22.2: Common Challenges with Urban Soils

Typically, the first challenges you are likely to find with urban soils are compaction, the presence of concrete, construction materials and other trash, and the presence of toxic compounds. The basic causes of compaction in urban settings are very similar to those discussed in Chapter 6, such as traffic from heavy vehicles. However, in urban settings, it is oftentimes construction activity rather than the use of farm equipment that causes compaction and soil degradation. Because construction jobs are often done on tight schedules, the compaction potential of working on wet soils is likely to be ignored. Also, construction regularly involves either removing topsoil or adding fill to build up the ground level, along with the use of very heavy equipment (Figure 22.1). All of this results in disturbed, compacted soils low in organic matter and biological activity (Figure 22.2) In addition, construction debris and chemical waste materials left behind in many cases become part of the soil matrix, frequently raising the pH (because concrete contains lime).

Figure 22.1. A history of construction activity at an urban site oftentimes results in compacted soil that contains both debris and contaminants. Photo by Francisco Andreotti.

There are many kinds of toxic compounds that can be present in urban soils, and they can come from a variety of sources depending on the location and land-use history of a property (Figure 22.3). Addressing the presence of toxic compounds is critical not only because urban farms and gardens produce food for human consumption, but also because urban operations tend to emphasize educational programming. If members of the community are going to visit an urban farm on a regular basis, especially children, it is essential to resolve any problems related to toxic compounds in the soil.

Figure 22.2 Construction activities cause bare compacted soil with potential for water and wind erosion, and challenges with revegetation.

Figure 22.3 This former storage site in Portland, OR, was contaminated with volatile organic compounds. Photo courtesy the U.S. Environmental Protection Agency.

While all of these problems are solvable, their solutions might prove time consuming and expensive, depending on their severity. For example, the opportunity to use forms of tillage to reduce compaction may be limited in urban settings due to unique aspects of urban farming, such as the presence of underground utilities, a lack of space for heavy equipment or the cost. Therefore, if you are thinking about growing food or developing green spaces on an urban property, you should carefully evaluate the condition of its soils first and develop a plan for resolving any problems you identify.

Soil Contamination

Soil contamination is much more prevalent in urban areas than in agricultural ones. In urban soils, lead is the most common contaminant to pay attention to. It is prevalent due to its long-time use in gasoline (banned since 1989 in the United States) and paint (banned since 1978 for residential use). But there is a wide range of other contaminants from current and past land uses that could pose problems, such as petroleum products and legacy pesticides (lead arsenate, copper sulfate, etc.) (Table 22.1). Cases that are of special concerns include former industrial sites, areas along major roads, recent construction sites, waste disposal sites and junkyards. In some cases, contaminants can end up on a property from distant sources through atmospheric deposition (the process by which particles and gases in the air, such as those that come from tailpipe emissions, settle on the ground or in bodies of water).

People are exposed to soil contaminants through different possible pathways:

• Ingesting soil. The risk is greatest when the soil is left bare, especially with chemicals that are concentrated at the soil surface. This is especially of concern with children because they like to play in soil and may put dirty hands in their mouths.
• Breathing volatiles and dust. When winds or human activity sweep up bare contaminated soil, contaminants may enter the lungs and become absorbed into the body. Chemicals that stay at the surface are most susceptible to wind erosion. Fine soil particles themselves can also damage the respiratory system. Again, children are at greater risk of inhaling contaminated dust because of their behavior.
• Eating food grown on contaminated soil. The food that is grown at a contaminated site can expose people to toxic compounds in two ways: either contaminated soil finds its way onto a vegetable that is eaten without being properly washed or peeled, or the crop absorbs contaminants through its roots. Also, food crops grown using pesticides may contain residues of these chemicals and can expose people when they are eaten.
• Exposure through the skin. Skin is generally an effective barrier against contaminants, but in extreme cases a person may be impacted through rashes or blisters. Pesticide contaminants can also pass through the skin.

