial activities. In addition, use of raised-bed with new soil and safe gardening practices, such as the use of gloves and changing clothes before entering homes, can help to reduce exposure.

1. Introduction

Community gardens can provide various benefits, such as healthy and sustainable food options, green space, and opportunities for community engagement. Community gardens can help to fill vacant lots and open space with something beneficial to everyone in the surrounding area [1]. While motivations for a local community garden may vary, many are constructed in food apartheid areas, which the Congressional Research Service (CRS) highlights as a term to describe areas that are subject to structural conditions that limit food access, such as grocery stores divesting in low-income and colored neighborhoods [2]. Such areas tend to be in regions with lower socioeconomic status and areas comprising underserved and underrepresented communities [3]. Community gardens have been a tactic used in the fight against obesity, diabetes, and to promote overall nutrition by integrating youth into the process of growing and consuming personally grown produce [1,4]. In poverty-stricken urban areas, especially, youth are more likely than their affluent peers to experience physical and mental health problems, a poor-quality diet, an unhealthy body weight, academic difficulties, and engage in criminal activity [4]. Certain issues putting pressure on urban areas may require extensive social reform, but the benefits of community gardens may help alleviate some stressors. However, in looking to improve these urban areas as a safe space for youth and adults alike, it is important to ensure that community gardens are safe and healthy locations to grow produce.

A growing concern in community garden development in urbanized zones is trace metal contamination in soil. Urban soils may have elevated levels of trace metals and metalloids such as Pb, Cd, and As, which have been determined to have the potential for severe health risks [5,6]. The hand-to-mouth behavior associated with produce gardens, combined with the potential of inhalation of trace metals in soil dust, can lead to elevated blood Pb levels. Elevated blood Pb can lead to negative health effects such as inhibited brain development and anemia [7]. In urban areas, trace metal contamination is contributed by both current and historical anthropogenic sources, such as lead-based paints, automotive emissions, and local industries such as smelters and manufacturing [8,9]. Trace metal exceedances have been found in several community garden samples in the United States, with 10% (Pb), 9% (As), and 2% (Cd) of sampled New York City gardens exceeding New York State Department of Environmental Protection Soil Cleanup Objective standards for “Restricted Residential” use [10]. Several studies have found a high uptake of trace metals by various fruits and vegetables that can pose a risk to human health [6,11,12,13].

While there are several studies detailing the evidence of trace metal concentrations exceeding standards, it is important to highlight that the provided standards are highly variable and inconsistent in recommended levels for cleanup. Concentrations at which the soil is considered contaminated and unsafe for human health vary at different scales and by exposure method, such as ingestion, inhalation, or dermal exposure, and there is a great need for consistent guidelines on trace metal contamination to make it easier for gardeners to understand at what level their soil is unsafe for use [14].

Pittsburgh, Pennsylvania, is a historically industrial city with a legacy of trace metal contamination through its leadership in the steel industry between the 1880s and 1980s. However, trace metal contamination in and around the city of Pittsburgh is not a new discovery, as previous research has found significant metal contamination of Ohio River sediments in 1977 and 1987, as well as trace metal concentrations above natural background levels in a Pittsburgh lake-core [15,16]. Previous studies found that the trace metal deposition occurring after Pittsburgh’s role as a primarily industrial city has a long legacy, and it is, thus, important to continue to monitor these pollutants to minimize potential human exposure and reduce health risks. Pittsburgh has had 347 documented mills, coke ovens, and foundries located along the Monongahela, Allegheny, and Ohio Rivers, allowing for the easy transport of products (Figure S1) [17]. Spatial distributions of trace metals in soils have been previously correlated with industrial activities, with Cd concentrations elevated near historical coking operations, and recognized Pb elevations in locations with historical Pb smelters [18].

In the growing concerns over the health and safety of community gardens, it is important to implement testing methods that are both effective and accessible for local communities to use to determine if their soil is safe for growing produce. Inductively Coupled Plasma Mass Spectrometry (ICP-MS) is a well-accepted, reliable, and conventional method for determining trace metals. However, X-ray fluorescence (XRF) analysis has more recently been seen as a quicker alternative compared to the laboratory-intensive process of ICP-MS. XRF analysis can be performed in situ, potentially increasing the speed, cost-effectiveness, and accessibility of soil testing across a wide range of environments, allowing for the rapid delineation of any contaminated areas and quicker remediation response [19]. The drawbacks of XRF analysis include its high detection limit, and critical penetration depth for fluorescence emission for lead in soil may be limited to the top layer [19,20]. However, it has been found that preparing the field samples through drying, grinding, and sieving has significantly improved the quality of the XRF data [19,20]. In this study, a total of 54 soil samples from nine community gardens in Pittsburgh, Pennsylvania, and adjacent areas were sampled and analyzed for seven trace metals (Cd, Cr, Cu, Ni, Pb, V, Zn) and one metalloid (As) using both XRF and ICP-MS methods. We focused on only these eight elements in this study because these trace elements are one of the major health concerns in community garden soils, and also due to the availability of standards/recommendations for these elements in the soils.

