Connections: Understanding Urban Soil Lead

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Courtesy of BioCycle Magazine

There is no such thing as unleaded when talking about soils in urban areas. If a car had ever been driven near or paint had been applied to a home built on the soil prior to 1975, that soil will have measurable concentrations of lead. It can be detected in soils at the subparts per million range. Lead can cause developmental delays and reduced mental capacity. In short, lead is bad. But does a measurable level of lead in soil mean that you should be terrified to touch it? More importantly these days, does it mean you should be afraid to grow food in it?

This is a critical issue for organics recycling for a number of reasons. Much of the impetus for decentralized composting in urban areas comes from urban farmers who realize that compost is the key to soil health and successful gardening. In areas where centralized collection of feedstocks is the norm, these same growers are potential customers for finished products. Their potential to start composting and to use compost in their gardens and urban farms will also increase if a fear of lead contamination doesn’t stifle their efforts. That in part may depend on whether organics recyclers and healthy soil professionals can explain that using compost not only grows great vegetables, but serves to protect the consumers from any threat of lead contamination. To be a good explainer, some basics about the hazards associated with excess lead in soil must be well understood.

So what are the risks associated with elevated soil lead? The big concern is getting that lead in the stomach. The best/worst way to do that is to eat the lead contaminated soil directly. That may not sound too appetizing to readers of this column, but to a little kid it likely sounds a whole lot better than eating some kale. Children will also be less likely to wash their hands before eating — another way to accidentally consume some soil.
Lead Solubility

Getting soil lead into the stomach is not the whole story. Once in there, that lead has to become soluble and be absorbed by the body in order to pose a risk. Here again, it isn’t so simple. The potential for the lead to come into solution depends on two factors: how acidic the receiving stomach is and the form of lead in the soil. When a stomach has no food in it, it will be highly acidic. Hence that source of discomfort known as acid indigestion. Once some food is in there, the pH goes way up and the solubility of the lead goes way down.

The solubility of the lead can be changed by changing the mineral form of the lead. The same way that the properties of carbon change, based on the chemical structure, the properties of lead change according to what it is bonded to. Lead carbonates (cerrusite) are some of the most soluble forms of this element. Lead phosphate, in particular pyromorphite, is one of the least soluble forms. Once in their gut, the lead in children’s stomachs will be more efficiently absorbed into their systems than any lead in an adult stomach. So kids eating dirt are the primary risk associated with elevated soil lead. The risk assessment associated with lead in soil is all based on the potential for a little kid playing in the dirt on an empty stomach.

How about plant uptake? Here again, the main problem is associated with soil. Plants do not readily take up lead, however, many plants readily take up dirt. Here I am talking about plants like lettuces where some soil can adhere to the leaves and make for a crunchy salad. It is noted in the literature that carrots take up lead actually inside of the plant rather than just adhering to the surface. Lead concentrations in carrots, while higher than in other vegetables, are still in the low parts per million range. There has not been any research to look at how much of that lead will come into solution and be absorbed by the body when the carrot is eaten. Based on previous work with lead uptake in full versus empty stomachs, the answer is very, very little.

How much harm the lead does if anyone does eat the dirt or the carrot and lead is absorbed into their system depends also on how often people snack on soil. A one time exposure will not cause harm. Lead toxicity is chronic and not acute. That means that harm only results when soil and carrots are eaten frequently over time.

Read the full article in BioCycle Magazine

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