Some contaminants are highly adsorbed by soil particles, especially when the soil is around neutral pH. These contaminants typically remain close to the soil surface, although over time they may mix slightly into the soil due to biological activity or any form of digging or tilling. In the case of lead, the risk of exposure from contact with contaminated soil is significantly higher than from a crop that has absorbed the metal from the soil. This is because plants absorb minimal amounts of lead, especially when pH is neutral. You are much more likely to expose yourself to lead from dirty hands, breathing in dust, or from produce that isn’t adequately cleaned. However, lead can accumulate in roots, so growing root crops in lead-contaminated soils should be avoided.

Other contaminants include organic compounds like industrial solvents, pesticides and petroleum products. Industrial solvents like trichloroethylene (TCE) move readily through the soil and can reach groundwater. Some pesticides can remain in the soil for many years and slowly percolate into groundwater. Over time, certain organic compounds are degraded by microorganisms in the soil. Petroleum products tend to stay near the surface.

Obviously, contaminants that stay at the surface pose a larger risk of human exposure, especially if they also suppress vegetation and are therefore more prone to wind and water erosion (Table 22.2). But in that case the contaminants can also be more readily removed by scraping the top layer of soil (and replacing it with good topsoil or compost). Contaminants that readily leach to groundwater may pose a problem through drinking water. Again, the highest risks are with children. They are also more sensitive to toxic contaminants than adults.

Table 22.1 Common Contaminants in Urban Soils Based on Previous Land Use

Land Use	Common Contaminants
Agriculture, green space	Nitrate, pesticides/herbicides
Car wash, parking lots, road and maintenance depots, vehicle services	Metals, PAHs*, petroleum products, lead paint, PCB* caulks, solvents
Dry cleaning	Solvents
Existing commercial or industrial building structures	Asbestos, petroleum products, lead paint, PCB caulks, solvents
Junkyards	Metals, petroleum products, solvents, sulfate
Machine shops and metal works	Metals, petroleum products, solvents, surfactants
Residential areas; streets; buildings with lead-based paint; where coal, oil, gas or garbage was burned	Metals, including lead, PAHs, petroleum products, creosote, salt
Stormwater drains and retention basins	Metals, pathogens, pesticides/herbicides, petroleum products, sodium, solvents
Underground and aboveground storage tanks	Pesticides/herbicides, petroleum products, solvents
Wood preserving	Metals, petroleum products, phenols, solvents, sulfate
Chemical manufacture, clandestine dumping, hazardous material storage and transfer, industrial lagoons and pits, railroad tracks and yards, research labs	Fluoride, metals, nitrate, pathogens, petroleum products, phenols, radioactivity, sodium, solvents, sulfate
*Polycyclic aromatic hydrocarbons (PAHs) are a class of toxic chemicals produced when coal, oil, gas, wood and garbage are burned. Caulks containing harmful polychlorinated biphenyls (PCB) were used in schools and other buildings that were renovated or constructed from approximately 1950–1979. Source: Boulding and Ginn (2004)

Table 22.2 Health and Environmental Effects of Common Soil Contaminants in Urban and Industrial Areas

Contaminant Type	Examples	Comments
Metals	Cadmium, zinc, nickel, lead, arsenic, mercury	Adsorbed by soil at the surface unless physically incorporated. Sometimes a gas. Affect the central nervous system and mental capacity with long-term effects.
Radioactive Materials	Radon, uranium, plutonium, cesium, strontium	Mostly soil adsorbed or gaseous. Degrade over long time periods. Acute toxicity in high doses; cancer.
Industrial solvents	Chlorinated organics like PCE, TCE, DCE	Can leach to groundwater or be volatile. Slowly decompose in soil. Affect the central nervous system and mental capacity.
Petroleum products	Benzene, toluene, ethylbenzene, xylene, kerosene, gasoline, diesel	Risk from drinking water and inhalation from volatilized product. Irritation; affect the central nervous system and mental capacity.
Salts	Sodium chloride	Cause sodic soil conditions, aggregate breakdown and compaction.
Agricultural inputs	Nitrates, pesticides/herbicides	Impaire water quality. Irritation; affect the central nervous system and are associated with cancer.
Other organic and inorganic pollutants	PCB, asbestos, drugs and antibiotics	Associated with cancer; sometimes acute toxicity and central nervous system. Affect aquatic biology and drug resistance.