Lead (Pb) Bioaccessibility and Mobility Assessment of Urban Soils and Composts: Fingerprinting Sources and Refining Risks to Support Urban Agriculture

Municipal Compost Public Health, Waste Management, and Urban Agriculture: A Decadal Study of Fugitive Pb in City of Boston, Massachusetts, USA

Compostable materials constitute roughly half of waste generated globally, but only 5% of waste is actually processed through composting, suggesting that expanding compost programs may be an effective way to process waste. Compostable waste, if properly collected and processed, has value-added end use options including: residential and park landscaping, remediation of brownfield sites, and as growing media in urban agriculture (UA). Since 2001, our lab has partnered with The Food Project, a non-profit focused on youth leadership development through urban farming. From 2006 to 2022 we collected compost materials that were delivered to the farm from a variety of local sources and analyzed a suite of biogeochemical properties including lead (Pb) concentrations, organic carbon, and grain size distribution. Pb concentrations of Boston's municipal compost always exceeded the current City of San Francisco soil and compost purchase standard (80 μg/g). In 2012 Boston's composting program was halted when it exceeded the 400 μg/g Environmental Protection Agency's Pb in soil benchmark. Urban Pb is geomobile and must be managed to minimize resuspension and transport of fines whose Pb concentration is often elevated compared to bulk compost. Consequently, urban farmers have to source lower Pb compost from suburban suppliers at significantly greater cost. Over a 15 year period and through several city vendor contracts, Pb concentrations in municipal compost remain at levels that warrant continued surveillance and risk assessment.

Complexities in attributing lead contamination to specific sources in an industrial area of Philadelphia, PA

Globally, lead (Pb) contamination is one of the top ten chemical exposure issues affecting public health. The identification of specific Pb sources provides valuable information to determine assignment of liability for site cleanup, improve sampling plans and develop remedial strategies. This paper examines Pb concentrations and Pb isotopic data from samples collected at and near the site of a Pb paint production facility with a long operating history. Although high soil Pb concentrations were found at the site, Pb concentrations in surrounding neighborhoods did not simply decline with distance from the site. We evaluated soil concentrations and isotopic mixing lines to explore potential sources of Pb pollution. Three-isotope plots showed overlap of site samples and the surrounding neighborhood, consistent with pollution from the facility affecting offsite soils. A major challenge in separation of potential sources, however, is that the isotopic signatures of other potential Pb sources fall within the range of the soil data. The long operational site history, soil disturbances, the presence of nearby smelters, and other local and remote sources affect identification of lead sources. This analysis demonstrates that source attribution can be confounded by incomplete site and material sourcing information. An integrated approach that includes in-depth site characterization and an evaluation of historical activities (e.g., Pb ores used over time, amounts of Pb emitted by all area smelters, land use changes, and soil disturbances) is important for determining source attribution. This analysis provides insight into future site investigations where soil lead contamination has resulted from a long industrial history in an urban setting.

Sources, pathways and concentrations of potentially toxic trace metals in home environments

Despite ongoing concerns about trace metal and metalloid (trace metals) exposure risks from indoor dust, there has been limited research examining their sources and relationship to outdoor soils. Here we determine the concentrations and sources for potentially toxic trace metals arsenic (As), chromium (Cr), copper (Cu), manganese (Mn), lead (Pb) and zinc (Zn) and their pathways into homes in Sydney, Australia, using home-matched indoor dust (n = 166), garden soil (n = 166), and road dust samples (n = 51). All trace metals were more elevated indoors versus their matched garden soil counterparts. Indoor Cu and Zn dust concentrations were significantly more enriched than outdoor dusts and soils, indicating indoor sources were more relevant for these elements. By contrast, even though Pb was elevated in indoor dust, garden soil concentrations were correspondingly high, indicating that it remains an important source and pathway for indoor contamination. Elevated concentrations of As, Pb and Zn in garden soil and indoor dust were associated with home age (>50 years), construction materials, recent renovations and deteriorating interior paint. Significant correlations (p < 0.05) between road dust and garden soil Cu concentrations, and those of As and Zn in soil and indoor dust, and Pb across all three media suggest common sources. Scanning electron microscopy (SEM) analysis of indoor dust samples (n = 6) showed that 57% of particles were derived from outdoor sources. Lead isotopic compositions of soil (n = 21) and indoor dust (n = 21) were moderately correlated, confirming the relevance of outdoor contaminants to indoor environments. This study illustrates the source, relationship and fate of trace metals between outdoor and indoor environments. The findings provide insight into understanding and responding to potentially toxic trace metal exposures in the home environment.

