Golder Removes Contaminated Sediments at Prince Rupert Harbor
Carolina Johansson and Robert McLenehan of Golder Associates were project manager and project director, respectively Neil McQuitty was GAIA project manager.
The site was contaminated in 1977 when a dockside industrial transformer failed, discharging PCB-containing oil into the harbor. Immediate dredging was prohibited because of concerns for re-suspension of the contaminant.
Tides of up to eight meters, cross-current of up to two knots and the need to remove a seven-meter-thick contaminated sediment mass increased the challenges of the project.
Golder proposed low turbidity hydraulic auger dredging as a solution for this site. After rigorous review, this proposal received approval from the regulators, and the dredging of more than 50,000 cubic meters of sediment began.
Fraser River Pile & Dredge Ltd. of New Westminster, BC, was commissioned to carry out the dredging operations, under GAIA's supervision. Fraser River used the clamshell dredge with spud barge R.E. McKenzie and the barge MBD 36. John Smith was Fraser River's project manager and Jason Selgensen was site superintendent.
Because of the tidal fluctuation and currents, a spud barge was used. Because the contaminated sediment body was accurately defined, the spuds could be placed where they would not create a sediment plume upon retraction. The barge was also equipped with cables for position control.
The dredge head was mounted on a Hitachi EX800 barge-mounted excavator with a 72 foot long-reach boom. Power for pump and auger were supplied by an American Piledriving Equipment Model 630 hydraulic power pack capable of providing up to 630 hp (470 kW).
The DOP 2320 submersible dredge pump, with ten-inch suction and discharge was able to pump between 500 and 1,000 cubic meters per hour. Rated at 150 kW (approximately 200 hp), we found it operated well at 30 to 40 percent solids. Keeping the water-to-solids ratio low increases dredging efficiency, and reduces turbidity at the auger head.
The dredge head is made by the Damen Shipyards Group of Gorinchem, Netherlands for use specifically in low turbidity dredge applications. Hydraulic doors on the dredge head were used so that only sediment ahead of the dredge was pulled into the auger. This configuration, in conjunction with accurate position control, controlled dredge operation, and low water-to-solids ratios, resulted in average turbidity readings of less than five Nephelometric Turbidity Units (NTU) within five meters of the dredging operations.
Positioning control was accomplished through contaminant delineation and accurate positioning technology. The delineation program was composed of 84 boreholes and more than 500 samples, collected at various depth intervals, which were analyzed for PCB concentrations. For each borehole, the concentration data were interpolated/extrapolated over the full range of elevations of interest, and extracted at 0.2 m elevation intervals.
The location and concentration data were then summarized for each elevation interval in data tables. The data were input to the program Surfer, to create a regular grid of interpolated data points, and a set of contours of the PCB concentrations. The grid data tables for each elevation interval were compiled into a single 3-D grid data file.
A Trimble 5700 RTK Global Positioning System was used to provide precise positions and real-time tide corrections. A series of digital inclinometers was mounted on the excavator arms to provide angle information needed to solve the complete boom and stick geometry. Another inclinometer was mounted on the dredge head to provide orientation information to the operator. Orientation was obtained from a CSI Vector Sensor system mounted on the excavator roof, using a two meter offset between the antennas.
The achieved horizontal and vertical accuracies for the dredge head positioning were checked by periodic total station surveys, and were checked through a range of tide cycles and excavator arm orientations. In all cases these surveys showed the system accuracies were within five to 10 centimeters for both vertical and horizontal position.
Using a modified version of Dredgepack software, the dredge operator had a real-time computer display showing the position of the excavator, the geodetic elevation of the dredge head, and the geodetic elevation of the targeted slice of contaminated sediments. This allowed for the precise removal and segregation of different levels of contamination.
Continuous-recording turbidity sensors were deployed around the dredge and dredge area. Environmental monitors could instantly see if there were any discrepancies from permitted operating conditions, and then stop, slow or alter the dredging operations until the problem was resolved. This real-time control is a great improvement over the past, when variances could not be detected right away, and environmental effects might not be recorded until several hours or days after an event.
The low turbidity hydraulic auger dredge successfully removed more than 50,000 cubic meters of contaminated sediments from the environment, and work stoppages due to turbidity exceedences were rare. Over 98 percent of the PCB-containing sediment was removed from the work site, leaving a mass of less than one kilogram of PCBs out of an estimated 210 kg released to the environment. Sediment removal rates ranged from 200 to 6,500 cubic meters per week, averaging 2,800 cubic meters. Several factors affected productivity, including obstacles, equipment maintenance and repair, and the deliberate slowing of dredging in areas containing high PCB concentrations.
The success of this project led to its application at a second site in Kitimat, British Columbia. The low turbidity generation observed during the application at the Prince Rupert site led to the quick permitting of this approach in Kitimat. In addition, dredging was permitted to proceed outside of fisheries work windows, during a time period when dredging would not typically be approved.
The result of the application of low turbidity hydraulic auger dredging and associated technologies at these sites in Northern British Columbia has been the successful removal of sediments, the minimization of sediment re-suspension, and for the waters around Prince Rupert, the removal of a serious environmental threat.