The major objectives of this study were to compare measurements by two different methods of XRF and ICP-MS, and to assess the extent of trace metal contamination in community gardens across Pittsburgh by comparing them against the available standards and guidelines. Despite being a city with a legacy of industrialization, to our knowledge, no published studies assessing trace metal contamination in community gardens in Pittsburgh have been found, except one recent publication by our research group [21]. In the previous study [21], we compared heavy metal contamination from two major urban centers, Philadelphia and Pittsburgh, including suburban Philadelphia locations, in the state of Pennsylvania. In this study, we focused only on Pittsburgh by sampling more urban gardens in Pittsburgh than the previous study and comparing the measurement with two different methods, XRF and ICP-MS, while comparing the soil samples collected from raised beds and the ground.

2. Materials and Methods

2.1. Soil Sampling

Soil samples were collected from nine Pittsburgh-area community gardens in September 2022 (Figure 1). The garden sites selected for this study were based on accessibility, variety of urbanization level based on the visually observed density of development surrounding the garden, distance from known trace metal sources, and sites provided by the Grow Pittsburgh Grower’s Map [22]. Gardens ranged from 5–18 years in operation, with sizes ranging from approximately 405 to 2430 square meters. Information specific to all gardens is provided in the Supplementary Information (Table S1). There were no major differences in aesthetics or functioning of the gardens, as all sampled gardens utilized both raised beds and ground-planting. Soil samples were collected from both raised beds and the ground. Soil samples were more frequently replaced in raised beds than in the ground soil. To represent the entire garden, six separate soil samples were taken from each site: three different soil samples from various sections of the ground and three different soil samples from various sections of the raised beds. Mean, standard deviation, and range of concentration from three different soil samples taken from the raised bed and ground of each community garden are presented. Samples were collected from the top three inches of the soil using a plastic spoon and placed into a plastic bag until further analysis. The plastic spoon was cleaned with water and a paper towel between each sample to minimize possible contamination between samples. Soil samples were analyzed first by XRF and then by ICP-MS.

2.2. Sample Preparation

Approximately 10 g of each soil sample were weighed and dried. The drying process included approximately 3 days of air drying at room temperature and 1 full day of oven drying at 60 °C to remove moisture from the soil. After the soil was dried, samples were first sieved to remove larger materials such as rock or woody debris, and then a mortar and pestle were used to grind the samples into a fine powder to be prepared for both XRF and ICP-MS analysis.

2.3. Trace Metal Analysis

2.3.1. XRF Analysis

XRF cups (Chemplex Industries, Inc., Palm City, FL, USA) were filled with fine-ground soil sample (<10 g), and plastic wrap was used on the scan side of the cup in place of traditional XRF film. Cups were scanned on 31 October, and 2 November, 2022, with the Olympus Vanta Handheld X-Ray Fluorescence Spectrometer utilizing Beams 1 (90 s), 2 (30 s), and 3 (90 s) to acquire concentration values for all metals and elements of interest. One scan was conducted per sample, with a total scan time of 210 s. For quality control, a blank test was run between every garden using Olympus blank. Results for blank tests were checked to ensure there was no contamination for the elements of interest. Besides a blank test, scans were also made to a certified reference standard (OREAS 70b) [23]. The detection limits of ICP-MS and XRF instruments are provided in Table S2 and Table S3, respectively. The XRF methodology for this study is based on instructions provided on Vanta Handheld XRF GeoChem Training module provided by the manufacturer (Olympus Corporation of the Americas, Waltham, MA, USA).

2.3.2. ICP-MS Analysis

To minimize variations of soil samples analyzed by XRF and ICP-MS, the same soil sample that was used for XRF analysis was used for subsequent analysis by Inductively Coupled Plasma Mass Spectrometer (ICP-MS, Model 7900, Agilent Technologies, Santa Clara, CA, USA) analysis according to US EPA method 6020A [24]. Approximately 0.5 g of soil sample was weighed out for use in microwave acid digestion (MARS 6^TM, CEM Corporation, Matthews, NC, USA). In microwave acid digestion, 0.5 g of each soil sample and 10 mL of nitric acid (67–70%, ultrapure, for trace element analysis) (HNO₃) were put into 55 mL of Mars Express Teflon vessels, which were then placed into the Mars 6 One-Touch Technology and ran at 175 °C for 20 min. After microwave acid digestion was complete, each sample went through a process of three dilutions using de-ionized water and a 0.45 µm syringe filter to prepare for ICP-MS analysis. Every sample run in the ICP-MS used internal standards of Scandium, Germanium, Yttrium, Indium, Terbium, and Bismuth, and a standard check was run after every 10 samples. We used standard reference material (SRM 2711a) Montana Soil II to check the accuracy of the measurement by ICP-MS. When three replicate soil samples of SRM 2711a were measured by ICP-MS following the microwave acid digestion process, we found high recovery for Ni (84%), Cu (87%), Zn (94%), As (89%), Cd (91%), and Pb (94%) but low recovery for V (60%) and Cr (50%).