Can the mouse model successfully predict mixed metal(loid)s bioavailability in humans from contaminated soils?

Mouse models have been employed by many scientific research groups worldwide to predict the bioavailability of metal (loid)s and other chemicals in humans. Their suitability for predicting mixed metal (loid) bioavailability has been questioned and debated for decades by many research teams. In this study soils contaminated by lead (Pb) and arsenic (As), either in the field or by spiking in the laboratory, were used in bioavailability and bioaccessibility tests. The spiked soils were aged for more than a year prior to testing to achieve steady state and eliminate soil ageing effects, as reported in previous research. The bioavailability of, firstly, Pb in the presence of As and secondly, As in the presence of Pb was determined using mice. Furthermore, bioaccessibility was determined using a range of in vitro methods: relative bioaccessibility leaching procedure (RBALP), the Unified Bioaccessibility Research Group Europe (BARGE) method (UBM) gastric and intestinal phases, and the National Institute for Public Health and the Environment (RIVM) gastric and intestinal phases. The correlations between Pb and As bioavailability and their in vitro bioaccessibility when they were present in mixtures were analysed. The results indicated that the bioavailability of Pb in mice kidney tissues significantly correlated with bioaccessibility of Pb in RBALP (p < 0.01), UBM gastric (p < 0.01) and intestinal phases (p < 0.01) and RIVM gastric phases when Pb is present in metal (loid) mixtures. Results of the current study reveal that the RBALP, and UBM gastric and intestinal phase were by far the best methods for predicting the RB of Pb when it is present in metal (loid) mixtures. Consequently, the mouse model can successfully explain the in vivo in vitro correlation (IVIVC) of Pb when it is present in metal (loid) mixtures. However, we did find that a mouse model may not be the best one to explain the IVIVC of As when it is present in metal (loid) mixtures.

Metal and Pb isotope characterization of particulates encountered by foraging honeybees in Metro Vancouver

Honeybees and their products are useful biomonitors of metal distribution in urban centres. This study investigates particulate sources that foraging honeybees encounter in Metro Vancouver. Metal concentrations and lead (Pb) isotope compositions were measured in topsoil (top 2 cm, n = 14) colocated with existing research hives and in particulate matter ≤10 μm (PM₁₀, n = 27) collected throughout Metro Vancouver (British Columbia, Canada) during honeybee foraging hours over the course of one year (2018-2019). Topsoil served as a proxy for resuspended/coarse PM and, together with PM₁₀, covered the size range of particulates collected by foraging bees both actively (pollen) and passively (dusts). Particulate matter ≤ 2.5 μm (PM_2.5, n = 7) was collected on Whistler Mountain during two transpacific events (in spring 2014) to estimate the possible effect of transpacific particulate input on the Pb isotope composition of Western Canada aerosols. Metal concentrations and Pb isotopes in topsoil and PM from this study and bees and hive products from previous studies (collected in 2014-2019) reveal similar spatial trends: there were elevated amounts of some metals associated with anthropogenic activity (e.g., Pb, Zn, Sb) and less radiogenic Pb isotope compositions in most samples collected nearer to the city centre in comparison to samples collected in more suburban or rural areas. Bees and hive products have a smoothing effect on the spatiotemporal variability of the data; metal concentrations and Pb isotope compositions vary less in hive products than in PM, presumably because bees interact with multiple environmental domains while foraging. Wildfire smoke and transpacific input are phenomena that cause measurable shifts in Pb isotope compositions of PM, but not in hive matrices. The findings highlight important considerations to make (i.e., the smoothing effect) when linking public health data and decisions with environmental data from hive products in urban centres.

Arsenic and lead in soil: impacts on element mobility and bioaccessibility

Long-term brown coal mining contributes to risk element contents in soils surrounding coal basins. However, there is a lack of bioaccessibility characterization of the risk elements in the soils at the impacted locations for estimation of the potential health risk, in relation to the effects of soil particle size and element origin. In this study, soils from different geological areas (geogenic vs. anthropogenic) were sampled around the Most brown coal basin, Czech Republic. These soils were passed through sieves to obtain seven aggregate size fractions. For an estimation of the oral bioaccessibility of As and Pb in the size fractions, the physiologically based extraction test was applied, whereas the potential pulmonary bioaccessibility of the elements was estimated by using both Gamble's and Hatch's tests. The results showed that the geochemical pattern of the investigated elements clearly separates the soil samples collected from the mountain region (mineralization from geogenic processes) from those of the basin region (extensive coal mining). For As, the results indicated that it poses higher risks in the anthropogenically affected basin region due to its higher gastro-intestinal and pulmonary bioaccessibility in soil samples in this area. A higher bioaccessibility of As in the soils was recorded in the finer grain size fractions, which are usually air-borne and can be easily ingested and/or inhaled, leading to potential health risks to humans and livestock. The opposite pattern, with a higher content on coarse particles, was recorded for Pb, indicating a potential risk of livestock in the non-forest mountainous areas.

