Effectiveness of washing in reducing lead concentrations of lettuce grown in urban garden soils.

Urban gardeners contribute to sustainable cities and often take great care to limit exposure to soil contaminants like lead (Pb). Although best management practices (BMPs) like mulching to reduce soil splash can limit crop contamination, they may not eliminate all contamination for leafy greens, which trap soil particles. How effective is washing at removing Pb contamination from leafy greens when using BMPs? Are certain washing techniques more effective than others? We present results from two experiments addressing these questions. We grew lettuce (Lactuca sativa L.) in homogenized high-Pb (∼1,150 mg kg-1 ) and low-Pb (∼90 mg kg-1 ) soils in Brooklyn, NY, and Ithaca, NY. Our results show that washing can remove 75-94% of Pb from lettuce, including that remaining after the use of contamination-reducing BMPs. It was estimated that washing removed 97% of Pb deposited by splash, which is the dominant source of Pb, and removed 91% deposited by downward deposition. All washing techniques were effective at reducing Pb levels, with differences in effectiveness ranked as: commercial soak > vinegar soak > water soak (and water rinse not significantly different from vinegar or water soak). Washing crops grown in low-Pb soils is also important. Without washing, lettuce grown in low-Pb soil may still have Pb levels above the European Commission comparison value. We offer these empirical findings and recommendations in support of urban growers.

Soil lead (Pb) and urban grown lettuce: Sources, processes, and implications for gardener best management practices.

Urban community gardeners employ a range of best practices that limit crop contamination by toxicants like lead (Pb). While Pb root uptake is generally low, the relative significance of various Pb deposition processes and the effectiveness of best practices in reducing these processes have not been sufficiently characterized. This study compared leafy lettuce (Lactuca sativa) grown in high Pb (1150 mg/kg) and low Pb (90 mg/kg) soils, under three different soil cover conditions: 1) bare soil, 2) mulch cover to limit splash, and 3) mulch cover under hoophouses to limit splash and air deposition, in a New York City (NYC) community garden and a rural site in Ithaca, New York (NY). The lettuces were further compared to greenhouse (Ithaca) and supermarket (NYC) samples. Atmospheric deposition was monitored by passive trap collection through funnel samplers. Results show that in low Pb soils, splash and atmospheric deposition accounted for 84 and 78% of lettuce Pb in NYC and Ithaca, respectively. In high Pb soils, splash and atmospheric deposition accounted for 88 and 93% of Pb on lettuces, with splash being the dominant mechanism. Soil covers were shown to be effective at significantly (p < 0.05) reducing lettuce Pb contamination, and mulching is strongly recommended as a best practice.

The heavy metal budget of an urban rooftop farm.

Urban rooftop agriculture is a growing enterprise in the US with the goal of providing high quality, healthy, locally grown produce for city dwellers. However, air pollution abatement and the purification of stormwater are among the ecosystem services emphasized in studies of conventional green roofs. If rooftop farms actually capture pollutants, then accumulation of heavy metals in the soil could pose a problem over time. This study reports the heavy metal concentrations in soil, atmospheric deposition, and drainage output of 8 metals from the Brooklyn Grange Navy Yard Farm, rooftop vegetable farm in New York City, USA. Drainage of Pb and Mn were 6% and 14% of atmospheric bulk deposition, respectively, meaning that the Grange could be a net sink for Pb and Mn. Although there were small scale hotspots in the soil, farm-wide averages for heavy metal concentrations never exceeded guideline levels, and relatively low concentrations of Pb and Ba in the soil suggest that rooftop soils may be less vulnerable to contamination related to traffic and construction. In comparison to the growing seasons, we found relatively high concentrations of Pb and Cr in the soil during fallow periods when the soil was bare. To reduce the atmospheric deposition of heavy metals to soil, it is important to cover the soil with mulch, and discard the used mulch and unmarketable portion of vegetables, instead of recycling them via composting for soil amendments.

Estimated lead (Pb) exposures for a population of urban community gardeners.

Urban community gardens provide affordable, locally grown, healthy foods and many other benefits. However, urban garden soils can contain lead (Pb) that may pose risks to human health. To help evaluate these risks, we measured Pb concentrations in soil, vegetables, and chicken eggs from New York City community gardens, and we asked gardeners about vegetable consumption and time spent in the garden. We then estimated Pb intakes deterministically and probabilistically for adult gardeners, children who spend time in the garden, and adult (non-gardener) household members. Most central tendency Pb intakes were below provisional total tolerable intake (PTTI) levels. High contact intakes generally exceeded PTTIs. Probabilistic estimates showed approximately 40 % of children and 10 % of gardeners exceeding PTTIs. Children's exposure came primarily from dust ingestion and exposure to higher Pb soil between beds. Gardeners' Pb intakes were comparable to children's (in µg/day) but were dominated by vegetable consumption. Adult household members ate less garden-grown produce than gardeners and had the lowest Pb intakes. Our results suggest that healthy gardening practices to reduce Pb exposure in urban community gardens should focus on encouraging cultivation of lower Pb vegetables (i.e., fruits) for adult gardeners and on covering higher Pb non-bed soils accessible to young children. However, the common practice of replacement of root-zone bed soil with clean soil (e.g., in raised beds) has many benefits and should also continue to be encouraged.

Arsenic and Lead Uptake by Vegetable Crops Grown on an Old Orchard Site Amended with Compost.

