Contaminated Soil Treatment – How To Clean Contaminated Soils

By: Susan Patterson, Master Gardener

The key to growing a healthy garden is clean, healthy soil. Contaminants in soil can quickly lead to an array of problems, so determining possible causes of soil contamination beforehand and learning how to clean contaminated soils is very important.

What is Soil Contamination?

Before you begin to plan and construct your garden, it’s always wise to have a soil sample analyzed. The quality of soil can be affected by many things. It is important to determine what nearby land was used for in the past and assess the impact of any nearby industry.

Oftentimes, the causes of soil contamination result from dangerous chemicals that find their way into the soil and disrupt the soil structure. Contaminants in soil that is taken up by plants or comes in contact with garden fruits and vegetables can cause health problems. Soil test results will indicate the quality of the soil and the causes of soil contamination, if any.

Possible Contaminants in Soil

Urban dwellers should be particularly concerned with a number of possible soil contaminants including lead, which has been used in paint and as an additive to gasoline; cadmium, which results from burning coal and garbage; arsenic, which is used in wood preservatives, weed killers, pesticides, and fertilizers.

If you live close to an industrial or commercial site, it’s wise to have your soil checked for metals and cyanides, benzene, toluene, and other chemicals associated with gas station leaks. Rural residents should also check for past and present industries and pesticides.

How to Clean Contaminated Soils

While cleaning contaminated soil is not “literally” possible, some things can be done to reduce the toxic impact. Adjusting the soil pH to as close to neutral as possible will help reduce the negative impact of contaminants.

Contaminated soil treatment also includes adding plenty of rich organic matter to the soil and a healthy top-dress of peat moss, compost, or aged manure. This practice will help protect plants from damage.

Always be sure to wash any fruits or vegetables before you eat them. If contaminants are a problem, you can also plant in raised beds made with untreated lumber. This will allow you to add your own healthy soil.

Taking appropriate measures for cleaning contaminated soil beforehand can lead to a healthy garden for you and your family.

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Minimizing Risks of Soil Contaminants in Urban Gardens SoilFacts

Gardening in urban areas is increasingly popular in North Carolina. Growing food locally (which requires less energy for packaging, transit, and storage), connecting to nature at your doorstep, engaging children in agriculture, and controlling the amount and types of pesticides and fertilizers applied to your food are among the many wonderful reasons to garden in the city. But urban gardening poses potential risks. Before planting, city gardeners must evaluate how a prospective site was previously used, to identify potential safety hazards—including chemical contaminants—in the soil.

This publication alerts prospective gardeners to some of the most common contaminants in urban soils, such as lead and other toxic metals, solvents, pesticides, and total petroleum hydrocarbons. Readers will learn how to minimize potential risks to gardeners and to those who consume garden produce. The document includes information regarding site characterization, common contaminants, soil testing, interpretation of results, and strategies for reducing exposure risks.

To ensure quality site assessment and analyses, it may be necessary to engage trained professionals. Links to certified professional soil scientists, environmental consultants, and laboratories are provided.

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What Are the Dangers of Contaminated Soil?

Anyone working in close contact with contaminated soil, as gardeners do, is at risk of chemical exposure through skin absorption, as well as through inhalation of soil particles. Plants absorb chemicals from the soil – and not just in their roots, but in their shoots, leaves, fruits, and seeds, too – passing on the toxicity to people who eat the produce.

Depending on the contaminant, low level exposures may result in nausea, dizziness, fatigue, headaches, and rashes exposure at higher levels can result in neurological conditions, reproductive disorders, birth defects, and an increase risk of cancer.

Children, pregnant women, the elderly, and anyone with compromised health are especially vulnerable. Gardens are a wonderful opportunity for kids of all ages to learn and play, but small children are prone to sucking on dirty fingers, or even consuming soil directly, which poses a much more acute health risk than simply touching contaminated soil or even eating produce grown in it.

Lead Contamination in Urban Gardens

I have received many e-mails concerning the presence of lead in urban soils. Since September is prime time for beginning to build gardens at schools and community organizations, now is a good time to address the issue.

