Remediating Urban Soils: How biosolids can reduce exposure to toxic lead

 The element lead (Pb) caught media attention in January 2021. With headlines on politics and covid screaming, missing the news coming out of Flint, Michigan, would not have been too surprising. But the event should have easily caught the attention of the those of us who have worked in the public water sector. The Zoom-based court line-up of elected and appointed officials, including an ex-governor, charged with causing exposure of lead to children in Flint, Michigan, was eye-popping.  The NBC News headline read: “Ex-governor, 8 other former Michigan officials charged in Flint water crisis: Charges stem from ‘the largest criminal investigation in the history of the state of Michigan,’ the attorney general says.”  Public infrastructure, improperly managed or intentionally mismanaged by government leaders, can harm lives and can earn no-joke jail time.

Childhood exposure to the element lead is arguably a public health and environmental issue of the greatest consequence facing public managers. Exposures occur in multiple media -- air, water, and soil media – thereby crossing regulatory domains.  Emissions arise from legacy sources that are invisible more than releases from current activities. Lead exposure is old news, not the current hot button, as it is an issue that has afflicted civilization literally for millennia.  Financial and media resources currently drawn to PFAS might be better focused on lead if the goal is meaningful benefits to public health.

How big a problem is childhood lead exposure? Despite long-standing programs, such as abatement of lead paint in old homes and ending use of leaded gasoline, children in impoverished communities are harmed even today by lead, and the impact on their lives will be felt decades into the future. An article like Association of Childhood Blood Lead Levels With Cognitive Function and Socioeconomic Status at Age 38 Years and With IQ Change and Socioeconomic Mobility Between Childhood and Adulthood casts lead toxicity in an urgent light for its long-term effects. Even relatively low levels of exposure are concerning. This point is made in Low-Level Environmental Lead Exposure and Children’s Intellectual Function: An International Pooled Analysis: “We conclude that environmental lead exposure in children who have maximal blood lead levels < 7.5 μg/dL is associated with intellectual deficits…. childhood lead exposure was associated with lower cognitive function and socioeconomic status at age 38 years and with declines in IQ and with downward social mobility. Childhood lead exposure may have long-term ramifications.” 

Despite the attention to Flint, drinking water is not the major source of childhood exposure to lead. The article Children’s lead exposure: Relative contributions of various sources  points to dust and soil as the large risk: “the Flint water crisis, first recognized in 2015, continue to be regularly covered by the press … [but] lead contamination of soil has not received even a fraction of this consideration.” Urban soils are a large source of unregulated childhood exposure to lead. Lead concentrations in inner-city soils as a factor in the child lead problem bluntly concludes “Our findings pose environmental and public health issues, especially to children living within the inner-city.”

Wherever scientists look at soils in old urban centers, they easily find lead at concentrations above the EPA Safe Soil Level of 400 mg/kg. This has been true In New York City (Trace Metal Contamination in New York City Garden Soils and  Lead in New York City's soils: Population growth, land use, and contamination), in Cleveland (Management Options for Contaminated Urban Soils to Reduce Public Exposure and Maintain Soil Health), in Baltimore (Biosolids compost amendment for reducing soil lead hazards: a pilot study of Orgro® amendment and grass seeding in urban yards) and in Philadelphia (Relationship Between Total and Bioaccessible Lead on Children’s Blood Lead Levels in Urban Residential Philadelphia Soils).

The concern for lead risks is raised when urban soils are used for community gardening.  Many cities have programs to support gardening while explicitly addressing soil toxicity risks. The US EPA has been on the front lines of providing advice to city gardeners. EPA’s publications BROWNFIELDS AND URBAN AGRICULTURE: Interim Guidelines for Safe Gardening Practices and REUSING POTENTIALLY CONTAMINATED LANDSCAPES: Growing Gardens in Urban Soils are core documents.  Many cities have their own guidance document, for example “Soil Safety and Urban Gardening in Philadelphia.” Soil scientists have offered advice on safe gardening approaches: Lead in Urban Soils: A Real or Perceived Concern for Urban Agriculture? Another scientist proposes avoiding vegetables known to accumulate lead and other metals (Increased risk for lead exposure in children through consumption of produce grown in urban soils).

But community gardening is only one of several risks to children posed by lead-bearing soils. The journal article Mechanisms of children’s soil exposure concluded: “Soil is a source of contaminant exposure that must be evaluated when assessing environmental conditions and children’s health.” The survey of NYC soils shows that backyard soils have high metal levels: “Many of the soils analyzed exceeded the limits for Pb, Cr, As, and Cd levels. Higher percentages of home gardens are contaminated than community gardens.” The exposure pathway to children is more direct in backyard soils than in community gardens. The report Resuspension of urban soils as a persistent source of lead poisoning in children: A review and new directions  explains “… recent work on particulate resuspension and the role of resuspension of Pb-enriched urban soils as a continued source of bio-available Pb both outside and inside homes…. A strong seasonal relationship is found between atmospheric particulate loading and blood Pb levels in children….”