2.4. Enrichment Factor

Enrichment factor (EF) was used to determine the likelihood of anthropogenic influence on trace metal concentrations in the soil. Equation (1) was used separately on both the ground soil and raised bed samples to determine EF values.

$$\mathrm{E}\mathrm{F}={\displaystyle \frac{{\left(\frac{\mathrm{M}\mathrm{e}\mathrm{t}\mathrm{a}\mathrm{l}}{\mathrm{R}\mathrm{E}}\right)}_{\mathrm{s}\mathrm{o}\mathrm{i}\mathrm{l}}}{{\left(\frac{\mathrm{M}\mathrm{e}\mathrm{t}\mathrm{a}\mathrm{l}}{\mathrm{R}\mathrm{E}}\right)}_{\mathrm{b}\mathrm{a}\mathrm{c}\mathrm{k}\mathrm{g}\mathrm{r}\mathrm{o}\mathrm{u}\mathrm{n}\mathrm{d}}}}$$

(1)

“RE” is the reference element, and “Metal” is the concentration of element of interest. Typically, aluminum is used as the reference element, but since aluminum concentrations were not obtained in this analysis, EF values were calculated using rubidium as a reference element, as previously done by other studies [25]. Elemental concentrations for background were obtained from soil crust [26].

2.5. Statistical Analysis

Statistical analysis was performed in R using packages corrplot and ggcorrplot (R Core Team, 2021). A Shapiro–Wilk test was run in R to test the normality of the data. Mann–Whitney U-Test was performed in R to determine if there was a significant difference between raised and ground sample concentrations of trace metals for all gardens. We used R to analyze correlation tests between trace metal measurements by ICP-MS and XRF. Then, we present the results based on ICP-MS measurements.

2.6. Limitations

It is known that spectral interferences by peak overlaps can occur in portable XRF spectrometers [27] and polyatomic interferences can occur in arsenic analysis by ICP-MS [28], but the corrections are not made in this study.

3. Results and Discussion

Mean, standard deviation, and range concentrations for eight elements (As, Cd, Cr, Cu, Ni, Pb, V, and Zn) analyzed by ICP-MS from 54 soil samples collected across nine community gardens are provided in

Table 1. Trace metal and metalloid contamination in community garden soils across the United States.

City	Concentration (mg/kg)		Contamination Source
Pittsburgh, PA (n = 54) Metric: Mean ± standard deviation (Range) (This Study)	Raised Bed As: 8.58 ± 0.73 (7.83–10.24) Cd: 0.71 ± 0.16 (0.48–0.99) Cr: 31.10 ± 8.06 (21.92–48.84) Cu: 43.05 ± 10.76 (27.11–59.80) Ni: 18.61 ± 2.08 (14.64–21.78) Pb: 53.67 ± 23.34 (27.00–99.31) V: 20.34 ± 2.91 (15.56–24.85) Zn: 161.68 ± 31.84 (109.27–227.62)	Ground soil As: 9.74 ± 3.29 (5.50–17.53) Cd: 0.87 ± 0.62 (0.42–2.40) Cr: 31.57 ± 29.05 (14.24–107.74) Cu: 41.28 ± 12.89 (26.13–63.69) Ni: 22.13 ± 17.52 (11.56–68.16) Pb: 72.72 ± 41.21 (24.06–131.61) V: 19.32± 6.11 (10.49–30.42) Zn: 173.17 ± 49.90 (114.64–256.11)	Smelting, Transportation, Industry
Pittsburgh, PA (n = 20) Metric: Range (Bassetti et al., 2023) [21]	As: 5.50–17.53, Cu: 11.6–77.9, Cd: 0.2–12.5, Pb: 83.1–232.9		Anthropogenic Sources
Philadelphia, PA (n = 33) Metric: Mean (Bassetti et al., 2023) [21]	Raised Bed As: 3.18 Cd: 0.45 Cr: 39.34 Cu: 109.87 Ni: 25.78 Pb: 64.25 V: 33.33 Zn: 473.10	Ground soil As: 4.58 Cd: 0.79 Cr: 31.22 Cu: 102.41 Ni: 15.93 Pb: 120.86 V: 27.38 Zn: 298.66	Smelting
New York, NY (n = 106) Metric: Median (Cheng et al., 2015) [10]	As: 7.6, Cd: 1.1, Co: 8, Cr: 39, Cu: 55, Ni: 21, Pb: 140, Zn: 169		Leaded Paint and Gasoline, Refuse Incineration
Baltimore, MD (n = 422) Metric: Mean (Mielke et al., 1983) [29]	Cd: 0.56, Cu: 17, Ni: 2.8, Pb: 100, Zn: 92		Industry, Incineration, Paints, Leaded Gasoline
Connecticut (n = 174) Metric: Mean (Stilwell et al., 2008) [30]	As: 4.2, Cd: <0.5, Cr: 14, Cu: 40 Ni: 12, Pb: 176, Zn: 163		Industrial Sites, Transportation, Manufacturing
Chicago, IL (n = 86) Metric: Mean (Witzling et al., 2010) [31]	Raised Bed Cu: 8.99 Pb: 60.7 Zn: 38.4	Ground soil Cu: 19.3 Pb: 224 Zn: 69.1	Undefined
Phoenix, AZ (n = 28) Metric: Mean (Holmes et al., 2018) [32]	Cd: 0.9–4.4, Pb: 39.8–127.9		Industry, Fertilizer