Lead is a naturally occurring element, like copper, iron, and gold, however it is highly toxic. Lead poisoning is a common pediatric problem. The heavy metal acts as a neurotoxin, and even minute amounts in a child's system can lower intelligence and slow neurological development. Children suffer more severe effects than adults from exposure to lead, and retain lead in their bodies much longer.

Most lead poisoning results from ingesting paint chips from walls or toys or from ingesting dust and dirt that contains high concentrations of lead. Very rarely are contaminated fruits and vegetables the culprit, but all precautions should be taken to limit lead exposure.

Checking Site History

Because lead is not biodegradable or mobile, once it's in the soil it remains a long-term source of contamination. Gardeners should find out the history of the garden site before planting edible crops. Use the city lot number to check land use and ownership records at the local city hall. However, if you know the site was a former waste dump or something similar, then don't waste your time researching because it won't be suitable for growing food.

Even in areas with an acceptable history, there could be high levels of lead in the soil from paint waste and chips, automobile emissions, or other industrial sources. Unfortunately, many urban gardens are near older painted buildings, next to gas stations, adjacent to highways, or located in industrial zones. Basically, any garden in an urban environment is at risk, so the site needs to be examined carefully before planting.

Testing Soil

Have the soil tested for lead. Some extension service offices, EPA offices, and private labs offer this service. Call them for instructions on collecting and delivering your soil sample. Inform them that you intend to grow edibles and need the soil checked for lead and other pollutants. Soils with lead levels below 300 ppm are typically deemed safe for growing vegetables. Most urban soils have levels in this safe-but-lead-still-present category.

"Cleaning" the Soil

Soil scientists use ornamental plants to help clean contaminated soil, a practice called "phytoremediation." Certain plants have the ability to pull heavy metals from the ground. In this way, the plants (phyto) help to remediate the soil. Ornamentals like sunflowers, thrifts, and blue fescue are some of the best at absorbing lead. Phytoremediation will not remove all the lead, but it can be a relatively inexpensive, aesthetically pleasing component of a professional soil remediation plan.

Although it is becoming a popular buzzword, phytoremediation is best left to experts. Because lead does not degrade, the metal simply moves from the soil to the plant tissues. If accidentally placed with other garden waste, this lead-filled debris would contaminate the whole compost pile. In fact, disposal of materials with heavy metals requires professional services. The best options for urban gardeners with more than 300 ppm of lead in the soil are to cover the ground, use raised beds, and leave phytoremediation to the scientists.

Taking Extra Precautions

It is important for gardeners to remember that soils are not the only source of lead poisoning. Lead deposition from automotive and industrial emissions is common. Studies show that more lead is deposited on the leaves from urban dust than is absorbed through the roots from urban soils.

Even though the site may be technically deemed "safe," urban gardeners should still take extra measures to prevent contamination of their crops. According to expert Jaime Zaplatosch, education projects coordinator of Openlands Project, an urban conservation organization in Chicago, these important steps include:

  • Laying heavy-duty landscape cloth over the ground and mulching thickly to keep kids from easily coming into contact with the soil
  • Supervising young children to ensure they do not eat dirt or unwashed vegetables
  • Placing gardens away from roadsides, painted structures, garbage dumps, and industrial zones
  • Growing fruiting crops, such as tomatoes, peppers, beans, and okra, rather than root crops, leafy vegetables, or herbs, which absorb more contaminants from the soil and air
  • Putting a fence or hedge between your garden and high-risk areas like highways. Barriers help block dust that may contain lead particles.
  • Adding organic matter to the soil
  • Maintaining pH around 6.5
  • Cleaning produce thoroughly before eating or storing
  • Washing hands immediately after gardening and before meals.

If Lead Levels Are High

In areas of high lead contamination, do NOT grow edibles in the ground. Instead, lay heavy-duty landscape fabric over the ground, construct raised beds at least 18 inches tall, and add fresh, uncontaminated soil. When building raised beds, choose natural materials over those that have been treated with preservative chemicals. Gardeners should take all precautions to help keep their vegetables free of toxins.