Legacy of anthropogenic lead in urban soils: Co-occurrence with metal(loids) and fallout radionuclides, isotopic fingerprinting, and in vitro bioaccessibility

Anthropogenic lead (Pb) in soils poses risks to human health, particularly to the neuropsychological development of exposed children. Delineating the sources and potential bioavailability of soil Pb, as well as its relationship with other contaminants is critical in mitigating potential human exposure. Here, we present an integrative geochemical analysis of total elemental concentrations, radionuclides of ¹³⁷Cs and ²¹⁰Pb, Pb isotopic compositions, and in vitro bioaccessibility of Pb in surface soils sampled from different locations near Durham, North Carolina. Elevated Pb (>400 mg/kg) was commonly observed in soils from urban areas (i.e., near residential house foundation and along urban streets), which co-occurred with other potentially toxic metal(loids) such as Zn, Cd, and Sb. In contrast, soils from city parks and suburban areas had systematically lower concentrations of metal(loids) that were comparable to geological background. The activities of ¹³⁷Cs and excess ²¹⁰Pb, coupled with their correlations with Pb and co-occurring metal(loids) were used to indicate the persistence and remobilization of historical atmospherically deposited contaminants. Coupled with total Pb concentrations, the soil Pb isotopic compositions further indicated that house foundation soils had significant input of legacy lead-based paint (mean = 1.1895 and 2.0618 for ²⁰⁶Pb/²⁰⁷Pb and ²⁰⁸Pb/²⁰⁶Pb, respectively), whereas urban streetside soils exhibited a clear mixed origin, dominantly of legacy leaded gasoline (1.2034 and 2.0416) and atmospheric deposition (1.2004-1.2055 and 2.0484-2.0525). The in vitro bioaccessibility of Pb in contaminated urban soils furthermore revealed that more than half of Pb in the contaminated soils was potentially bioavailable, whose Pb isotope ratios were identical to that of bulk soils, demonstrating the utility of using Pb isotopes for tracking human exposure to anthropogenic Pb in soils and house dust. Overall, this study demonstrated a holistic assessment for comprehensively understanding anthropogenic Pb in urban soils, including its co-occurrence with other toxic contaminants, dominant sources, and potential bioavailability upon human exposure.

Soil Lead Concentration and Speciation in Community Farms of Newark, New Jersey, USA

Farmed urban soils often bear legacies of historic contamination from anthropogenic and industrial sources. Soils from seven community farms in Newark, New Jersey (NJ), USA, were analyzed to determine the concentration and speciation of lead (Pb) depending on garden location and cultivation status. Samples were evaluated using single-step 1 M nitric acid (HNO3) and Tessier sequential extractions in combination with X-ray absorption fine structure spectroscopy (XAFS) analysis. Single-step extractable Pb concentration ranged from 22 to 830 mg kg−1, with 21% of samples reporting concentrations of Pb > 400 mg kg−1, which is the NJ Department of Environmental Protection (NJDEP) limit for residential soils. Sequential extractions indicated lowest Pb concentrations in the exchangeable fraction (0–211 mg kg−1), with highest concentrations (0–3002 mg kg−1) in the oxidizable and reducible fractions. For samples with Pb > 400 mg kg−1, Pb distribution was mostly uniform in particle size fractions of <0.125–1 mm, with slightly higher Pb concentrations in the <0.125 mm fraction. XAFS analysis confirmed that Pb was predominantly associated with pyromorphite, iron–manganese oxides and organic matter. Overall results showed that lowest concentrations of Pb are detected in raised beds, whereas uncultivated native soil and parking lot samples had highest values of Pb. As most of the Pb is associated with reducible and oxidizable soil fractions, there is a lower risk of mobility and bioavailability. However, Pb exposure through ingestion and inhalation pathways is still of concern when directly handling the soil. With increasing interest in urban farming in cities across the USA, this study highlights the need for awareness of soil contaminants and the utility of coupled macroscopic and molecular-scale geochemical techniques to understand the distribution and speciation of soil Pb.

