The county tax assessor’s office and the local city hall are important sources of tax records and permits that have been obtained for the property, which can help uncover past uses. Examples of prior uses of sites that may have caused soil contamination are parking lots, junkyards, auto repair or painting, carpentry, machine shops, dry cleaners, gas stations, railroad yards, and illegal dumping. The history of a site will help to determine what kind and how much soil testing is necessary. A site that has been primarily residential or used as green space is generally lower risk. A site that has had past industrial or commercial uses should be more carefully analyzed.The U.S. Environmental Protection Agency finding recommends that for urban areas, “at a minimum, the soil test should include pH, percent organic matter, nutrients, micro-nutrients, and metals, including lead” finding. This level of testing is adequate for a site that has been residential or green space. Most commercial soil labs can test for the most important heavy metals, including lead, arsenic, cadmium, chromium, and nickel. More testing may be appropriate for a site with a history of industrial or commercial use, which might include CAM-17 testing. finding An EPA-recognized laboratory is the best choice for this level of testing.It is possible that other types of tests may be necessary, such as testing for PAHs finding, a class of potentially toxic byproducts of incompletely burned garbage, oil, wood, coal and other organic materials. They can accumulate in soils and become a concern on a site that has been used previously as a car wash, parking lot, road and maintenance depot, or vehicle service station. Staff members at soils labs can be great sources of information. They are generally willing to talk on the phone about appropriate testing based on site history. However, there are instances where additional support may be necessary.According to the California Department of Toxic Substance Control finding, “brown fields are properties that are contaminated, or thought to be contaminated, and are under-utilized due to perceived remediation costs and liability concerns” finding. In cases involving brown fields that were formerly industrial or manufacturing sites, old gas station sites, and other situations, expert assistance may be necessary: these sites may have cleanup issues beyond what a community project can accomplish without technical expertise and a significant budget. These sites may be too expensive to test and remediate; help may be available, though, through state and local brownfield programs.
California DTSC oversees a voluntary cleanup program for brownfield sites. Some cities have brownfield programs that may be able to provide guidance, resources, and perhaps even help in securing funding for cleanup. Although urban agriculture is not yet a common reuse for brownfield sites,vertical tower for strawberries it is an area for further exploration. Local offices of the USDA Natural Resources Conservation Service finding may also be able to provide technical assistance and guidance on a case-by-case basis.Laboratories generally provide an online instruction sheet on how to sample the soils, prepare the samples, and mail them to the lab. Once you have selected a lab, review its website for instructions or speak to a staff member by phone. Detailed instructions on collecting soil samples are also available in several of the resources listed in the section “Sources of More Information.” Review these instructions carefully, since poorly collected or unrepresentative soil samples may not provide accurate information. Generally, it is essential to approach soil testing with a plan in mind. Making a simple map of a proposed site and noting areas with different characteristics helps you decide how many samples to collect.Most labs are receptive to questions regarding interpretation of test results. Soil testing will indicate whether plant nutrients are low and need to be raised for best plant growth and whether the soil pH needs to be adjusted. The soil test can also indicate whether there are higher than acceptable levels of heavy metals or other contaminants, depending on what tests were requested.Heavy metals are the contaminants most commonly tested for, and there is more guidance available about heavy metals, especially lead, than for many other contaminants. Even so, there is no one standard as to what constitutes safe levels of heavy metals in urban agriculture. Most guidelines that exist are created for residential use scenarios and contact with soil through skin exposure or accidental ingestion, rather than being formulated for gardening or farming. Still, they provide some guidance about what is acceptable. One standard to consider is the California Human Health Screening Level finding, which is used by the City and County of San Francisco and the City of San Jose in their guidance on lead hazard assessment for urban gardens and farms.
For lead, the HHSL is 80 parts per million finding. Anything lower is considered below the level of concern for human health. Another standard is U.S. EPA’s Soil Screening Levels finding for residential use, which consider lead to be a hazard at levels of 400 ppm or more. In 2013, the U.S. EPA Technical Review Work group finding for Lead provided a bit of clarification specifically for gardening and deemed soil lead at less than 100 ppm to be low risk for home-grown produce.The U.S. EPA TRW chart with specific, tiered soil lead recommendations for gardening is available in table 2. These standards are advisory only; check with local municipalities finding to find out whether more specific requirements have been established. Two related products can be considered for use in improving existing garden soil. Biochar is a charcoal product that when mixed with soil adsorbs heavy metals and binds them tightly, making them less available in the soil. It has other potential benefits, including holding water and releasing plant nutrients. The second product, activated charcoal, is sometimes mixed with soil to adsorb contaminants including certain pesticides, herbicides, and petroleum products. Both can raise soil pH, which could pose a problem if soil is already alkaline. See the section “Sources of More Information” to learn more about these materials before applying them.The most cautious strategy, perhaps most likely to be considered when dealing with a brownfield site, is to remove existing soil from the growing area and replace it with soil that is certified safe. However, this expensive procedure is out of the range of what most community groups or individuals can afford. Contaminated soil must be disposed of according to law, which can be expensive, in addition to the costs involved with soil excavation, removal, and replacement. A more common approach is to build raised beds and fill them with clean soil. A study of backyard, school, and community garden soils in San Francisco showed that raised bed gardens had significantly lower levels of arsenic, cadmium, and lead than in-ground gardens finding. Another study, conducted in Chicago, measured significantly less lead in raised bed gardens than in-ground gardens finding.To create raised beds, urban farmers can build frames of redwood or other lumber, brick, concrete, rocks, or any other sturdy material that won’t leach contaminants into the soil. Treated lumber should be avoided since lumber treatments may cause copper or other metals to leach into the soil. Water-permeable fabrics can be applied as a barrier between the on-site soil and the imported soil used to fill raised beds. Landscape fabric is permeable to liquids and air, unlike black plastic. Landscape fabrics are made from various materials, including non-woven polypropylene, woven fabric, biodegradable paper mulch, or flexible geotextile fabric. Once the frame has been lined with landscape fabric it should be filled with “clean” soil.
