By Sally Brown, University of Washington
Abstracts of these resources are available in the searchable Information Portal offered to Northwest Biosolids members.
The concurrent decline of soil lead and children's blood lead in New Orleans
Constructed soils for mitigating lead (Pb) exposure and promoting urban community gardening: The New York City Clean Soil Bank pilot study
Effect of biosolids processing on lead bioavailability in an urban soil
Case studies and evidence-based approaches to addressing urban soil lead contamination
Long-Term in situ reduction in soil lead bioavailability measured in a mouse model
Lead contaminated soils in inner cities is a wide spread problem with historical roots. Back in the day lead (Pb) was added to paints to make them dry faster and last longer and to gasoline to stop engine knocks. That all stopped by the time I was in middle school (aka a very long time ago). But that lead doesn’t go anywhere. If you sample soils in a city, particularly in the older parts of that city, there is a good chance that the total soil Pb will measure well over background, likely in the 100s of ppms.
The big concern with Pb in soils is for little kids. The kids are at risk first because they find dirt a lot more appetizing than grownups and second because their bodies are so much more efficient at absorbing the metal. Lead exposure can cause a range of problems including impaired brain function and development. We typically measure Pb concentration in blood and as our understanding of the impact of elevated blood Pb has expanded, the regulatory limits for acceptable blood Pb have decreased. At mining sites, where there is a legacy of soil Pb contamination from mining and smelting, the EPA Superfund program has had a dig and haul policy- dig out the contaminated soil and put it in a repository. Replace it with clean stuff (see Pb graffiti below). That approach doesn’t work so well for Brooklyn, Oakland, St. Louis….
First- where would you put all that soil from Brooklyn (remember that trees do grow there- https://en.wikipedia.org/wiki/A_Tree_Grows_in_Brooklyn_(novel))? Then what would you put in its place? This month’s library is all about soil Pb in urban areas and how composts and biosolids are the solution to this problem. The first article is a new one from Howard Mielke and others. Dr. Mielke is a professor at Tulane and he believes that the appropriate/ safe concentration of Pb in soils is about zero. He is well respected but very cautious. For some perspective, the normal long standing EPA regulatory value for soil Pb is 300 ppm for areas where children are likely to be exposed. This paper follows the Pb concentration in soils in New Orleans and notes that as they have gone done, children’s blood Pb levels have also decreased. Much of the decrease is attributed to Hurricane Katrina which deposited new materials on surface soils, effectively reducing the Pb concentrations. This article shows a link between the areas where flooding occurred, effectively reducing the soil Pb concentration and decreases in children’s blood Pb levels.
The next article, also co- authored by Dr. Mielke takes us to Brooklyn. Here the focus is eggplants, rather than trees. The authors are looking for some clean dirt to use in community gardens. There is limited composting done in NY and despite Pam Elardo moving from the King County WWTD to lead the NYC wastewater program, most of the biosolids are landfilled. They found some sediment and had some food/yard compost donated from a local group. All manufactured soils had low Pb and the plants grown in these gardens were also very low in Pb. I would be more than happy to send you a paper that I co-authored with Ganga Hettairachchi and Rufus Chaney about the use of composts to reduce concerns about Pb in urban agriculture- but it has been in the library before and here I wanted to show that Mielke who is so worried about urban soil Pb sees compost and clean soils as a solution.
From here we go to the way back time machine and paper #3. This starts the tale of why composts and biosolids are not just good for urban soils because they are typically low in Pb, but because they actually make the Pb in the soils less dangerous. This paper reports on finding from a WEF study. I did this work while working with Rufus Chaney at USDA. We took different types of biosolids and added them to a high Pb soil from Baltimore. We tested changes in Pb availability by feeding the soil to mice and by using different soil extracts. What we found was pretty impressive. The biosolids compost produced using high iron biosolids (DC Water) reduced not only total Pb concentrations but also the portion of the Pb that was dangerous. We saw about a 30% reduction in Pb bioavailability. Important to note here that the high lime biosolids actually increased the Pb availability. This was the beginning of a larger body of work showing that biosolids can reduce the hazards associated with Pb by changing the mineral form of the metal. The high lime biosolids made more of the lead convert to lead carbonate (cerrusite) which is easily dissolved in an acid stomach. The high Fe biosolids caused the Pb to partition to iron oxides and to organic matter. I can send you more papers on this stuff- just let me know. But the point of this work comes out in the 4th paper.
For the 4th paper we go back to Dr. Mielke. I had known him through Rufus and when he was in Seattle he looked me up. We had lunch with his daughter, a medical Dr. at UW and I took him down to Tagro. He had a tour of the facility and we went to see some of the community gardens that had the Tagro delivered. He thought that this biosolids stuff was a great idea for the Pb problem. We ended up bringing several bags of the potting soil back to his daughters’ house in Magnolia. It made enough of an impression on him that he asked me to contribute to a review article on biosolids as a solution for urban Pb contaminated soils. That review article is #4 in the library. Very nice to see that biosolids are considered as a solution by one of the most cautious and highly respected scientists in the field.
Finally we get to paper #5. So they work well in the lab but do they stand the test of time? People might wonder about the efficacy of biosolids in binding soil Pb. As part of the work that started with the 2003 paper, we ended up testing that high Fe biosolids compost at a smelter contaminated site in Joplin, MO. We set up field plots testing a range of different types of phosphorus, compost, and iron rich materials, alone and in combination. We had the compost shipped out from DC Water. Here it is below right on top of the plots.
Below is a shot of the field site itself.
That was back in about 1997. Turns out that scientists went back and collected more soil from the field site recently. More mice were fed and the Pb speciation was determined using X-Ray adsorption spectroscopy. The compost (with added P, they didn’t test the compost alone treatments) worked better than ever. In fact the compost (as well as the high Fe material + P) were more effective than the phosphorus treatments at reducing the Pb availability. Phosphorus has long been seen as a gold standard here as it can precipitate with the Pb to form pyromorphite- a highly insoluble Pb mineral. The reduced adsorption of Pb by the mice was accompanied by changes in the mineral form of the metal, as measured in the fed soil as well as the mice feces.
Take home of the library- soil Pb in urban areas is a problem. Biosolids and composts- as long as they are low in Pb and not high in lime, are the solution. I love being the solution.