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Soils And Urban Agriculture:

Land Use and Contaminants tableexcerpted by Jack Kittredge from the EPA publication “Brownfields and Urban Agriculture” and from “Using Historical Records to Assess Environmental Conditions at Community Gardens” by Robert Hersh and from “Toolbox for Sustainable City Living” by Scott Kellogg and Stacy Pettigrew

Overview

Across the country, communities are adopting the use of urban agriculture and community gardens for neighborhood revitalization. Sites ranging from former auto-manufacturers, industrial complexes, and whole neighborhoods, down to small individual lots, including commercial and residential areas, are being considered as potential spots for growing food.

Redeveloping any potentially contaminated urban property (often referred to as brownfields), brings up questions about the site’s environmental history and the risks posed by a proposed reuse. At this time there are no definitive standards for soil contaminant levels that are safe for food production. EPA has long-established soil screening levels for contaminated site cleanup, but these threshold-screening levels usually serve as a starting point for further property investigation and do not factor in plant uptake or bioavailability.

How clean is clean for gardening activities.

Clean-up and reuse of any contaminated site is based on risk assessment and exposure scenarios – the levels of contamination present and how a person can be exposed to that contaminant, based on the intended reuse. These criteria for residential, commercial and industrial reuse are based on potential exposure: length of time spent on the site, types of activities performed on the site, and potential contamination pathways such as inhalation, ingestion, or possible skin contact with contamination.

Step-By-Step Guidelines

The following process proposes a series of questions you need to ask and the information you need to gather in order to make decisions while implementing an urban agriculture project. This model may be applied to any urban agriculture project on any brownfield site, and may be of value for other reuses where contact with soil may be higher, such as parks or recreational areas.

The previous use of the property and those surrounding it will be the major deciding factor on how cautious you should be before gardening. The more historical information learned about a site’s previous uses, the more informed decisions can be made during garden development.

We can infer possible types of contamination based on the previous use of the property. For example, residential areas may have unsafe concentrations of lead where the presence of older housing stock or structures indicates lead-based paint was present. Industrial areas may be high in heavy metals such as cadmium, mercury, chromium and arsenic. Heavy metals are elements, the basic building blocks of matter. They cannot be broken down any further by regular natural processes. If left alone, heavy metals present in soils remain indefinitely. Excessive exposure to heavy metals can result in a number of negative health effects, including organ damage, birth defects, and immune system disorders.

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 in different ways to create substances with varying chemical properties. Some molecular contaminants found in soils are pesticides (dieldrin, chlordane, glyphosate), fuels (diesel, gasoline), and byproducts of industry (PCBs, dioxin). Polycyclic aromatic hydrocarbons (PAHs), a group of chemicals formed during the incomplete burning of coal, oil, gas, wood, garbage, or other organic substances, can be found at former residential properties as well as commercial and industrial properties from fires or combustion processes. PAHs stick to soil particles and are found in coal tar, crude oil roofing tar, wood smoke, vehicle exhaust, and asphalt roads. Sites previously used for parking may have high concentrations of petroleum from leaking oils and fuel, and gas stations may have had leaking underground storage tanks that can cause contaminated groundwater and soils, or poor indoor air quality. Even greenspace or agricultural uses may have hotspots from over-fertilized ground, pesticides, or animal feed spills.

Mycoremediation, bacterial remediation and compost bioremediation are the most appropriate methods for treating molecular contaminants. The natural metabolic processes of bacteria and fungi are capable of breaking the molecular bonds of contaminants, making them 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.

Sanborn mapIdentify Previous Use — What is the history of your proposed site?

Maps and Photographs — One of the most valuable sources of land use information is fire insurance maps made and published by the Sanborn Map Company. These maps are detailed and beautifully illustrated, and at a scale of 50-feet-to-one-inch they show building footprints, gas lines, underground storage tanks, pipelines, prevailing wind direction, railway corridors, and other information for some 12,000 U.S towns and cities starting in 1867 and continuing to the present. Perhaps the most important features to locate on these maps are the drains, where facilities released effluent that may have contained heavy metals, solvents, and other contaminants from production processes. No other published maps show such detailed urban land use information.

Historic Sanborn maps can be accessed in a number of ways. They are typically found in the archives and special collections of city halls or in public and university libraries. Most Sanborn maps have also been digitized by Environmental Data Resources, and can be searched online through latitudinal and longitudinal coordinates for a fee. See http://www.edrnet.com/environmental-services/sanborn-maps.