. We focused on these eight elements because these are of main health concerns, and the standards are available for these eight elements. Concentrations for each of the nine gardens are provided in Table S4 (Supplementary Section).

3.1. Comparison of Two Measurement Methods: ICP-MS and XRF Concentrations

There were high correlations (R² > 0.80) for six elements (As, Cr, Cu, Ni, Pb, and Zn) (Table S5) measured by XRF and ICP-MS. The highest R² value of 0.96 was observed for Ni. Compared to other elements, poorer correlation was observed for Cr and V. This may be due to the fact that only nitric acid was used for the digestion of soil samples. The addition of HCl following the EPA method 3051A may improve the detection of these elements. These two elements had a lower recovery percentage for this method when tested against SRM 2711a Montana Soil II. For Pb concentrations, there was one data point that stood out as a clear outlier in the correlation plot, so this data point was removed to see if the relationship was significantly stronger without this point (Figure 2; Table S5). Without the outlier in the Pb dataset, the high R² value of 0.973 indicates that XRF analysis can be used as a reliable alternative method for Pb measurement. However, a linear regression slope of 1.232 indicates that XRF tends to overestimate the Pb measurement over ICP-MS. Since Pb is an element of high concern in soil pollution due to its potential for adverse health effects, this is important to recognize that XRF values are comparable to ICP-MS values.

Although there was a high level of correlation, XRF measurements tended to overestimate concentration values (As, Cr, Cu, Ni, Zn, and Pb), as indicated by slopes greater than 1 (Figure 2; Table S5). The overestimation may be explained by the only acid-digestible fraction of trace metals that the ICP-MS method is able to detect compared to the total metal content measured by the XRF method. A previous study showed that the nitric acid-digestible fraction of the measured trace metals was, on average, 71% of the XRF-detectable content for the five trace metals analyzed [33]. We found similar results with the elements of interest, on average, returning XRF values 68% of the ICP-MS concentration values. Other studies have also reported overestimation by the XRF method compared to the ICP-MS method due to the latter method only measuring the acid-digestible part of the soil [34,35].

A good correlation was found between Pb concentrations measured by XRF, as well as ICP-AES and AAS [19]. Several other studies have found a good correlation between XRF and ICP-MS. Previous study found a high correlation for As, Ba, Co, Cu, Mo, Nb, Pb, Rb, Sr, U, and Zr, and a poor correlation with Cr, Ni, and V [36]. This study also found a low correlation for V but found a strong correlation for Cr and Ni. The XRF method can be considered as a suitable alternative for the measurement of certain elements, such as As, Cr, Cu, Ni, Pb, and Zn, when the soil samples are properly prepared for analysis (dried, ground, and sieved). The XRF analysis in this study was performed ex situ, where samples were carefully prepared for analysis following drying, grinding, and sieving, so the conclusions are applicable only for physically processed samples. Therefore, such preparation will be required for XRF. When high contaminant levels are found, further analysis should be conducted by ICP-MS for confirmation because of overestimation by XRF.

3.2. Comparison with Other Studies

Several studies have previously analyzed trace metal concentrations in community garden soils due to the potential health risks (Table 1). Mean concentrations obtained in this study are in similar ranges to other urban locations around the United States. In comparison to other studies that included cities with industrial histories, most metal concentrations fall into a similar range of values and are largely attributed to industrial histories and transportation in these urban centers. Lead concentration found in this study is lower than New York [10], Connecticut [30], and Chicago [31] (Table 1). Both New York and Connecticut had multiple sources, including incineration and industries. Pittsburgh concentrations are similar to other cities, such as Philadelphia, Baltimore, and Phoenix (Table 1). All these cities have histories of industrial pollution, which indicates that trace metal pollution in Pittsburgh is not unique to this city but a problem that several cities face as they establish community gardens in areas previously and possibly currently hosting industrial sites.