If you are not growing edibles, you don't need raised beds however, for the protection of the kids, always lay landscape fabric and top it with mulch. Soils with lead concentrations that are not suitable for veggies can still grow great shade trees, flowering shrubs, and cut flowers.

These simple steps can allow urban gardeners to safely grow food and flowers. In most situations, the nutritional, psychological, aesthetic, ecological, and spiritual benefits of gardening will far outweigh the efforts needed to reduce the risk of contamination. Because our environment is not pristine, gardening in an urban area is definitely more challenging, but the rewards are that much sweeter.

Newbie Urban Gardeners May Not Be Aware Of Soil's Dirty Legacy

Graze the Roof is a community-produced garden that grows vegetables on the rooftop of a church in San Francisco. Sergio Ruiz/Flickr hide caption

Graze the Roof is a community-produced garden that grows vegetables on the rooftop of a church in San Francisco.

The majority of Americans now live in cities, which means we have very little to do anymore with the production of our food.

But there's a reversal of that trend afoot, as more city people decide that they want to cultivate crops and raise some livestock. After all, there are few things more satisfying than biting into a bunch of tender, red radishes you grew yourself, or a fresh egg from the backyard.

According to figures just released by the National Gardening Association, the number of Americans growing food in urban areas increased 29 percent between 2008 and 2013 from 7 million to 9 million people.

But many of these city-dwelling gardeners still aren't aware of how to grow food in urban soil safely, according to a study by researchers at Johns Hopkins University's Center for a Livable Future.

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Grow Your Own Vegetables, City Dwellers, But Do Mind The Lead

Most gardeners understand that the soil in big cities is often contaminated with lead — and know to get their soil tested. But the researchers' interviews with 70 urban gardeners in Baltimore revealed that most are pretty clueless about how to avoid other types of contaminants — like heavy metals and asbestos — from getting into their vegetables.

Part of the problem is that "there might be contaminants that [gardeners] can't test for," says Brent Kim, a program officer at the Center for a Livable Future. Most soil tests look for lead, cadmium and arsenic, he tells The Salt. But they don't test things like petrochemicals left behind by cars, or cleaning solvents, which might have seeped into the soil from an old laundromat.

Many of these chemicals — including the cleaning solvents laundromats used back in the day, and chemicals found in the exhaust of cars — are carcinogenic, and they're dangerous to ingest or even breathe in. Asbestos left over from a building demolished years ago can cause lung problems, as well. And children are especially vulnerable to all of these substances, Kim says.

So if you're thinking of starting an urban garden, Kim says, once you've have found a plot of land, you should learn its history. What's now an empty plot or a backyard might once have been a parking lot, a gas station or the site of a chemical spill, he says. "Knowing the site history will give you some clues about what might be in that soil," he says.

And testing the soil and investigating the land's history are crucial whether or not you're planning on using raised beds. "People tend to think raised beds are going to solve their contamination problem," Kim says. But contaminated soil could easily get kicked onto your plants, especially if the beds are low to the ground.

"Another consideration is you have to be careful about the materials that you're using to build a raised bed," Kim says. Recycling wood from an old construction site might seem like a good, eco-friendly idea. But that wood could be treated with chemicals you don't want touching your fruits and veggies, Kim says.

Even after taking all those precautions, gardeners can never be 100 percent sure that they're in the clear, Kim says. So it's always a good idea to use gloves while gardening, and wash all your produce thoroughly. And if your young kids like to help out in the garden, you might want to clean them up as well.

All of this advice applies to all urban gardeners, Kim says, whether you're in charge of a small backyard plot or a giant operation, like 140-acre Hantz Farms in Detroit.

According to the report, in Baltimore, over a quarter of the gardeners were relative newbies, with less than five years of experience.

Kim says there's no quick way to bridge the knowledge gap, but providing gardeners with the right resources is a first step. He and his colleagues have created an urban gardening guide. Baltimore residents can also check out an interactive map of the city, which highlights sites where the risk of contamination is high.

Other great resources include the U.S. Department of Agriculture's online guide.