Changes in land use can also be detected through aerial and historic photographs. The oldest available aerial photography dates back to the 1920s, and the most common sources are the U.S. Geological Survey’s Urban Dynamic Research Program, state natural resources and transportation departments, and regional, county, and city planning agencies. In addition, there are numerous commercial aerial photography studios that have large archives, but their rates are high compared to government agencies.

New technologies, however, make it easier to access historical images. The “time slider” feature in Google Earth allows one to compare satellite images of a city’s built environment at different points in time. Currently Google Earth has made images available from the mid-1970s to the present, though the time period varies with location.

City Directories — City directories can also be used to research past uses of a property. They are not telephone directories, but rather indexes that provide a record of changes in property occupancy at specific addresses going as far back as the late 19th century in many cities. Starting with the most recent directory and working backward, it is possible to develop a list of business operations at single address over decades. One could determine, for example, that a vacant lot that looks suitable for a community garden was previously used as a gas station after having been an auto body shop, or a dry cleaners, or some other use that might have led to soil contamination. One can broaden a search to include business operations on nearby properties if there is reason to believe that contamination from these properties may have migrated onto the target site.

City directories are often overlooked in researching the historical uses of a property, but they show the dynamic nature of urban development—that is, the boom and bust cycles of urban history. They can identify how these broad changes played out at specific addresses. City directories can be found in many major public libraries, as well as state archives.

Environmental Databases — While no comprehensive list of contaminated properties is available, one can search a number of online environmental databases. For example, the Right-to-Know Network’s website—rtknet.org—provides access to site-specific information on chemical and oil spills, as well as the locations of illegal dumping, through the Emergency Response Notification System database (ERNS).

The RTKNet site also links to CERCLIS (Comprehensive Environmental Response, Compensation, and Liability Information System), an EPA-maintained database that contains information on preliminary assessments, potential and actual hazardous waste sites, site inspections, and cleanup activities at thousands of sites across the country. Similarly, EPA’s Resource Conservation and Recovery Act Information System (RCRIS), contains extensive data on hazardous-waste-handler permits and activities, which can be searched by address and or zip code. A wealth of environmental information can be found online at the state level through the state’s environmental protection agency.

Polk City Directory pageHistorical documents as well as environmental databases are key components of a site investigation. But in many cases, there may be limitations or gaps in the historical and regulatory record. One way to address these limitations is to find out about the property from persons who live nearby. Neighbors are likely to have a wealth of knowledge about a potentially contaminated site, particularly if the property was used for unregulated activities, such as midnight dumping, illegal auto repairs, etc. In addition, one can interview local planners, town historians, previous site owners, and others who have some connection with the property.

Perhaps the most critical step in the process is to walk through and inspect the site thoroughly. One often finds conditions not reflected in official records and photographs. The site can be checked for indications of illegal dumping or the burning of garbage. The presence of building rubble, old foundations, backfilled areas, and spots where subsidence has occured all indicate areas potentially requiring further assessment. The property can also be checked for soil staining and chemical and gasoline smells.

Determine Whether Previous Use is High or Low Risk to Site Soil and Water

Once you feel you have an understanding of the previous uses of the site, determine whether that use is high or low risk for agriculture reuses, the likely crops or garden design, and sample the site accordingly. As a rule of thumb, recreational or residential previous uses are typically lower risk while commercial and industrial uses can be considered higher risk.

Perform Sampling

Low risk previous uses like residential areas, green space, traffic corridors and parking areas generally have a narrow band of likely contamination that allows for a basic sampling strategy. High risk uses, like manufacturing or rail yards, open up the possibility of many types of contamination over a wide area of the site, and require a more rigorous sampling strategy.

Not all types of contamination will have the same effect on you as a gardener or on your crops. Research on soil metal chemistry and plant uptake has found that most metals are so insoluble or so strongly attached (i.e. adsorbed) to the actual soil particles or plant roots, that they do not reach the edible portions of most plants at levels which would compromise human health when eaten.

Manage Risks

Perform Clean-Up

If results indicate that the existing soil is not safe for gardening activities and you are planning to plant in-ground, remediation may be necessary. Techniques most applicable for agriculture projects include physical (excavation, installing geotextiles, soil washing or soil vapor extraction) or biological (microbial, phytoremediation, or application of soil amendments).