Coupling Square Wave Anodic Stripping Voltammetry with Support Vector Regression to Detect the Concentration of Lead in Soil under the Interference of Copper Accurately

In this study, an effective method for accurately detecting Pb(II) concentration was developed by coupling square wave anodic stripping voltammetry (SWASV) with support vector regression (SVR) based on a bismuth-film modified electrode. The interference of different Cu²⁺ contents on the SWASV signals of Pb²⁺ was investigated, and a nonlinear relationship between Pb²⁺ concentration and the peak currents of Pb²⁺ and Cu²⁺ was determined. Thus, an SVR model with two inputs (i.e., peak currents of Pb²⁺ and Cu²⁺) and one output (i.e., Pb²⁺ concentration) was trained to quantify the above nonlinear relationship. The SWASV measurement conditions and the SVR parameters were optimized. In addition, the SVR mode, multiple linear regression model, and direct calibration mode were compared to verify the detection performance by using the determination coefficient (R²) and root-mean-square error (RMSE). Results showed that the SVR model with R² and RMSE of the test dataset of 0.9942 and 1.1204 μg/L, respectively, had better detection accuracy than other models. Lastly, real soil samples were applied to validate the practicality and accuracy of the developed method for the detection of Pb²⁺ with approximately equal detection results to the atomic absorption spectroscopy method and a satisfactory average recovery rate of 98.70%. This paper provided a new method for accurately detecting the concentration of heavy metals (HMs) under the interference of non-target HMs for environmental monitoring.

Evaluating the utility of elemental measurements obtained from factory-calibrated field-portable X-Ray fluorescence units for aquatic sediments

We assessed factory-calibrated field-portable X-ray fluorescence (pXRF) data quality for use with minimally-prepared aquatic sediments, including the precision of replicate pXRF measurements, accuracy of factory-calibrated pXRF values as compared to total digestion/ICP-OES concentrations, and comparability of calibrated pXRF values to extractable concentrations. Data quality levels for precision, accuracy, and comparability were not equivalent for element/analyzer combinations. All analyses of elements that were assessed for precision and accuracy on a single analyzer were both precise (<10% relative standard deviation) and accurate (r² > 0.85) for K, Ca, Ti, Mn, Fe, and Zn. Calibrated pXRF values for Al, K, Ca, Ti, Mn, Fe, Cu, Zn, and Pb were within ∼10% relative difference of total digestion/ICP-OES concentrations. Calibrated pXRF values for Fe, Cu, Zn, As, and Pb were within ∼20% relative difference of extractable concentrations. Some elements had a higher level of data quality using specific analyzers, but in general, no pXRF analyzer had the highest level of data quality in all categories. Collectively, our data indicate that a wide range of factory-calibrated pXRF units are capable of providing high-quality total concentrations for the analysis of aquatic sediments.

VegeSafe: a community science program generating a national residential garden soil metal(loid) database

VegeSafe is a national community science initiative aimed at characterising soils in Australian residential gardens and community gardens. The program has been operating for over 5 years and has generated soil metal(loid) data from over 8600 residential garden and community garden soil samples, submitted by almost 2000 community scientists. The VegeSafe program represents the largest archive of soil metal(loid) data and associated metadata for residential garden soils in Australia. Samples were collected across Australia, with 61% of samples collected from NSW (n = 5284), Victoria (VIC) 20% (n = 1688) of samples and Queensland (QLD) 7% (n = 592) of samples. Soil metal(loid) data obtained by analysis of bulk soil samples by portable X-ray florescence spectrometry (pXRF) for As, Cu, Cr Mn, Pb and Zn showed spatial patterns of greater soil metal(loid) concentrations around city areas, particularly in NSW and VIC. The Australian Health Investigation Levels for low-density residential land uses (HIL-A) were used in this study as guideline values for soil metal(loid) concentrations. Overall, there was a relatively small number of HIL-A exceedances in the dataset, with most metal(loid)s exceeding their HIL-A concentration in < 5% of incidences. The notable exception to this was for Pb, which had HIL-A (300 mg/kg) exceeded in 27% (n = 1427) of samples in NSW, 17% (n = 280) in VIC and 10% (n = 61) in QLD. Through the power of community engagement and community science, the VegeSafe program presents an unprecedented insight into soil metal(loid) concentrations in Australian residential gardens.