University of Maryland Extension

Soil contamination assessment and risk management

Neith Little, Extension Agent, Urban Agriculture

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This topic brings up a lot of fear and strong emotions, which makes it extra important to make decisions using good information about how soil contamination works and about your specific situation.

What are soil contaminants?
Most contaminants of soil that might pose a risk to human health fall into one of two categories: the so-called “heavy metals” (lead, arsenic, cadmium, chromium, copper, mercury, selenium, zinc) and complex organic compounds (solvents, pesticides, creosote, petroleum). Heavy metals are naturally occurring elements and are relatively easy to test for—many soil labs offer a test that will measure all of these metals for $40 to $100. Labs that offer tests for organic compounds are more rare, and the tests are more expensive and are specific to different types of organic contaminants.

Organic compounds eventually break down over time into smaller molecules, but metals are elements—they’re as small as they’re going to get in a human time-scale—so once they’re in a soil you’re pretty much stuck with them. On the other hand, some organic compounds can be taken up by plants’ roots into the parts of plants that people and animals eat. But at normal soil pH most metal contaminants bind so tightly to the soil that plant roots cannot take them up. This means that if your soil has heavy metals in it, the most important risk management tactic is to keep the soil itself out of people’s bodies: avoid growing crops who’s edible portions contact the soil, and minimize dust that could be inhaled.

Knowledge is power
The good news is that soil contamination is very site-specific, and many sites are relatively fine. Surveys of lead in soils across Baltimore, MD have found that only a small percentage of sites surveyed have lead levels higher than the EPA’s threshold of 400 ppm (Mielke et al. 1983; Yesilonis et al. 2008). This is likely because after the fire of 1904, Baltimore required most buildings to be made of brick, which is less likely to have exterior lead paint (interior lead paint is another story). Thus atmospheric deposition from leaded gasoline was probably the largest source of lead outdoors. Johns Hopkins University’s Center for a Liveable Future is in the middle of an ambitious Safe Urban Harvests study in Baltimore, so keep an eye out for updated data soon.

So does this mean you don’t need to worry? No, this means that it’s worth spending the money to test your soil for heavy metals so that you will know what you are dealing with. If you are early in the process of finding a place to grow, getting the soil tested for heavy metals will help you find a site to grow that has less metal contaminants. And if you already have a growing site, testing will give you information you need to decide whether you need to take steps to reduce your risk.

Even if you are farming in a rural area, depending on your site history it might be worth testing for heavy metals. Many old fruit orchards have high lead and arsenic contamination, because from 1890 to 1947 lead arsenate was commonly used as a pesticide in fruit orchards across the US (Schooley et al. 2008).

How much is too much?
This is the tough question, and unfortunately the answer is not cut and dry.

Legally, there are no federal standards for acceptable levels of contaminants in agricultural soil. The EPA sets thresholds for contaminants in brownfield sites that must be cleaned up before being returned to public use (EPA 2001, EPA 2002), but they will be the first to admit that those thresholds were never intended to be used for agricultural decision-making (EPA 2011).

States and local municipalities sometimes set their own standards for soil contaminations. The Maryland Department of the Environment (MDE 2008) and the New York State Department of Environmental Conservation have set local Soil Cleanup standards (NYSDEC & NYSDH 2006). Baltimore City has a draft Soil Safety Policy for Food Production which requires growers who want to make food production the official zoned use of their site to first test for soil contamination and have a plan to reduce risks based on the results of those tests (Baltimore Office of Sustainability 2014).

When weighing the risk posed by soil contaminants at your specific site, consider federal and local standards, expected local background levels in uncontaminated soils, and whether children will be gardening at your site.

Risk mitigation
At the pH that is best for crop growth, metals bind tightly to soil particles (Brennan and Lindsay 1996; Elliot et al. 1986) and are unlikely to be taken up into the edible part of a crop (Brown et al. 2016; Chaney et al. 1984). Rice is an important exception to this rule, because it is prone to taking up arsenic into the rice grains (Wang et al. 2003).

This means that on sites with metal contaminants, the most important mitigation goal is to keep the soil out of people’s bodies. Tactics that minimize how much farmers and gardeners are exposed to contaminated soil will also reduce the risk to customers. Relatively easy risk management strategies are to

  • avoid growing crops where the edible portion contacts the soil (root crops, lettuce),
  • keep bare soil covered with mulch, cover crops, or grass (under crops, in walkways, adjacent to growing areas),
  • minimize how much you disturb the soil (low-till or no-till),
  • avoid tracking soil into vehicles and living areas (wear gloves, use designated clothes and shoes for farming/gardening, wash dirty boots before leaving the growing area),
  • in high risk situations (very high contaminant levels, children gardening, growing root crops or lettuce) grow in raised beds filled with a soil or growing medium that is known to be uncontaminated.

What about phytoremediation?

Over recent decades, many growers and researchers have been excited by the prospect of using plants, particularly sunflowers, to selectively take up contaminants from the soil and remove them (phytoextraction).The bad news is that lead is usually so tightly bound to the soil that trying to extract it with plants would take centuries, and you'd still need to figure out what to do with all that contaminated plant matter.The good news is that because lead binds so tightly to the soil, it is much more practical to phytostabilize, by adding organic matter and other materials to make the lead even less bioavailable and more dilute in the soil. For a good summary of the research, see Blaustein (2017).

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