Soil assessment for urban agriculture: a Vancouver case study

Introduction

Urban agriculture is becoming more prevalent in North American cities. With the rise of urban farming comes the need to evaluate the soil and the urban environment to assess potential risks of metal contamination due to pollution in urban-grown food. Community gardens and urban farms are often established on “brownfields,” which are abandoned lots where past industrial and commercial activity have potentially caused contamination by metals or other chemical products. These areas also usually experience high levels of pollution due to industrial activity and traffic density, which contribute to metal contamination as well. 

Metals tend to remain for soils in long periods of time because they are not easily washed away by water or taken up by plants. Metal compounds naturally occur in soils, but can also be deposited into soils through metal waste and air pollution. Exposure to heavy metals can be very damaging or even lethal. Humans are easily exposed to toxic metals by touch, ingestion and inhalation, such as while working in an urban farm, or by consuming crop plants that have become contaminated while growing. 

This study also sought to provide recommendations for assessment of urban farm sites, to reduce the risk of metal contamination.

Research Process

From October 2011 to February 2012, the researchers assessed metal soil contamination in different established or potential farm sites in Vancouver. They measured how much metal was originally in the soils and how much each site accumulated over the course of several months, and then looked at how much metal was transferred into the vegetation growing in these three sites. 

The researchers chose three study sites in Vancouver which varied greatly in history and activity: 

    1.    The UBC Farm: a 24 ha university research site located on the South Campus of UBC. The farm has been managed for the past 40 years and the region has experienced large-scale development in the past decade. 


    2.    The 16 Oaks community garden: established in 2008, prior to which the site was vacant for approximately 10 years. This site experiences high levels of traffic and borders a residential area.


    3.    The Hastings site: a brownfield site; this lot has been vacant for the past forty years. Prior to vacancy the site served as a scrap metal yard. Sand, gravel and metal-containing sand fill has been added in attempts for remediation.


The researchers looked at five metals as indicators of contamination: zinc, lead, nickel, magnesium and copper. 

Results

The total metal concentrations were highest at the 16 Oaks community garden site and lowest at the UBC Farm. This is likely due to historical and current differences in activity and traffic levels between the two locations, as the UBC Farm has lower traffic volume with no industrial history while the community garden is situated in a high-traffic neighbourhood. Atmospheric metal deposition levels from the air and rainwater were also lowest at the UBC Farm and highest at the Hastings brownfield. Researchers attributed this to the influence of traffic and history of metal waste at the brownfield. 

Nickel and magnesium were associated most with the original “parent” soil (not from the added topsoil) while zinc, lead and copper seem to be most associated with human activity, including vehicle traffic and industrial activity. Levels of nickel and magnesium were found at near-normal levels across all sites. The UBC Farm had close to normal levels of all five metals tested. The 16 Oaks community garden site had higher levels of zinc and lead, and the brownfield had higher levels of zinc, lead and copper. Traffic, metal waste and industrial by-products all contribute to elevated metal concentrations in urban soils. These findings display the significance of atmospheric deposition in metal contamination. 

Metals were found in higher concentrations in the shoots of the Kentucky bluegrass samples than the roots, and the levels at which they were found reflected that of the soils they grew in. This suggests that growing plants in soils contaminated with metals can cause the bioaccumulation and translocation of metals into above-ground (shoots) plant parts, which could have negative impacts on human health. 

This study revealed three factors to be relevant for urban soil metal assessment: site history, what metal deposits affect the site, and the original soil or parent material of the site. 

The study proposes a three-stage, holistic approach to assessing potential urban farm sites: 1. Observing the site, looking at city archives and speaking to community members. Refer to guidelines for assessing site history created by other Canadian cities (Toronto and Montreal). 2. Evaluate the sites posing medium to high levels of concern, including physical testing of the soil. 3. Metal concentrations found should then be evaluated according to provincial or municipal regulations.