Assessing Urban Impacted Soil for Urban Gardening
Urban gardening is gaining momentum in North America. Urban gardening can provide broad health, environmental, social and economic benefits.
Often the land available for increasing the urban land base for community gardening are lands that are vacant, abandoned, or previously used for purposes other than food production. Despite a growing interest to garden on these lands, previous and current activities on or next to these sites might have resulted in con-tamination of the soil.
This guide is a decision-support tool used to identify areas that may be contaminated but could be suitable for food production and to identify appropriate ex-posure reduction actions based on the condition of the site.
Step 1 – Establish a Level of Concern
The initial step of the guidance is to assess the likelihood that the soil quality for a garden may be of concern due to contamination from past activities. The appropriate Level of Concern is identified by conducting a site visit and researching the land use history to determine if various indicators are present.
A site visit is conducted by walking through and inspecting the site thoroughly. The site is walked through and checked for indications of illegal dumping or burning of garbage. The soil is turned over with a shovel in the areas intended for gardening and checked for soil staining (discoloration, usually dark patches) and odors (chemical and gasoline smells).
A site history is researched by searching available city records and asking local neighbors for information about the past and current use of the site and adjacent properties.
Each indicator is associated with a level of concern. The indicator of greatest concern defines the level of concern for the site as a whole. The table to the upper right lists the various indicators, the appropriate Level of Concern, and the recommended next steps for the garden site.
For sites that have been characterized as Medium Concern, go to Step 2. For all other gardens, go to Step 4.
Step 2 – Sample and Test the Soil
If the planned garden on a Medium Concern site is larger than 13 by 13 ft, the soil should be tested to determine the concentrations of soil contami-nants. The cost of a raised bed garden of this size is less than soil sampling, thus it is not cost effective to conduct soil testing for gardens that are smaller than this size. We recommend that small gardens in the Medium Concern category go to Step 4. For larger gardens, the depth of soil to be sampled is 0 to 40 cm.
We developed the above streamlined list of contaminants of concern (COCs) for the Medium Concern sites. The cost to analyze each composite sam-ple for all the parameters listed is approximately $250. The number of required composite samples is determined by the size of the garden. For a community garden 1 to 2 samples covers 225 to 450 m2, respectively. The average community garden is 280 m2. Thus, most community gardens will require 2 samples at a cost of approximately $500.
If the indicators identified during the site visit and site history suggest that the soil might be contaminated by other soil contaminants not on our stream-lined list of COCs, then the site should be treated as a site of High Concern (Go to Step 4).
Step 3 – Interpret the Soil Tests
In Step 3, the Exposure Reduction Tier for the garden is determined by comparing the soil concentration of each COC with the Soil Screening Values (SSVs) on page B-8.
The SSVs define the three risk levels, and are used to interpret the soil test data as follows:
• If the concentrations of all of the COCs are below the respective SSV 1, then the site requires Tier 1 Exposure Reduction;
• If the concentration of any COC is above the SSV 1 but does not exceed the SSV 2, then the site requires Tier 2 Exposure Reduction; or,
• If the concentration of any COC is above the SSV 2, then the site requires Tier 3 Exposure Reduction.
Step 4: Mitigate the Risks
There are many simple and inexpensive actions gardeners can easily take to reduce their exposure to urban soil contaminants depending on the risk level for the site. The illustration on page B-8 summarizes the recommended exposure reduction measures for the gardens that are required for Tier 1, 2 or 3 Exposure Reduction.
Through regular gardening practices gardeners already do many of the activities outlined in Tier 1 and 2 Exposure Reduction risk levels. For example, gardeners add soil and organic matter to their gardens on an annual basis to improve the yield of their garden. These behaviours, year after year, re-sult in a reduction in both the concentration and bioavailability of soil contaminants. In addition, gardeners turn over their soil at least twice a year, aer-ating their soils and exposing deeper soil to sunlight (two mechanisms that degrade and reduce organic soil contaminants). These practices over many years significantly reduce the concentration and the bioavailability of soil contaminants.
Existing gardens on lands that are in the Low Concern category should continue to use Tier 1 Exposure Reduction measures. Existing gardens in the Medium Concern category should use Tier 2 Exposure Reduction measures, with the exception of avoid or restrict growing produce. There is no need to test the soils. Existing gardens in the High Concern category should follow the soil testing indicated for Medium Concern sites.