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Sediment Remediation

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CN evaluated numerous remedial alternatives to remove the impacted sediment from the bottom of the Makami River. CN determined the area requiring active remediation from the conclusions presented in the Comprehensive Ecological Risk Assessment (ERA). The remedial alternatives were evaluated to determine which method would be the most effective, most easily implemented, and least intrusive to the natural environment. Minimizing disturbance to existing fish habitat was a critical consideration in selecting the most appropriate method. The substrate at the bottom of the Makami River is a mixture of large deposits of fine-grained sediment and sporadic collections of cobbles and boulders. As the oil components are more associated with fine-grained sediment, it was critical to maximize removal of the fine-grained sediment, while leaving the cobbles and boulders in place to maintain the shape of the river channel and to minimize the disturbance to the aquatic habitat. The most effective way to remove the sediment was using two different hydraulic vacuum methodologies in series in wet conditions. This phased approach maximized the removal of the fine particles where the oil was entrained, while maintaining the shape of the river channel and minimizing the disturbance to fish habitat. All other evaluated remedial methodologies were very difficult to implement and would have been detrimental to fish habitat.

The sediment removal work was conducted from August 19 to October 7, 2015. The first hydraulic vacuum unit was a barge-mounted unit designed to remove large deposits of fine-grained sediment. Once the first hydraulic vacuum unit was finished, a smaller diver-operated vacuum unit removed the sediment from in between large boulders and cobbles in the river channel. The final in-stream remediation works included excavator removal of remnant fill material leftover from the temporary rail bridge that could not be removed using the vacuum units. Sediment samples were collected and analyzed throughout the sediment removal activities. At this stage, localized areas requiring further sediment removal were identified.  Based on the depth and location of the deposits, an excavator was used to remove these sediments.

During the sediment removal, the sediment-water mixture was vacuumed from the bottom of the river and pumped to large bags called GeotubesTM, staged in two containment areas. The bags allowed the water to slowly seep out, leaving only dry sediment in the GeotubesTM. The sediment was placed in trucks and shipped to the disposal facility in Cartier. The water from the bags was pumped to a treatment unit where oil components were removed. The treatment of this water was completed under an Environmental Compliance Approval provided by the Ministry of Environment and Climate Change (MOECC). The treated water was tested regularly to ensure that it was suitable to discharge to the natural environment, and all sample results were provided to the MOECC. The treated water was discharged to the Makami River downstream of the Old Gogama Road Bailey Bridge. 

Once the sediment removal was complete, confirmatory sediment samples were collected. Results from these samples indicated that the remaining sediment did not pose an unacceptable risk to ecological receptors and that no additional sediment remediation was required.

After the sediment removal was complete, boulders, cobbles, and pea gravel materials were placed in the deepest sections of the river bed to restore the pre-derailment channel shape. These materials were selected as they are suitable for the aquatic species in the area. Through natural channel processes, new clean sediment from upstream areas of the river will naturally accumulate in this area over time in a similar way to the pre-derailment condition. 

Prior to commencing sediment removal activities, sediment control measures were installed within and downstream of the work zone to prevent disturbed sediment from moving downstream. CN collected turbidity readings within the river water (indirect measurement of stirred up sediment) throughout the sediment remediation work to evaluate whether the sediment control measures were working as designed. Sediment control measures were adjusted or added throughout the work as needed.