With a progressively educated population becoming more aware of the inherent dangers of the conventional food supply, urban farming has become hugely popular. However, more people are also becoming aware of contaminated soil and how heavy metals pose potential risks to their food crops. As backyard gardening continues to explode in popularity, we must ask how contaminated is our soil?
Many municipalities in many countries are embracing urban agriculture. Others are banning it due the threat it places on the food industry. How dare members of the public grow their own food? That is preposterous and should be illegal according to many government officials backed by food industry lobbyists.
Take Julie Bass’ ordeal with the city of Michigan. Michigan, as you probably know, has been the state worst hit by the depression. Only four people still have jobs. So Julie decided to cut down on her food bill by growing her own vegetables.
One would think city officials would applaud her initiative and self-sustaining choice. Instead, they favor grass sprayed with lots of chemicals as the preferred form of suitable, live plant material. So they charged her with a misdemeanor. She is considerable a non-law abiding citizen simply because she chose to grow her own vegetables on her own property.
This type of bureaucratic nonsense is actually spurring a movement where citizens are demanding the right to grow their own fruits and vegetables without restrictions. In Oakland, California efforts are underway to modify the city’s outdated zoning code to be more permissive of urban farming. A planning commission recently met to officially initiate the process which attracted hundreds of enthusiastic residents. Chicago, Cleveland, Seattle, and San Francisco are among the many cities that have already enacted similar changes.
While all share the common goal of encouraging sustainable local food production, no two cities have approached the issue of soil contamination in quite the same way. Not only is there no agreement about how to encourage — or even require — urban farmers to handle soil safely, but there’s also a lack of consensus among regional, state, and federal agencies as to what actually qualifies as safe.
On one hand, the cities must protect citizens from the dangers of lead poisoning, and on the other it’s tasked with supporting urban agriculture in areas known to contain contaminated soil. Striking the proper balance will mean the difference between disease, disappointment, and a thriving 21st-century city.
Soil remediation is a billion dollar market. In the last 10 years, the field has grown annually and the number of contaminated sites discovered has almost doubled.
“In 1950, we didn’t know anything about pollution,” recalls Guy Mercier, a researcher at the INRS Centre Eau Terre Environnement [Research Centre on Water, Earth, and the Environment]. Many industries just dumped their waste on their own property or in nearby waterways.
Lead in urban soils is commonly referred to as a legacy pollutant. It is most closely correlated with two sources: lead paint and leaded gasoline. Neither is still in use; lead paint was outlawed in the 70s, and leaded gasoline’s six-decade run ended in the mid-80s. But their effects remain, especially in older neighborhoods near historic freeway corridors. Lead contamination in soil can also result from lead plumbing and prior commercial and industrial land uses.
Soil contamination is either solid or liquid hazardous substances mixed with the naturally occurring soil. Usually, contaminants in the soil are physically or chemically attached to soil particles, or, if they are not attached, are trapped in the small spaces between soil particles.
Contaminants in the soil can hurt plants when they attempt to grow in contaminated soil and take up the contamination through their roots. These contaminants can then adversely impact the health of animals and humans when they ingest, inhale, or touch contaminated soil, or when they eat plants or animals that have themselves been affected by the contaminated soil.
Animals ingest and come into contact with contaminants when they burrow in contaminated soil. People can ingest and come into contact with contaminants when they dig, garden or come in contact with the contaminated soil. Certain contaminants, when they contact our skin, are absorbed into our bodies. When contaminants are attached to small surface soil particles they can become airborne as dust and can be inhaled.
Once pesticides enter soil they spread at rates that depend on the type of soil and pesticides, moisture and organic matter content of the soil and other factors. A relatively small amount of spilled pesticides can therefore create a much larger volume of contaminated soil. For example, approximately 30 tonnes of pesticides buried on a site in Yemen in the 1980s contaminated over 1500 tonnes of soil.
The cornucopia of screening levels proposed by various public agencies is obscure and inconsistent. For example, the California Office of Environmental Health puts its hazard assessment level at 80 parts per million for ingested soil, while the California Department of Toxic Substances Control says soil becomes hazardous material when lead surpasses 1,000 parts per million. Between them is a bevy of guidelines and limits for lead exposure in food, water, and soil in a variety of contexts. They tend to provide more confusion than guidance. The defining criteria for polluted status seems to change frequently for scientific, social, economic, or political reasons.
