News and information for the worldwide dredging industry

Bookmark and Share Email this page Email Print this page Print

Contractor Leaves Spill Site Free of Contaminants

Last Fall, contractor D.A. Collins Environmental removed 3000 cubic yards of contaminated sediment from the Genesee River at Rochester, New York. The cleanup, using Cable Arm's dredge cell method, was so thorough that capping with clean material, as originally planned, was not necessary.

The contamination was caused by a train derailment on December 23, 2001 that occurred approximately 150 feet from the river. A freight train consisting of 43 cars derailed resulting in 27 cars coming off the tracks three of which were tank cars carrying acetone (2) and methylene chloride (1). The spill consisted of 14,000 gallons of acetone and 16,000 gallons of methylene chloride. The accident occurred a mile from the mouth of the river into Lake Ontario.

As a result, the incident contaminated 28,000 tons of soil and 1.4 million gallons of water. These materials were removed and treated within eight months by the train company under an Interim Remedial Measure (IRM) of the Voluntary Cleanup Program (VCP) with the New York Department of Environmental Conservation (NYSDEC).

The spill contaminated sediments in the river adjacent to the spill. Though the water immediately above the contaminated sediments had returned to drinking water quality a month after the accident, about 3000 cubic yards of bottom sediment in an area of about 1/3 acre remained contaminated with methylene chloride.

Due to the isolation of the impacted sediments (24 feet below the surface) and unaffected water quality of the river, a proposal was made to allow the area to clean itself through natural attenuation. But by spring of 2004, the contaminants still remained, and environmental engineering firm AMEC Earth & Environmental was retained to design and manage a cleanup project.

Scientists set up eight sediment-sampling events to measure the impact on the river channel and to evaluate the effectiveness of natural attenuation. Over this time period, the concentrations of methylene chloride had decreased, but remained elevated above background levels.
Based on these results, AMEC prepared and submitted a remedial Action Selection/Design Report to the NYSDEC describing an approach to remove approximately 3000 cubic yards of impacted sediment.

The NYSDEC and U.S. Army Corps of Engineers approved the proposed dredging plan and issued the appropriate permits.
Site mobilization began in early September 2004, and the project was completed in three months. AMEC retained D.A. Collins Environmental to perform the dredging. Collins contacted Ray Bergeron of Cable Arm Clamshell to provide an environmental dredging bucket and ClamVision positioning system. Bergeron shipped a 3.5 cubic yard environmental bucket with an eight by nine-foot footprint. This bucket achieved one foot of penetration per cycle.

Collins installed the bucket on an American 7260 crane, which they mounted on a system of Flexi-Float barges. Before the project began, it was discovered that increased river current was making it impossible to isolate the dredging area with silt curtains as originally planned. Therefore AMEC project manager Tim Ahrens submitted a letter to NYSDEC explaining that both curtain manufacturer Elastec, and Cable Arm advised that an alternative method be applied. The dredge cell method devised by Cable Arm had been effective on a similar project in the St. John's River in Florida, he explained.

NYSDEC approved the change, and D.A. Collins leased additional Flexi-Float barges to form an open cell on the dredging barge, protected by silt curtains extending 10 feet below the surface.

The dredge was positioned so the dredge cell faced downstream, breaking the current and further guarding against re-suspension of the contaminants. Using the real-time Clamvision, the operator utilized a monitor display in the crane cab to place each bucket in an exact XYZ position. The filled buckets were transferred and discharged into material barges, following procedures to minimize spillage. The bucket was rinsed in a wash tank before re-entering the water column for the next grab.
This method, which combines a closed design bucket that can dredge with minimum re-suspension, a well-trained operator, and on-site technical support personnel who understand the process, makes possible the removal of highly contaminated materials at reasonable cost, said John Lajeunesse of Cable Arm.
Following this project, D.A. Collins moved to the Bass River in Massachusetts to remove coal tar from the river bed. Lajeunesse is providing technical support on that project, also, and stressed that the effectiveness of the method has drawn the attention and approval of citizens living near the sites.

The dredged material in the Genesee project was transported to an off-loading area about a quarter-mile upstream, where standing water was removed and the sediment amended with Portland cement. The sediment was transferred to land for staging and testing, then transported offsite to appropriate disposal facilities.
AMEC created a Community Air Monitoring Plan (CAMP) that was in use throughout the project. It incorporated real-time data collection with laboratory analysis verification. Sampling consisted of eight perimeter monitoring stations and five community sampling locations, totaling thirteen different sampling points.

Because the dredging and sediment off-loading areas were a quarter mile apart, it was necessary to provide perimeter monitoring around each activity. The monitoring stations consisted of photo-ionization detectors (PID) and particulate monitors. Each unit logged data at pre-determined intervals, and was connected to a visual alarming system that would notify staff in the event of an exceedance. The equipment was inspected and calibrated several times throughout the workday and downloaded daily.
In addition to the perimeter monitors, community monitoring stations were established in a nearby school, marina and adjacent neighborhoods. These locations were equipped with an integrated sampling pump that collected air samples for laboratory analysis by methods OSHA 80 (GC/FID) and OSHA 69. No exceedances of criteria specified by the CAMP and NYSDOH occurred during the project.

River water quality was monitored at established sampling locations 300 feet upstream and downstream of the dredging area. One sample at each location was collected daily to monitor chemical concentrations in the water column. Turbidity was also monitored to ensure suspended sediments did not escape the work zone, with samples collected and documented every two hours during dredging. Sample results for turbidity were instantaneous, while chemical concentrations were available within 24 hours. Analytical data verified that the monitoring criteria in the approved work plan for both parameters were never exceeded.

When target dredging depths were reached, sediment end-point samples were collected to evaluate and document chemical concentrations and dredging effectiveness.

Results were submitted to the NSYDEC and USACE for approval. Based on these results, it was determined that the goals established in the work plan had been achieved and that the project had been successfully completed. Following confirmation of the sample results by regulators, post-excavation hydrographic surveys were conducted to document and verify dredge depths and sediment volume removed.

Ray Bergeron and John Lajeunesse stress that the following criteria are necessary for successful environmental dredging: operator confidence and ability to follow procedures to utilize the system, all crew members know the project criteria, no downtime due to equipment or software failure (good maintenance and backup equipment available), operator capable of using (startup/shutdown/troubleshoot) Clamvision positioning system, equipment is well maintained, safety awareness in all crew members.

Add your comment:
Edit Module