Today’s urban soils have been poisoned by years of irresponsible use of toxic metals and chemicals. These contaminants pose a great risk to human health and have rendered much of the land that could potentially be used for urban food production dangerously unfit. Not only can plants grown in polluted soil pass pollutants on to those who eat them, but mere exposure to contaminated dirt and dust can be hazardous. The problem is so extensive that along with gaining access to land, contaminated soil is one of the greatest obstacles to growing food in the city. (p.180)
Bioremediation is the process of using the natural abilities of living organisms (typically plants, bacteria, and fungi) to speed the degradation or assist in the removal of contaminants. In relatively short periods of time, bioremediation can break down chemicals or absorb metals that otherwise might persist indefinitely or take years to degrade. Bioremediation differs greatly from conventional ex-situ (off-site) methods of soil remediation. Typically, such methods destructively excavate huge amounts of contaminated soil. The soil is then either landfilled and replaced with sterile fill dirt or transported to a facility for chemical- and energy -intensive treatment and then returned, all at great financial cost. Such investments tend not to benefit poor communities. In contrast, bioremediation is an in-situ (on-site) process. All treatments are done on location. No soil is removed from the site. Bioremediation’s processes result in an improvement or overall quality and health of the soil.
Using bioremediation to treat contamination is not radical in and of itself – it is an accepted method in mainstream engineering. What is radical is an approach that uses techniques that are cheap and simple and can be carried out by people with little to no background in science and without being dependent on engineers and massive funding. These techniques give people a genuine hope of proactively cleaning the soil in their backyards, community gardens, playgrounds and parks.
The two most general categories of soil contaminants are heavy metals and molecular contaminants. Heavy metals are elements. Elements are the basic building blocks of matter. They cannot be broken down any further by regular natural processes. The Periodic Table lists all known elements including oxygen, hydrogen, carbon. The elements lead, cadmium, mercury, chromium, and arsenic are called heavy metals. If left alone, heavy metals present in soils remain indefinitely. While some heavy metals, like iron and magnesium, are essential nutrients in small amounts, humans and most other life forms did not evolve with heavy metals present in the high concentrations found today. Fueled by the demands of industry, enormous amounts of heavy metals have been brought to the surface of the planet by extractive mining and concentrated through smelting and refining. Excessive exposure to heavy metals can result in a number of negative health effects, including organ damage, birth defects, and immune system disorders.
Since they cannot be destroyed, there are limited methods for treating elemental contaminants. Phytoremediation and compost remediation are the bioremediation methods most commonly used to treat heavy metal contamination. Phytoremediation accumulates metals in certain metal-loving plants that are then removed and disposed of elsewhere. Compost binds up metals with organic molecules in the soil, reducing the percentage that is absorbed by plants or human tissue. Molecular contaminants are made up of molecules: elements bound together is different ways to create substances with varying chemical properties. Some molecular contaminants found in soils are pesticides (dieldrine and chlordane), fuels (diesel and gasoline), and by-products of industry (PCBs and dioxin). Others, like polycyclic aromatic hydrocarbons, can result from either human or natural events, such as fires or volcanic eruptions. (p.182)
Mycoremediation, bacterial remediation, and compost remediation are the most appropriate methods for treating molecular contaminants. The natural metabolic processes of bacteria and fungi are capable of ripping apart molecular contaminants into benign components, which they then use as food. These processes occur naturally over time, but the rate of degradation can be accelerated by adding beneficial organisms to a site and providing the proper habitat and nutrients. Bioremediation can be broken down into four categories: bacterial remediation, mycoremediation, phytoremediation, and compost remediation. (p.184)
Some plants used for phytoremediation:
– Lead (in order of effectiveness): Indian and Japanese mustard, Scented Geranium, Corn, Pumpkins, Sunflowers, Pennycress, Amaranth, Nettles, Tomatoes,
– Arsenic: Chinese Brake Ferns
– Nickel and Cadmium: Scented Geranium
From Kellogg, S., Pettigrew, S. (2008). Toolbox for Sustainable City Living. Cambridge : South End Press.