The potential for lead (Pb) and arsenic (As) transfer into vegetables was studied on old orchard land contaminated by lead arsenate pesticides. Root (carrot), leafy (lettuce), and vegetable fruits (green bean, tomato) were grown on seven "miniplots" with soil concentrations ranging from near background to ≈ 800 and ≈ 200 mg kg-1 of total Pb and As, respectively. Each miniplot was divided into sub-plots and amended with 0% (control), 5% and 10% (by weight) compost and cropped for 3 years. Edible portions of each vegetable were analyzed for total Pb and As to test the effect of organic matter on transfer of these toxic elements into the crop. Vegetable Pb and As concentrations were strongly correlated to soil total Pb and As, respectively, but not to soil organic matter content or compost addition level. For Pb vegetable concentrations, carrot ≥ lettuce > bean > tomato. For As, lettuce > carrot > bean > tomato. A complementary single-year study of lettuce, arugula, spinach, and collards revealed a beneficial effect of compost in reducing both Pb and As concentrations in leafy vegetables. Comparisons of all measured vegetable concentrations to international health-based standards indicate that tomatoes can be grown without exceeding standards even in substantially Pb- and As-contaminated soils, but carrots and leafy greens may exceed standards when grown in soils with more than 100-200 mg kg-1 Pb. Leafy greens may also exceed health-based standards in gardens where soil As is elevated, with arugula having a particularly strong tendency to accumulate As.

Concentrations of lead, cadmium and barium in urban garden-grown vegetables: the impact of soil variables.

Paired vegetable/soil samples from New York City and Buffalo, NY, gardens were analyzed for lead (Pb), cadmium (Cd) and barium (Ba). Vegetable aluminum (Al) was measured to assess soil adherence. Soil and vegetable metal concentrations did not correlate; vegetable concentrations varied by crop type. Pb was below health-based guidance values (EU standards) in virtually all fruits. 47% of root crops and 9% of leafy greens exceeded guidance values; over half the vegetables exceeded the 95th percentile of market-basket concentrations for Pb. Vegetable Pb correlated with Al; soil particle adherence/incorporation was more important than Pb uptake via roots. Cd was similar to market-basket concentrations and below guidance values in nearly all samples. Vegetable Ba was much higher than Pb or Cd, although soil Ba was lower than soil Pb. The poor relationship between vegetable and soil metal concentrations is attributable to particulate contamination of vegetables and soil characteristics that influence phytoavailability.

Lead (Pb) and other metals in New York City community garden soils: factors influencing contaminant distributions.

Urban gardens provide affordable fresh produce to communities with limited access to healthy food but may also increase exposure to lead (Pb) and other soil contaminants. Metals analysis of 564 soil samples from 54 New York City (NYC) community gardens found at least one sample exceeding health-based guidance values in 70% of gardens. However, most samples (78%) did not exceed guidance values, and medians were generally below those reported in NYC soil and other urban gardening studies. Barium (Ba) and Pb most frequently exceeded guidance values and along with cadmium (Cd) were strongly correlated with zinc (Zn), a commonly measured nutrient. Principal component analysis suggested that contaminants varied independently from organic matter and geogenic metals. Contaminants were associated with visible debris and a lack of raised beds; management practices (e.g., importing uncontaminated soil) have likely reduced metals concentrations. Continued exposure reduction efforts would benefit communities already burdened by environmental exposures.

Impact of microbial diversity on rapid detection of enterohemorrhagic Escherichia coli in surface waters.

Enterohemorrhagic Escherichia coli (EHEC) are a physiologically, immunologically and genetically diverse collection of strains that pose a serious water-borne threat to human health. Consequently, immunological and PCR assays have been developed for the rapid, sensitive detection of presumptive EHEC. However, the ability of these assays to consistently detect presumptive EHEC while excluding closely related non-EHEC strains has not been documented. We conducted a 30-month monitoring study of a major metropolitan watershed. Surface water samples were analyzed using an immunological assay for E. coli O157 (the predominant strain worldwide) and a multiplex PCR assay for the virulence genes stx(1), stx(2) and eae. The mean frequency of water samples positive for the presence of E. coli O157, stx(1) or stx(2) genes, or the eae gene was 50%, 26% and 96%, respectively. Quantitative analysis of selected enriched water samples indicated that even in samples positive for E. coli O157 cells, stx(1)/stx(2) genes, and the eae gene, the concentrations were rarely comparable. Seventeen E. coli O157 strains were isolated, however, none were EHEC. These data indicate the presence of multiple strains similar to EHEC but less pathogenic. These findings have important ramifications for the rapid detection of presumptive EHEC; namely, that current immunological or PCR assays cannot reliably identify water-borne EHEC strains.

tir- and stx-positive Escherichia coli in stream waters in a metropolitan area.

Diarrheagenic Escherichia coli, which may include the enteropathogenic E. coli and the enterohemorrhagic E. coli, are a significant cause of diarrheal disease among infants and children in both developing and developed areas. Disease outbreaks related to freshwater exposure have been documented, but the presence of these organisms in the urban aquatic environment is not well characterized. From April 2002 through April 2004 we conducted weekly surveys of streams in the metropolitan Baltimore, Md., area for the prevalence of potentially pathogenic E. coli by using PCR assays targeting the tir and stx(1) and stx(2) genes. Coliforms testing positive for the presence of the tir gene were cultured from 653 of 1,218 samples (53%), with a greater prevalence associated with urban, polluted streams than in suburban and forested watershed streams. Polluted urban streams were also more likely to test positive for the presence of one of the stx genes. Sequence analysis of the tir amplicon, as well as the entire tir gene from three isolates, indicated that the pathogenic E. coli present in the stream waters has a high degree of sequence homology with the E. coli O157:H7 serotype. Our data indicate that pathogenic E. coli are continually deposited into a variety of stream habitats and suggest that this organism may be a permanent member of the gastrointestinal microflora of humans and animals in the metropolitan Baltimore area.