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Sediments
Sediment is material that, because of its weight, settles at the bottom of a
liquid. Commonly, the word is used to describe the dirt, silt, soil, or polluted
material that settles at the bottom of a lake, river, or stream.
The Issue
Sediments are the habitat for macroinvertebrates or bottom-dwelling species at
the lower end of the aquatic food web. When sediment becomes polluted, it
contaminates macroinvertebrates, fish, birds, and mammals, including humans, who
consume fish and waterfowl. Sediment pollution in one area can also be a
continuing source of further pollution downstream unless it is cleaned, removed,
or controlled.
Condition of Concern
The condition of the St. Clair River has been documented since the 1950s.
Studies in 1968, 1977 and the mid-1980s revealed highest concentrations of
polluted sediments are within 100 meters (328 feet) of the Ontario shore near
industrial discharges. Evidence indicated that sediment-contaminating substances
originated in both Ontario and Michigan.
Evidence of Concern
Studies in 1957 showed that bottom sediments along the Ontario shoreline were
moderately to severely contaminated with a variety of chemicals, which were
attributable to industrial activity. The zone of sediment contamination
decreased from the entire length of the St. Clair River 64 kilometers (40 miles)
in 1968 to about 20 kilometers (12.4 miles) in 1977, 12 kilometers (7.4 miles)
in 1985, and 6 kilometers (3.75 miles) in 1990. These improvements are
attributed to fewer discharges and natural remediation, which includes
biodegradation (biological decomposition), being covered by new sediment from
upstream, or movement of polluted sediments to downstream locations.
Studies along the Michigan shore in the 1980s showed sediments
with:
• Excessive
arsenic, chromium, iron, nickel, and manganese downstream from Pine River;
• Moderate
concentrations of oil and grease adjacent to Port Huron and Marine City;
• Heavy
concentrations of copper and iron in 23 sampling locations;
• Heavy pollution
at two sites immediately downstream of the Canadian National Railroad tunnel.
Sources in the St. Clair River have contributed to elevated
sediment concentrations of hexachlorobenzene, octachlorostyrene, PCBs and
mercury in Lake St. Clair.
Goals and
Strategies
Using information from these studies, BPAC identified six goals and strategies
for sediment remediation:
• Identify and
control contamination sources
• Delineate type
and extent of contamination (that is, concentration and total volume)
• Evaluate
potential for natural restoration
• Determine
costs, funding sources, and permits required to remove and dispose of sediments
• Determine the
physical and chemical conditions of cleanup areas relative to available
remediation options
• Monitor areas
before and after cleanup to evaluate cleanup effectiveness
While each of these six steps is critical, the first –
identification and control of contamination sources – is most critical. Failure
to eliminate the source of pollution before initiating a clean-up program could
result in recontamination.
Remedial
Measures
Four remediation options or technologies are typically available to clean up
contaminated sediments:
1.
Natural Remediation (No Action) – Allow time and natural river
processes – decay, biological decomposition, coverage by clean sediments, and
downstream transport – to reduce negative environmental effects.
2.
Contain Sediment In Place (Capping) – Use cleaner sediments or
a synthetic cap to cover moderately contaminated sediments and prevent contact
with aquatic organisms and further movement of contaminated particles.
3.
In-Situ Treatment – Inject chemicals into contaminated sediments to
stimulate biodegradation of contaminants, converting them to a less toxic form,
and solidify or stabilize the sediment.
4.
Removal (Dredging) – Physically remove the contaminated
sediment by means of mechanical buckets, centrifugal pumps, or entrained air or
water. Dredged contaminated sediments must be disposed in a specially
constructed Confined Disposal Facility (CDF) or other suitable treatment.
Current
Conditions
As a result of ongoing efforts and naturally occurring changes, contaminated
sediments are now limited to locations in the upper part of the St. Clair River
adjacent to historical industrial discharge points.
A remedial project at the Dow Scott Road landfill and the Scott
Road and Cutoff drains eliminated the last remaining discharge point of
contamination in the St. Clair River. This project was a prerequisite for
subsequent sediment remediation.
Studies by the Ministry of the Environment and Energy (MOEE),
Environment Canada (DOE), and the Sarnia Lambton Environmental Association (SLEA)
(formerly the Lambton Industrial Society, from 1994 to the present, identified
the most contaminated sediments as being located in three priority zones.
In June 2000, as a result of historical discharges from their
facilities, Dow Chemical Canada Inc. engaged MOEE and DOE to develop a plan to
remediate contaminated sediments adjacent to their manufacturing complex on the
banks of the St. Clair River. Based on detailed sediment sampling and evaluation
of remedial technologies, Dow Chemical determined that it would hydraulically
dredge approximately 28,000 cubic meters of sediments in an area 3000 feet (925
m) long and 125 feet (39 m) wide. This activity is occurring in three phases:
·
Phase 1 – A pilot project to assess the removal and treatment technology was
completed in June 2002.
·
Phases 2 and 3 occurred in 2002 and 2003, successful completion of the project
was achieved early in 2004.
Two other priority
sediment zones, located in the upper St. Clair River, will be subject to a risk
assessment to determine the need for remediation.
Accomplishments
Studies in the 1990s indicate that concentrations of contaminants in sediments
have decreased over the past 20 to 30 years. Significant reductions in mercury,
lead, oil, grease, and PCBs are the result of changes in industrial processes,
additional wastewater treatment, improved housekeeping operations, and spill
prevention initiatives. The natural processes within the river have also led to
improvement of its sediment.
Future Endeavors
Successful management of historically contaminated sediments is a key remaining
priority in the St. Clair River Remedial Action Plan. This action will result in
improved water quality and progress toward achievement of RAP goals.
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