"I see these urban growing spaces as these oases in the middle of these urban environments," Kim says. They bring communities together, and they help people save money on fresh produce.

"Urban growing spaces are amazing," he says. "Let's keeping doing this, but let's do it safely."

Guidance for Air (Vapor Intrusion) Cleanups

Vapor intrusion is the general term given to the migration of hazardous vapors from any subsurface vapor source (i.e., contaminated soil or groundwater) into the indoor air of an overlying building or structure. These vapors can enter buildings through cracks in basements and foundations, as well as through conduits and other openings in the building envelope. All types of buildings, regardless of foundation type (e.g., basement, crawl space, slab-ongrade), have openings that render them potentially vulnerable to vapor intrusion.

When and where vapor intrusion occurs, concentrations of vapors can increase gradually in amount in buildings or structures as time passes (i.e., “accumulate”). Depending upon site- and building-specific circumstances, vapors of potentially toxic chemicals may accumulate to a point where the health of the occupants (e.g., residents, workers, etc.) in those buildings could be threatened. In addition to their toxicity threats, methane and certain other vapor-forming chemicals can pose explosion hazards depending upon structure-, building-, and site-specific circumstances. Explosion hazards may pose an imminent and substantial danger to human health and public welfare.

Response actions to address vapor intrusion when it poses unacceptable human health risks typically entail a combination of: remediation to reduce or eliminate subsurface vapor sources engineered exposure (mitigation) controls for specific buildings to reduce vapor intrusion or reduce concentrations of vapor-forming chemicals that have already entered the building monitoring to assess and verify the performance and effectiveness of the remediation systems and engineered exposure controls and institutional controls (ICs) to restrict occupancy and land use and/or to alert parties (e.g., prospective developers, owners, and municipalities) of the presence of subsurface sources of vapor-forming chemicals and to foster operation, maintenance, and monitoring of the remediation systems and engineered exposure controls.

Guidance Documents Pertaining to Air (Vapor Intrusion) Cleanups

The following guidance document addresses the evaluation and management of a single exposure pathway – the vapor intrusion pathway. The intent of these guidance documents is to provide a tool to help the user conduct a screening evaluation as to whether or not the vapor intrusion exposure pathway is complete and, if so, whether it poses an unacceptable risk to human health. A complete pathway means that humans are exposed to vapors originating from site contamination. The documents provide limited recommendations for delineating extent of risk or eliminating risk.

This technical guidance presents EPA’s technical recommendations based on our current understanding of vapor intrusion into indoor air from subsurface vapor sources. One of its main purposes is to promote national consistency in assessing the vapor intrusion pathway.

This technical report presents a summary of indoor air studies that measured background concentrations of Volatile Organic Compounds (VOCs) in the indoor air of thousands of North American residences and an evaluation and compilation of the statistical information reported in these studies. The objective of this compilation is to illustrate the ranges and variability of VOC concentrations in indoor air resulting from sources other than vapor intrusion.

This report provides simplified simulation examples to illustrate how subsurface conditions and building-specific characteristics determine the distribution of vapor-forming chemicals in the subsurface, and the indoor air concentration relative to a source concentration. It was prepared to help environmental practitioners gain insights into the processes and variables involved in the vapor intrusion pathway and to provide a theoretical framework with which to draw inferences about and better understand the complex vapor fate and transport conditions typically encountered at actual, non-idealized contaminated sites.

This guidance focuses on releases of petroleum-based fuels (e.g., gasoline, diesel) from underground storage tanks (USTs), which are typically located at gas stations. This guide applies to new and existing releases of petroleum-based fuels from leaking USTs and to previously closed sites, where the implementing agency has reason to suspect that there may be a potential for petroleum vapor intrusion.

This report presents technical information about sites in the U.S. that have been investigated for vapor intrusion. The primary focus of the report is the evaluation of concentrations of chlorinated Volatile Organic Compounds in and underneath residential buildings based upon EPA's vapor intrusion database as of 2010. The technical information provided in this report may be useful for regulators, responsible parties, and others assessing and managing vapor intrusion investigation programs.

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