Many non-remedial options exist for sites with low levels of contamination, or sites with contamination exposure risks which can be controlled by planting above ground, including installing raised beds, gardening in containers, green walls or rooftop growing, and aquaponics.

Each remediation technique has unique benefits and drawbacks. Digging away the contaminated soil and disposing it in a landfill is the most effective technique for removing contaminants but can discard valuable topsoil. This is also the most expensive method, and replacing the contaminated soil with clean, non-industrial fill (that has been sampled for contaminants or has been certified as safe) can be cost-prohibitive to a non-profit gardener or community group. In-situ or on site remediation techniques or biological strategies may take multiple growing seasons or multiple applications, costly monitoring, and maintenance. Even remediation by amending with compost may be more involved than it sounds since composting needs to have preceded growing to create sufficiently healthy soil.

Construct physical controls

  • Build your garden away from existing roads and rail, or build a hedge or fence to reduce windblown contamination from mobile sources and busy streets.
  • Cover existing soil and walkways with mulch, landscape fabric, stones, or bricks.
  • Use mulch in your garden beds to reduce dust and soil splash back, reduce weed establishment, regulate soil temperature and moisture, and add organic matter.
  • Use soil amendments to maintain neutral pH, add organic matter and improve soil structure.
  • Add topsoil or clean fill to ensure the soil is safe for handling by children or gardeners of all ages and for food production. Your state or local environmental program, extension service, or nursery may be able to direct you to providers of “certified safe” soils, or to recommended safe sources for gardening soil.
  • Build raised beds or container gardens

– Raised beds help improve water drainage in heavy clay soils or low-lying areas. They also create accessible gardening locations for many users and allow for more precise soil management.

– Foot traffic should not be necessary in the bed, so the soil does not become compacted and soil preparation in the coming years is minimized.

– Your state or local city agency may recommend using a water permeable fabric cover or geotextile as the bottom layer of your raised bed to further reduce exposure to soils of concern.

– Raised beds can be made by simply mounding soil into windrows or by building containers. Sided beds can be made from wood, synthetic wood, stone, concrete block, brick or naturally rot-resistant woods such as cedar and redwood.

Begin Farming

Whether it is a long-term or an interim use, simply greening a once-blighted or vacant property and improving the soil structure has real effects on the economic and social value of land and community health. It can also reduce the runoff of urban soil, silt and contaminants into stormwater systems by allowing greater infiltration of rain into soils improved with added compost and soil amendments. The ability to grow food or horticultural crops such as flowers or trees on this newly greened area will produce multiple beneficial effects to those who may farm it. Healthy eating, increased physical activity, reduction of blight, improved air quality and improved quality of life are all nearly immediate health benefits from urban agriculture.

 




Phytoremediation:  Using Plants to Clean Up Soils

Thlaspi caerulescens, or alpine pennycress, is a small, weedy member of the broccoli and cabbage family. It accumulates metals in its shoots at astoundingly high levels.

Thlaspi caerulescens, or alpine pennycress, is a small, weedy member of the broccoli and cabbage family. It accumulates metals in its shoots at astoundingly high levels.

Phytoremediation is the use of green plants to remove pollutants from the environment or render them harmless. Current engineering-based technolo-gies used to clean up soils — like the removal of contaminated topsoil for storage in landfills — are very costly and dramatically disturb the landscape. But the “green” technology of using plants to take up heavy metals and radioisotopes can, in certain situations, provide a more economical approach and one that is less disruptive as well.

Certain plant species — known as metal hyperaccumulators — have the ability to extract elements from the soil and concentrate them in the easily har-vested plant stems, shoots, and leaves. These plant tissues can be collected, reduced in volume, and stored for later use. In addition, of course, while acting as vacuum cleaners these unique plants must also be able to tolerate and survive high levels of heavy metals in soils — like zinc, cadmium, and nickel.

We are at the early stages of identifying and learning about the transport and tolerance mechanisms of these plants. One for instance, Thlaspi caer-ulescens, or alpine pennycress, is a small, weedy member of the broccoli and cabbage family and thrives on soils having high levels of zinc, nickel, cobalt and cadmium. Researchers have been studying the underlying mechanisms that enable T. caerulescens to accumulate excessive amounts of heavy metals.