3.3. Standards Relevant to Community Garden Soils

Standards and guidelines for trace metals are required in residential soils to assess risk from exposure. Soil safety policies (SSPs) have been implemented in urban agriculture sites nationwide, but guidance to urban growers has been found to be inconsistent, vague, or even lacking in reference to what safe levels of contaminants in these scenarios may be [37]. The lack of consistent recommendations and guidelines for lead in soil, and not easily accessible test services for urban gardeners, make it difficult to assess and understand the risk for urban gardeners [31]. In a review of SSPs in the United States [37], only 10 cities with policies pertaining to soil testing had guidance values to interpret such testing, and these values ranged widely across cities. Due to the ambiguity and lack of consistent guidelines, standards for soil contamination and clean-up for residential soils were derived at various scales, including state, national, and some international scales (

Table 2. Various standards regarding trace metal contamination in residential soils that pose a health risk to humans and are considered a need for remediation. The lowest concentration standard for each metal is indicated by bold font [38,39,40,41,42,43].

	Soil Metal Concentrations for Residential Gardening Soils (mg/kg)
Standard	V	Cr	Ni	Cu	Zn	As	Cd	Pb
1.  US EPA Clean-Up Required (US EPA)	550	390	1600	--	23,000	0.4	78	200 *
2.  Pennsylvania DEP (PA DEP)	1100	37	4400	7200	66,000	12	110	500
3.  New York State DEC (NY DEC)	--	22 **	140	270	2200	16	2.5	400
4.  New Jersey Remediation Standards (NJ)	390	--	1600	3100	23,000	19	71	200
5.  Canada (CCME)	130	64	50	63	200	12	10	140
6.  United Kingdom (UK)	--	130	130	--	--	37	22	200

* The US EPA is currently recommending a reduction in the residential screening level for lead in soils to 200 ppm (US EPA 2024) [44]. ** Hexavalent chromium. 1. Generic soil screening level for ingestion. United States Environmental Protection Agency. 2021. 2. Medium-specific concentrations (MSCs) for inorganic regulated substances in soil for residential soil. Direct contact numeric values. Pennsylvania Department of Environmental Protection. 2023. 3. Restricted use soil cleanup objectives for residential soil. Department of Environmental Conservation. State compilation of codes, rules, and regulations of the state of New York. 2023. 4. 2021 Adopted residential soil remediation standard for the ingestion–dermal exposure pathway. New Jersey Department of Environmental Protection. 2021. 5. Canadian Soil Quality Guidelines for residential/parkland soil. Canadian Soil Quality Guidelines for the Protection of Environmental and Human Health. Canadian Council of Ministers of the Environment. Update 7.0. September 2007. 6. Heavy metal guidelines in soil. Contaminated Land Exposure Assessment (CLEA) soil guideline value. UK. 2009.

).

Standards for the concentration of certain trace metals in soils used for gardening across the United States are not widely known nor standardized for use by the entire country, but certain metals have been focused on more often to create standards and recommendations that will not pose a serious threat to human health if the metals are found at or below that concentration. Measurements made in community garden soils in Pittsburgh were compared against national standards, state standards, and standards of Canada (Canadian Council of Ministers of the Environment (CCME)) and the United Kingdom (Table 2). Different guidelines were more stringent on certain metal concentrations, which proves valuable in creating a comprehensive comparison of the lowest perceived safe level for exposure to these trace metals. Canada and the Pennsylvania Department of Environmental Protection (PADEP) provide standards for all eight elements, but Canada has the most stringent standards for most elements, requiring the lowest concentrations for half of the trace metals (

Table 3. Average trace metal concentration measured by ICP-MS in Pittsburgh community gardens exceeding the most stringent standard in residential soils, as indicated by bold font. Location of these gardens is shown in Figure 1. Column headings are different available standards, which are also provided in Table 2.