One way of ensuring that soil is clean is to purchase topsoil or planting mix certified by the Organic Materials Review Institute finding. OMRI performs an independent review of products intended for use in certified organic production, handling, and processing. OMRI reviews producer-submitted products against the National Organic Standards and generates a list of acceptable products.Local urban farmers or gardeners may have recommendations on potential sources of quality soil. Some vendors may be able to provide documentation that soil has been tested, and if not, a sample of the soil could be sent to a lab for testing to be sure that it is not contaminated.Other types of containers besides raised beds can be used in urban agriculture. For example, some commercially available growing systems are self-watering, for smaller-scale projects. Others are mesh “socks” that are filled with growing medium and placed on top of the soil. These types of “instant gardens” may be especially appropriate for short-term projects. Although tires are sometimes used as planters, this practice is not recommended, since tires can contaminate soil with leached metals as they degrade over time.Many sites have created successful urban agriculture projects through depaving. For example, in 2006, volunteers removed 5,000 square feet of asphalt from the grounds of Carthay Center Elementary School in Los Angeles and created a thriving garden for the entire community. To improve the hardened clay soil, tons of soil, mulch, and compost were donated and tilled into the soil by volunteers. The garden consists of raised beds and in-ground beds, as well as a stone fruit orchard, citrus orchard, tropical garden, butterfly garden, and poetry garden. If a site plan calls for depaving, test for heavy metals and other contaminants prior to asphalt removal. To test soil beneath asphalt, cut triangular holes in the asphalt with a hand-held concrete saw to expose the soil. Then remove the asphalt triangles, along with any sub-grade debris or stones, and obtain the samples. Replace the pavement triangles and add sand or pea gravel afterward to eliminate a tripping hazard. If soil tests show heavy metals above recommended levels, consider raised beds or large containers rather than removing the asphalt.As urban farming in all its forms becomes more popular, municipalities are beginning to consider a variety of policies related to land use for this purpose, including requirements for soil testing and remediation. Generally,vertical growing the decision faced by municipalities is whether to require soil testing and a remediation plan where appropriate, or to make recommendations and provide educational materials on best practices. Requiring soil testing can create challenges for municipalities if they do not have staff available and qualified to review and evaluate soil test results and remediation plans. Some municipalities require soil testing for contaminants finding for community gardens. Anyone contemplating establishment of such a garden or farm should check with the local city or county environmental health department, parks and recreation department, or other appropriate agency. Municipal decision makers should ensure that policies do not create insurmountable barriers for urban agriculture.
Many community groups pursuing community gardens or other urban agriculture projects have limited funds to conduct extensive site analysis for contaminants and perform any needed remediation. At the same time, contamination of urban soil is an important environmental health consideration, yet it is possible to have urban agriculture that is both safe and cost-effective. Soil testing is extremely important and should be facilitated and encouraged by municipalities. Ideally, this would include subsidizing the cost of soil testing and providing assistance with interpretation of soil tests and development of simple remediation plans. Minimally, it would involve having a list of best practices that urban farmers would agree to implement at their site. Education is important in every case. Many best management practices, such as adding organic matter and managing soil pH, are important strategies for ensuring safe soils but are not practical to handle via policy. Cities and counties should explore forming partnerships with the local UC Cooperative Extension office to provide educational resources and training on soil management for urban agriculture. UC Cooperative Extension has trained Master Gardener volunteers available who may be able to help provide education at the local level. Cities should also foster a connection between their own brownfield program, if one exists, and urban farmers. Municipal brownfield programs should be encouraged to work with urban farmers to identify potential sites and support testing and remediation for urban agriculture projects. This strategy is being used successfully in several U.S. cities, including Milwaukee, Wisconsin, and Kansas City, Missouri.Cround water is a critical resource for California water management. Stored in aquifers, water from rainy seasons can be used during dry and hot summers and supports water users through droughts if it is replenished in wet years.