Plants Which Can Clean Up Various Contaminants

Plants Which Can Clean Up Various Contaminants

It apparently possesses genes that regulate the amount of metals taken up from the soil by its roots and deposited at other locations within the plant. These genes govern processes that can increase the solubility of metals in the soil surrounding the roots as well as the transport proteins that move metals into root cells. From there, the metals enter the plant›s vascular system for further transport to other parts of the plant and are ultimately depos-ited in leaf cells.

Thlaspi accumulates these metals in its shoots at astoundingly high levels. Where a typical plant may accumulate about 100 parts per million (ppm) zinc and 1 ppm cadmium, and would be poisoned with as little as 1,000 ppm of zinc or 20 to 50 ppm of cadmium, Thlaspi can accumulate up to 30,000 ppm zinc and 1,500 ppm cadmium in its shoots, while exhibiting few or no toxicity symptoms.

Whereas in normal plants zinc transporter genes, for instance, appear to be regulated by the zinc levels in the plant, in Thiaspi some sort of mutation has enabled these genes to stay maximally active at all times, independent of zinc levels until they are raised to very high concentrations.

Fortunately, zinc, nickel, cobalt and cadmium are metals that can be economically extracted from the shoots of Thiaspi, providing a viable process for removing these metals as soil contaminants. The crop would be grown as hay, and the plants cut and baled after they’d taken in enough minerals. Then they’d be burned and the ash sold as ore. Ashes of alpine pennycress grown on a high-zinc soil in Pennsylvania yielded 30 to 40 percent zinc — which is as high as high-grade ore.

Phytoremediation and Radioactivity

Phytoremediation has also shown promise as a means of dealing with radioactive elements. For soil contaminated with uranium, studies have found that adding the organic acid citrate to soils greatly increases both the solubility of uranium and its bioavailability for plant uptake and translocation. Cit-rate does this by binding to insoluble uranium in the soil. With the citrate treatment, shoots of test plants increased their uranium concentration to over 2,000 ppm — 100 times higher than the control plants.

The radioactive element cesium-137, with a half-life of 32.2 years, was released during the cold war by aboveground nuclear testing. Large land areas are now polluted with radiocesium and the costs of cleaning up just Department of Energy sites in the United States with engineering technology has been estimated at over $300 billion.

Amaranthus retroflexus is highly effective in removing radiocesium from soil.

Amaranthus retroflexus is highly effective in removing radiocesium from soil.

Fortunately, phytoremediation is an attractive alternative to these approaches. Studies showed that the primary limitation to removing cesium from soils with plants was its bioavailability. The form of the element made it unavailable to the plants for uptake. But now it appears that the ammonium ion is effective in dissolving cesium-137 in soils. This treatment increases the availability of cesium-137 for root uptake and significantly stimulates radioac-tive cesium accumulation in plant shoots.

And one plant species, a pigweed called Amaranthus retroflexus, has been found to be highly effective in removing radiocesium from soil. Researchers were able to remove 3 percent of the total amount in just one 3-month growing season. With plantings of two or three crops annually, the plant could clean up a contaminated site in less than 15 years.

Aluminum Tolerance in Acid Soils

Aluminum is the third most abundant element in the Earth’s crust and is a major component of clays in soil. At neutral or alkaline pH values, aluminum is not a problem for plants. In acid soils, however, a form of aluminum — Al+3 — is solubilized into a soil solution that is quite toxic to plant roots. Well over half the world’s 8 billion acres of arable land suffer from some degree of aluminum toxicity, including 86 million acres in the US.

Arabidopsis thalianaStudies have found that, in aluminum toxicity, the root tip is the key site of injury, leading to inhibited root growth, a stunted root system, and reduced yields or crop failures from decreased uptake of water and nutrients. In some plants, however, aluminum triggers the release of protective organic ac-ids from the root tip into adjacent soil. When released, these acids form a complex with the toxic aluminum, preventing the metal’s entry into the root. Wheat and corn, for instance, tolerate aluminum by excluding the metal from the root tip.

Researchers looking for better aluminum tolerance are studying Arabidopsis thaliana, or thale cress, a diminutive, weedy member of the mustard fami-ly. Mutants have been found which are quite aluminum tolerant and those mutations are being analyzed for genes conferring aluminum tolerance. It should then be possible to improve the tolerance of relatively aluminum-sensitive crop species, such as barley, or to further enhance the tolerance of existing aluminum-tolerant germplasm.