Soil is a major repository for lead and arsenic released by human activities. Some soils are naturally high in lead or arsenic, but many have been artificially enriched through a variety of means.
Contaminated soils contain total concentrations of elements exceeding the natural background level for local soils. Contamination is an intrinsic property of soil, and contaminated soils are easy to identify by chemical testing.
Gardening on lead- or arsenic-contaminated soil increases the likelihood of exposure to these two potentially toxic elements. There is no safe amounts for exposure of either element.
In adults, lead poisoning can cause high blood pressure and damage to the brain, nervous system, kidneys, thyroid, and blood. The body mistakes heavy metals like lead, arsenic, and mercury for essential nutrients such as calcium and iron and stores them in tissue, where they bind with cells and are readily absorbed. What’s more, there’s no cure beyond simply removing the source of contamination and allowing the body to clean itself out.
Diagnosing lead poisoning is tricky. Symptoms are unreliable, onset is gradual, and in children developmental problems may not surface until years later.
The EPA has found that living next to a coal ash disposal site can increase your risk of cancer or other diseases. Heavy metal poisons from may pose a serious threat to the health of any garden,leeching toxins into the soil and the plants themselves.
Identifying Lead and Arsenic in Soil
– Suspect soil lead contamination if the garden is within 20 feet of older buildings or other structures once painted with lead-based paints.
– Suspect soil lead contamination if the garden is within 100 feet of roadways and parking areas, particularly near high-traffic routes.
– Suspect soil lead and arsenic contamination if the garden is within 1 mile of existing or former smelters, fossil fuel-fired electrical power plants, or cement manufacturing facilities.
– Suspect soil lead and arsenic contamination if the garden is planted on a pre-1947 orchard site.
– Suspect soil lead and arsenic contamination if the garden is planted on or near tailings from current or former metal ore mines.
Chemical analysis of soil will confirm the presence of elevated concentrations of lead or arsenic.
Locate a soil testing laboratory and discuss requirements for soil sample size and containers before collecting samples. Your Local University or Cooperative Extensions may be able to help you find suitable laboratories.
Using a nonmetal tool, such as a plastic trowel or scoop, collect samples from the top 8 inches of the garden soil at several locations within the garden. Dump them all into a plastic bucket, mix the soil samples using a nonmetal tool until they are uniformly combined. Collect a subsample or composite sample from the soil mixture (usually about one cup volume) and place it in a plastic bag or other nonmetal sample container (often provided by the testing laboratory). Label the sample with your name, date, location, and depth of sample, using a permanent marker. Deliver the composite soil sample to a testing laboratory and request analyses for total lead and total arsenic concentrations.
Collect more than one composite sample from different areas within the garden if the garden is very large or if you expect contamination patterns to vary greatly. Use common sense when devising sampling plans. For example, if a garden is adjacent to an old building where lead paint might have been used, collect one composite sample from the garden area next to the building, where soil lead might be high, and one from farther away in the garden, where soil lead might be low. Map the sampling sites so you can relate the test results to the specific locations.
Testing laboratories normally report the lead and arsenic concentrations in units of milligrams per kilogram (mg/kg) or parts per million by mass (ppm). These units are numerically identical; that is, 10 mg/kg of a substance in a soil sample is the same as 10 ppm by mass of that substance.
Numerous interpretive standards exist for soil lead and arsenic. They often are contradictory because they reflect the varying objectives of the originating organizations and regulatory agencies. Most standards currently are undergoing review and therefore are subject to revision.
Garden Plant Selection
Crops respond differently to soil lead and arsenic depending on plant species and variety. Unfortunately not enough data are available to reliably rank plant species and varieties for growth, yield, and lead and arsenic uptake responses. A few general guidelines can be abstracted
from the scientific literature.
The quantities of lead found in most lead-contaminated soils typically are not high enough to reduce plant growth and yield. Elevated concentrations of soil arsenic can stunt plants and reduce yields. If sufficiently high, soil arsenic can cause plant death. Arsenic in plants bonds irreversibly with energy transport molecules, interfering with their activity. Plants containing excessive arsenic effectively “run out of energy.”
