Introduction
For decades, removing contaminated sediments through dredging has been restricted or prohibited in North America. The major concern has always been that dredging would resuspend the contaminated sediment in the water, spreading contamination and causing environmental impacts across even larger areas. Using dredging technologies proven effective in Europe for several years, recent work in Canada has demonstrated that dredging of contaminated sediments can now be safely undertaken. The success of this work opens the door to removing restrictions on hydraulic dredging in North America and presents opportunities for cleaning up contaminated sediments adjacent to industrial sites. It also greatly increases the flexibility of timing options for navigational maintenance dredging in environmentally sensitive areas.
What has changed?
In 2004, dredging commenced to remove approximately 50,000 cubic metres of PCB-contaminated sediments from a harbour area located near Prince Rupert, on the west coast of Canada. This sediment was contaminated in 1977 when a large industrial transformer associated with pulp mill operations failed and discharged PCB-containing oil into the harbour. At the time of the accident, immediate removal by dredging was prohibited because of concerns of re-suspending and further transporting the PCB-contaminated sediment. The harbour experiences tides of up to eight metres and cross-currents of one to two knots, which further increased this risk.
By 2004, complete removal was possible because of access to low-turbidity hydraulic dredging technology and dredging control systems not available in 1977. These technologies included:
• Detailed three-dimensional computer modelling: Analysis results from over 525 sediment samples, collected from 84 boreholes, were modelled so that sediments could be contoured by PCB concentration.
• Accurate dredge positioning control: The hydraulic dredging pump and auger was mounted on a long-reach Hitachi EX800, which in turn was mounted on a spud-barge. A global positioning system tied to instrumentation on the boom, dredge head, and barge recorded all movements and resulted in dredge head positioning control to +/-10 cm. In combination with sediment character isation and 3-D modelling, the sediment could be extracted in accordance with PCB concentrations, and sediments with hazardous levels of PCBs could be isolated and disposed of separately. This ability to segregate out the sediments with higher concentrations of contamination meant the rest of the sediment could be treated by less costly methods.
• Low-turbidity hydraulic dredging: The dredge pump and horizontal auger attachment, manufactured by Damen, were designed to minimise sediment re-suspension. Hydraulic doors on the auger were used so that only sediment ahead of the dredge was pulled into the auger. The pump could also handle a low water to solids ratio. The combination of dredge head design and low water to solids ratios resulted in localised water currents set up by the dredging operations. The outcome was that an average turbidity of less than five Nephelometric Turbidity Unit (NTU) was generated within five to ten metres of the dredging operations.
• Better sediment and water handling methods: Hydraulic dredging pumps a water and sediment slurry through a pipeline either to a barge or a shore facility. The slurry must be dewatered and the contaminated sediment removed and appropriately disposed of. Many contaminants, such as PCBs, polycyclic aromatic hydrocarbons, and many metals have a high affinity to organic particles. Therefore, designing an efficient sediment settling system can reduce or eliminate the need for expensive water treatment prior to its re-release into the environment. The range of flocculants to facilitate the process and their environmental safety is much greater today than was the case 20 years ago. The work conducted near Prince Rupert used four bottom-draining sediment dewatering cells, three sequential water settling ponds, and one clarifier for water polishing. Flocculent type and addition rate varied according to sediment type and PCB concentration.
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