Most Stringent Standard (mg/kg)	CCME	PA DEP	CCME	CCME	CCME	PA DEP & CCME	NY DEC	CCME
	130	37	50	63	200	12	2.5	140
Garden		Concentration (mg/kg)
	V	Cr	Ni	Cu	Zn	As	Cd	Pb
A	23.83	32.27 *	21.72	50.04	174.18	9.07	0.56	70.58
B	22.47 *	31.65 *	19.42 *	55.67 *	166.69 *	8.38 *	0.72 *	44.03 *
C	19.88 *	48.84 *	16.86 *	43.54 *	178.07 *	8.60	0.72 *	32.43 *
D	22.62 *	22.04 *	18.66 *	32.11	150.99	8.80	2.40	56.82
E	22.83	23.13	17.68	46.38 *	172.90	9.86	0.66	85.52
F	20.47	107.74	68.16	52.19	210.22	17.53	1.20	103.77
G	18.82	27.35	19.29	59.80 *	234.32	10.87	0.87 *	123.59
H	30.42	33.63 *	20.56	63.69	148.98 *	7.87 *	0.73 *	27.00 *
I	20.75 *	33.38	17.98 *	40.03 *	203.11	9.75	0.68 *	131.61

* Raised-bed soil.

). Since Canada is geographically close to the United States and has a similar industrial history, these standards can be applied in this study.

The concentrations of most trace metals in these nine gardens did not exceed several standards. In this analysis, the highest mean concentration of either the raised or ground samples was used for comparison, as it can be considered the best indicator of potential exposure risk at that garden. When compared to the most stringent guidelines (Table 2), Pb, Cd, and V levels did not show exceedances in any of the nine gardens (Table 3). Garden F showed exceedances for Cr, Ni, Zn, and As. Besides garden F, Cr exceeded in garden C, Cu in garden H, and Zn in gardens G and I (Table 3).

While only 3.7% of all samples exceeded the most stringent Pb standard (CCME: 140 ppm), under New Jersey guidelines, it is recommended that for concentrations over 100 mg/kg, such as in gardens F, G, and I, that best management practices are followed to minimize Pb exposure, such as applying phosphate fertilizer, maintaining a high pH for fruiting vegetables, and keeping soil mulched to minimize dust and Pb inhalation [41]. Nearly 15% of all soil samples exceeded this recommendation. In addition, as the sample size was limited, there is potential that other soil samples around the Pittsburgh area may have higher Pb concentrations. Therefore, it may be useful for future studies and analysis to interpolate where Pb concentrations may be elevated based on these samples and samples collected in a previous study [21].

Pb concentrations tend to be higher in the urban region of Pittsburgh (Figure 3). Overall, Pb concentrations were higher in the urban center of Pittsburgh, which are closer to past industrial usage and steel works (Figure 3). Garden F has higher Pb concentrations, likely due to the close vicinity to the glass industry, and this may have an influence on the surrounding areas. It is unclear how much area that the industry might cause higher concentrations in, but future studies can analyze more soil samples to confirm the impact of the historical sources near garden F. A study in Los Angeles, California, has found higher metal concentrations with proximity to road and higher lead with neighborhood age, suggesting the vehicle emission and lead paint sources, respectively [8].

3.4. Raised-Bed vs. Ground Samples

In all nine gardens, metal concentrations were not consistently higher in the ground or raised-bed samples (Table 1; Figure S2). However, in gardens A, F, and G, most elemental concentrations were greater in the ground samples than in the raised beds (Figure S2). In garden F, concentrations of Cr, Pb, Ni, and Cu were much greater in ground soil (Cr: 107.74 mg/kg; Pb: 103.78 mg/kg; Ni: 68.16 mg/kg; Cu: 52.19 mg/kg) than raised-bed soil (Cr: 28.82 mg/kg; Pb: 52.48 mg/kg; Ni: 21.78 mg/kg; Cu: 31.56 mg/kg). In comparison, gardens B, C, and H had the highest metal concentrations in raised-bed samples, but garden H have a much higher Cu concentration in the ground soil (63.69 mg/kg) than in raised-bed soil (35.56 mg/kg). There was a significant difference in elemental concentrations between raised-bed and ground samples for only Cr (p = 0.019; Table S6).

While there was no statistical significance in whether raised beds, on average, had different trace metal concentrations than the ground samples, some trends did occur. The difference between raised beds and ground soils may have been influenced by the factors specific to the garden sites, especially when considering the historical context of Pittsburgh and potential modern sources of trace metals. For example, in garden F, several metals have much higher average concentrations in the ground samples. Although garden F is located farther from the urban region of Pittsburgh, it is likely influenced by its proximity to the glass industry (Table S1). Previous studies analyzing Pittsburgh soils at random points throughout the region found that soils to the east were more contaminated with trace metals, likely due to a combination of wind patterns and geography in which the pollutants from industry may have been trapped and deposited in the river valleys [6]. Since raised-bed concentrations are much lower for several metals at garden F, this indicates that the use of raised beds at this garden, where the soil is likely to be changed frequently, may be effective at lowering trace metal concentrations in soil used to grow produce.