Risks from exposure to dust from backyard soil is usually outside the regulatory purview of environmental and health officials at local, state, or federal governments.  In Philadelphia, unexpectedly high blood lead levels in children resulted in a track down by investigative reporters to legacy soil contamination in gentrifying neighborhoods, where industrial lands were converted to home sites. The 2017 award-winning expose’ “In booming Philadelphia neighborhoods, lead-poisoned soil is resurfacing” included testing residential soils for lead, with some results as high as 10 times the EPA recommended soil level, and with neighborhood children having blood lead levels substantially above the 5 micrograms per deciliter action level.

Scientists have studied ways of reducing risks of high blood level levels of lead in children arising from contaminated neighborhood and backyard soils.  The 2004 review article Reducing Children’s Risk from Lead in Soil discusses chemical reactions in soil and in the human body that can reduce lead adsorption, also termed lead bioavailability or bioaccessibility.  The report Linking elevated blood lead level in urban school-aged children with bioaccessible lead in neighborhood soil explained that “bioaccessible Pb was a much stronger predictor of BLL [blood lead levels]."  Amendments rich in iron and aluminum appear to reduce lead bioavailability (Effect of soil properties on lead bioavailability and toxicity to earthworms). Another study (Variability of Bioaccessible Lead in Urban Garden Soils) pointed to phosphate and organic matter as significant for reducing lead bioavailability.  The 2004 article also targeted use of the nutrient phosphorus for reducing lead absorption in children, as phosphorus reacts with lead to form insoluble minerals that pass through the body.  The report Phosphorus Amendment Efficacy for In Situ Remediation of Soil Lead Depends on the Bioaccessible Method  looked at “Six phosphate amendments, including bone meal, fish bone, poultry litter, monoammonium phosphate, diammonium phosphate, and triple superphosphate….” One interesting recent paper suggests that a wastewater-derived phosphorus mineral, struvite, could reduce lead levels: Phosphate recycled as struvite immobilizes bioaccessible soil lead while minimizing environmental risk (“granular struvite can optimize trade-offs among soil Pb immobilization, crop Pb health risk, and P loss risk”). But the response of lead to phosphorus is not sure-fire. The journal article Soil solution interactions may limit Pb remediation using P amendments in an urban soil reports that “competing cations, such as Ca, Fe, and Zn, may limit low rate P applications for treating Pb soils.”

Success at reducing risks from lead-contaminated soils seems to improve with use of organic matter rich soil amendments. The study Management Options for Contaminated Urban Soils to Reduce Public Exposure and Maintain Soil Health tied reduced toxicity risks from organic-rich amendments to multiple factors. These characteristics include the vigor of the vegetative cover, the dilution of the toxic metals, the reduction of exposure of toxics at the soil surface, and the stability of soil aggregates for good drainage. The researchers also demonstrated that the soil amendments reduced exposures to toxic organic compounds: “mixing compost with the soil reduced benzo(a)pyrene content.”

Biosolids-based soil amendments have proved particularly effective because these products combine all three effective characteristics for reducing lead bioavailability – amorphous iron and aluminum, phosphorus in various mineral forms, and nutrient-rich organic matter.  The effectiveness of biosolids products had been first noted on research on brownfield and Superfund sites contaminated with lead and other toxic metals by mining and smelting activities. The paper  In situ remediation/reclamation/restoration of metals contaminated soils using tailor-made biosolids mixtures concluded “Tailor-Made Biosolids remediation of metal toxic soils allows effective and persistent  remediation at low cost….”  A review article of urban soil research (Case studies and evidence-based approaches to addressing urban soil lead contamination)  explained “composts, both biosolids and food and yard waste-based, have also been shown to reduce total soil Pb and Pb accessibility…. a trial program in Baltimore where the high Fe biosolids composts were added to Pb contaminated soils in 9 home gardens, reduced the bioaccessibility of lead in soil.  …Reduction in bioaccessible Pb is related to the absorption of Pb on high surface area Fe oxides.”  These results were confirmed in another study: Biosolids compost amendment for reducing soil lead hazards: a pilot study of Orgro® amendment and grass seeding in urban yards (“This study confirms the viability of in situ remediation of soils in urban areas where children are at risk of high Pb exposure from lead in paint, dust and soil”).

From all of this we have two big messages: health risks to children from lead-contaminated urban soils are significant, and the capacity of biosolids-based soil amendments to reduce health risks are persuasive. Yet, deployment of biosolids products to mitigate risks of exposure to urban soils is uncommon. A host of explanations can be offered: biosolids is a “tough sell” in urban communities; no regulations compel mitigation of lead in backyard soils; lead-based paint exposure in homes remain an urgent risk; and expensive local testing and demonstrations might be needed to support new programs. But does not the experience of the Flint water crisis teach us a lesson? The nexus of public policy in health and environmental management can collide with a vengeance when public leadership fails to act to protect the health of children. We biosolids managers have the knowledge and capacity to assume responsibility for advocating the fact that, when it comes to mitigating childhood lead risk, Biosolids is the Lead Story.