Green beans and other legumes appear to be most sensitive to soil arsenic contamination. They often fail to grow at soil arsenic concentrations which cause no deleterious effects on other plant species. Growth patterns of stone fruit trees such as peaches and apricots are very sensitive to elevated soil arsenic; apples and pears are less sensitive, and cherries are intermediate. Information about growth sensitivity of other crop species is sparse. The stunting effect of soil
arsenic may have horticultural benefits. Although the results are difficult to predict, arsenic stunting can control the size of ornamental plants and fruit trees.
The distribution patterns of lead and arsenic among various plant parts is highly variable. Seeds and fruits typically have lower lead and arsenic concentrations than do leaves, stems or roots. Roots and tubers usually have the highest lead and arsenic concentrations, with the skin having higher lead and arsenic concentrations than does the inner flesh. The lead content of roots correlates more closely to soil lead than does lead in leaves or stems, possibly because roots tend to retain absorbed lead and not transport it higher up into the plant. Tree fruits such as
apples and apricots contain very low lead and arsenic concentrations. Contamination of plant parts by lead- and arsenic-rich soil or dust can increase the apparent lead and arsenic content of that plant.
Organic arsenic compounds may be less toxic than inorganic arsenic compounds. Although comprehensive data about the distribution of chemical species in food plants are not available, preliminary reports suggest organic arsenic is predominant in fruits and vegetables, while
inorganic arsenic is more common in grains. Plants grown on sands and sandy loams have higher total arsenic contents than those grown on heavier-textured soils at equivalent total soil arsenic concentrations.
6 SOLUTIONS FOR DECONTAMINATION
1) Add Some Charcoal
If the contamination is recent, like a small chemical spill, try adding some activated charcoal the the soil. The charcoal will act as a sponge and soak up the chemical. When you are through, you simply remove the charcoal.
2) Add Fish Bones
Ground-up fish bones can be used to bind lead in soil and create stable compounds that are harmless to humans. Once mixed with the soil, decomposing bone fragments deposit phosphates that move through the soil and encapsulate tiny particles of lead. A six-inch layer of clean soil is poured over the top, leaving the dirt safe to plant in. An entire yard can be remedied with one treatment, which takes a few weeks to run its course.
3) Maximize the Soils Ability to Heal
When ridding your garden of contaminants and toxins, it is important to have good drainage. Adding compost and gypsum to the soil will improve drainage and encourage worms to inhabit the area. Worms are natural aerators, and will improve soils quality. If your soil is dense or has a high clay content, you may want to add sand or pea gravel to the area to improve drainage.
4) Grow Plants that Detoxify
Some plants act as natural detoxifiers and soil cleansers. Sunflowers have long been used to clean toxins. Another plant you can use is the ferns. Both act as leeches, pulling contaminants from the soil with their roots, and thus decontaminating it.
5) Bake the Soil by Solarization
Start by adding organic materials and fertilizers to the soil. Work these in well with a garden tiller or a spade. Rake the surface of your garden as smooth as possible. Get the ground moist before covering it. Do not over water the area; you just want the ground to be damp. Cover your garden with a layer of plastic. It is best to make the area as airtight as possible, and you may want to consider digging a trench around your garden to bury the plastic. If you need to use more than one piece of plastic, use a heat and weather resistant sealant to glue them together. Bury the sides of the plastic in the ground well. Pull the plastic tight as you go to make sure it is airtight. Leave the plastic in place over the winter months.
Bioremediation is a treatment process that uses naturally occurring microorganisms (yeast, fungi, or bacteria) to break down, or degrade, hazardous substances into less toxic or nontoxic substances. Microorganisms, just like humans, eat and digest organic substances for nutrients and energy. In chemical terms, “organic” compounds are those that contain carbon and hydrogen atoms. Certain microorganisms can digest organic substances such as fuels or solvents that are hazardous to humans. The microorganisms break down the organic contaminants into harmless products — mainly carbon dioxide and water. Once the contaminants are degraded, the microorganism population is reduced because they have used all of their food source. Dead microorganisms or small populations in the absence of food pose no contamination risk.
Keep in mind that no matter what urban farmers have to do to grow them, they’re better off enjoying their own fruits and vegetables than those bought at a grocery store. , In fact, lead contamination may be a minor issue when you consider the dangers of conventional agriculture when farmers are spraying their crops with pesticides and herbicides whose concentrations are far more lethal.
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