A study of community gardens in Philadelphia found higher As and Pb levels in ground samples compared to raised beds, and higher Zn, Cu, V, and Ni in raised beds compared to ground samples [21]. Soil samples collected from garden beds were more often (81%) below guidance values than non-bed samples (50%) in a study conducted in New York City [45]. Raised beds do not provide a long-term remedial solution, and continual monitoring of trace metals is still recommended, even for raised-bed soil samples [6]. A study of lead in urban gardens suggests that raised beds provide only a limited reduction in exposure and recommends the continual maintenance of raised beds to reduce lead exposure [46]. Raised beds can be a source of pollutants, such as arsenic, if they are made from creosote railroad tires or chromated copper arsenate (CAA)-treated timber [14].

In gardens B, C, and H, the higher concentrations in raised beds may also be indicative of continued atmospheric deposition of trace metals into the soils. All three of these gardens are located outside of the central peninsula of Pittsburgh, where steel mills and iron works were densely concentrated (Figure S1). Gardens B and H are in suburban areas with no local history of heavy industry, except coal mining near garden B (Table S1). Since they were outside the industrial zone, there were likely fewer trace metal deposits in the soil from steel operations and other smelters. The concentrations being higher in the raised beds likely indicates how modern emission sources, such as vehicle pollution, can affect raised-bed soils, in addition to the ground soils. Zn, Cu, and Pb are all associated with vehicular traffic, and their higher concentrations in raised beds in these three gardens are indicative that this modern source of trace metals is settling in the beds despite replacing the soil for each growing season. It is unknown where gardeners obtain their soil from, so the soil used in the raised beds may also be exposed to additional trace metals before use in gardening, which may cause higher concentrations than the ground soil around it. In addition, certain site-specific environmental conditions could have resulted in lower ground concentrations at these sites, as garden B implemented a type of plastic sheeting underneath their garden, and garden H appeared to have been built upon pre-existing concrete, which may limit the exposure to trace metals that have been historically present in this soil. In this study, it is unknown if the ground soil at each location is original topsoil or if it has been imported from other locations. Replacement of topsoil is a possibility, but existing contamination in the soil beneath any imported topsoil likely combines and contaminates new soil over time.

Gardens B, C, and H, regardless of raised bed or ground samples, also had lower mean Pb concentrations than all other gardens (Table 1; Figure S2). Previous studies have analyzed the impact of vehicle emissions on soil Pb levels, and it has been shown that Pb levels are elevated near high-traffic roads [47]. Most of the other gardens displayed higher Pb levels and are either located near highways or high-traffic city roads, further supporting the suggestion that traffic influences Pb levels. However, garden C is also located near a main highway in Pittsburgh and did not display this trend. It is recommended that if soil Pb levels are high in garden soil that gardeners implement raised beds that they can add clean soil and compost to each growing season, and gardens A, F, G, and I, on average, do display a decrease in Pb concentrations with the use of raised beds (Table 1; Figure S2) [40,45].

3.5. Correlation Analysis

In the ground samples, high correlation was observed between Ni and Cr (r = 0.99, p < 0.01), Cr and As (r = 0.71, p < 0.01), and Ni and As (r = 0.7, p < 0.001) (Figure 4). Pb was correlated with As (r = 0.44, p = 0.02) and Zn (r = 0.53, p = 0.004). In the raised beds, the highest correlations were observed between V and Ni (r = 0.76, p < 0.001) and Zn and Cu (r = 0.66, p < 0.001) (Figure S3).

High levels of correlation between metals may indicate common sources. The correlation between Ni and Cr in the ground samples indicates that they likely came from similar anthropogenic sources or lithological origins. Cr pollution is associated with steel and fossil fuel combustion, and Ni is associated with similar industries of smelting and mining, refining, and petroleum industries [48,49]. Previous studies found elevated Cr and Ni concentrations in surface soil due to their common contamination sources, both in agricultural and industrial settings [50]. In addition, both Cr and Ni are associated with glass and ceramic making, which is relevant when looking at the elevated concentrations at garden F due to the presence of a glass manufacturing industry [49,51]. The high correlation of Ni and Cr in the ground samples highlights the historical anthropogenic sources. Both Cr and Ni were correlated with As concentrations, and As pollution is associated with mining, smelting, coal combustion, and pesticide application [52]. Arsenic has similar emission sources as Ni and Cr, they likely had similar sources of anthropogenic emissions near these sampled gardens, especially in the context of Pittsburgh, where smelting, mining, and fossil fuel combustion are a major part of its history and development. In addition, As is also used as a fining agent in glassmaking, relating to the higher ground concentrations in garden F, which is potentially caused by proximity to the glass industry [53].

