لخّصلي

Clean-up of lead-contaminated sites: The Ontario urban
clean-up experience

The sources of lead to humans and to ecosystems are numerous: the toxicology of the metal exemplifies the need for a multimedia approach to the study and management of lead exposure. Nevertheless, elevated blood lead, which is the primary exposure indicator for vertebrates, especially for humans, has implicated lead in soil as a major immediate contributor. Whatever its origin, be it airborne lead from automobile exhaust, deposition from smelter emissions, dust from commercial or industrial processes, or flaking paint, lead in soil in urban areas has been seen as a threat to the health of humans, particularly for young children. The present case study addresses the clean-up of lead-contaminated soil in a articular
area of metropolitan Toronto, Ontario, Canada. Emissions from two secondary lead smelters, which processed and essentially recycled lead from batteries, pipes, and other waste materials, had contaminated the soil of residential land (Roberts et
al., 1974). Studies revealed concentrations of total lead in surface soil as high as
11,500mgkg-1 (Temple, 1978) in residential areas and up to 51,000mgkg-1 on industrial
land (Rinne et al., 1986).
In 1974, the Ontario Environmental Hearing Board was directed by a provincial
Order-in-Council to hold public hearings on lead contamination in the metropolitan
Toronto area. Soil replacement was recommended by this Hearing Board, for levels
at or above 3,000mgkg-1 of total lead on residential properties. Recommended
trigger concentrations for lead in the literature, in fact, range from 150mgkg-1 as a
546 Recovery, rehabilitation, and reclamation
level requiring “further investigation” to 600mgkg-1 as an “indication for clean-up”
(Moen et al., 1986) and a Toronto working group on lead had recommended a cleanup
level of 1,000mgkg-1. A final guideline of 2,600mgkg-1 was arrived at for the
smelter sites, based on a confidence interval for soil lead of ±13% (i.e., 2,600).
Based on this guideline, 67 sites, including frontyards, backyards, and laneways,
on 48 residential properties in the vicinity of one of the secondary smelters underwent
soil replacement during 1977 and 1978. The top 15cm of soil was replaced by
clean soil (with lead concentration no more than 7mgkg-1) at a cost of CAN$67,000.
There was considerable disruption and damage to gardens and fences for the residents
during this period. The company, while not admitting liability, donated a sum
for lead research to the University of Toronto. Clean-up would be useless if the
problem that contaminated the soil in the first place were not abated. Subsequent to
the discovery of the lead in soil problem, the company was in compliance with standards
for lead in air.
Following the soil replacement, lead in soil was determined with the following
results.
80% of the treated sites had lead