In the raised beds, high correlations between V and Ni, as well as Zn and Cu, indicate that different modern sources may be contaminating these soils. Cu and Zn are both associated with vehicular traffic, as Zn is related to tires and lubricant oils [54] and Cu contamination of topsoil is associated with particles from car brakes [55]. This correlation highlights how modern vehicular emissions may influence raised-bed concentrations of certain trace metals. Correlation between heavy metals is used to test their origin from similar sources or interdependency [56]. Lead is only associated with Zn and As in ground soil (Figure 4). Leaded gasoline and leaded paints remain the major sources of lead pollution, although both are banned in the United States [8]. Metals such as lead and zinc are also related to the smelters [57,58]. The use of chromated copper arsenate-treated wood and air deposition can increase As content in soil [8]. Heavy metals remain one of the major land contamination issues. In China, 80% of contaminated lands were due to heavy metals such as cadmium, mercury, arsenic, lead, and nickel [59].

3.6. Trace Metal Enrichment

Enrichment factor (EF) was calculated with rubidium as a reference element to determine likely additional anthropogenic influence of trace metal concentrations in the community garden soils when compared to crustal concentrations, where values above 1.5 indicate anthropogenic influence [24]. EF values were greater than 1.5 for Pb, Zn, Cu, Cd, and As in all ground and raised-bed samples, suggesting additional anthropogenic sources for these elements (Figure 5 and Figure S4). Enrichment factor analysis showed that three gardens (D, E, F, G, and I) stand out for anthropogenic pollution. Garden D had a high anthropogenic input to Cd concentrations in the ground sample, and garden I had a high anthropogenic input to Pb concentrations in the ground sample. In both the raised-bed and ground samples, Cu was still above the EF threshold of 1.5, but it indicates a lower contribution from anthropogenic sources compared to other trace metals. Gardens D, F, G, and I had EF values exceeding 25 for several trace metals, indicating severe enrichment and pollution by anthropogenic sources. EF values between 25–50 indicate severe enrichment [60]. EF values help to determine metals originating from anthropogenic activities [8,60,61]. Highest EF values were found for Cd in garden D and Pb in gardens F, G, and I. This indicates anthropogenic sources such as fossil fuel combustion, wear and tear of tires, leaded paint, and industrial sources [8,14,45,46,61,62].

Few metals (V, Ni, Cr) did not consistently exceed the EF threshold, indicating variance in anthropogenic contributions to the soil concentrations. According to the EF threshold of 1.5, V is considered to be at natural levels for all gardens. Ni exceeded this 1.5 threshold in gardens F, G, and I, and Cr exceeded this threshold in gardens F and G. For Ni, the anthropogenic influence in gardens F, G, and I can be attributed to manufacturing, coke ovens, iron works, and mining (Table S1) [63]. Garden F’s anthropogenic influence can largely be attributed to the glass industry local to the area, as Ni is often used in glassmaking [51]. Cr is also a common metal used in glassmaking, which can explain the high anthropogenic influence of Cr at garden F as well [51]. Furthermore, Ni and Cr concentrations are highly correlated in the ground samples, suggesting that they may be coming from a similar source (Figure 5). In addition, this correlation between Ni and Cr concentrations is evident with garden G also exceeding the EF threshold for these metals, potentially due to the coke oven and iron works in this location.

4. Conclusions

In this study, we measured and analyzed concentrations of eight trace metals and metalloids (As, Cd, Cr, Cu, Ni, V, Pb, Zn) from nine Pittsburgh-area community gardens. All nine gardens did not have Pb levels exceeding the recommended standard of 140 mg/kg. Only one garden exceeded the most stringent standard for Cr, Ni, Zn, and As. Concentrations of most metals were higher in the ground samples near the urban center of Pittsburgh.

With trace metal pollution becoming a growing concern in the use of community gardens, especially in urban areas, more soil testing should be implemented at the establishment of a new garden. Early soil testing can help guide garden choices regarding the use of raised beds. Continuous monitoring is important as well because it can recognize how modern sources may be polluting the soil and what precautions are required to reduce human exposure. Routine monitoring of trace elements in soils with a history of industrial legacy is recommended. The XRF method can make soil testing for community gardens more accessible and provide immediate results, as it has been shown to be a reliable method compared to the ICP-MS method. Collaboration between local universities and community gardens can be made to routinely test trace metal contamination in the community gardens.

Safe practices should always be followed, especially when trace metal concentrations are at any level of concern. Safe practices to avoid exposure and ingestion of heavy metals in soil include growing in raised beds filled with clean soil and compost, washing produce thoroughly, and being careful not to track soil indoors [64]. When very high levels of trace elements are found, appropriate remediation procedures should be conducted to reduce exposure. Collaborations between researchers, community organizations, and stakeholders can help to assess the level of soil contamination, to check compliance with the guidelines, to obtain funding for remediation efforts, and to educate urban gardeners about the soil contaminants.