Groundwater Metals Contamination and Construction Dewatering: Treatment Challenges and Solutions
Re-development of port land, previously occupied by Western Stevedoring’s (Western) terminal in North Vancouver, BC to the G3 Terminal Vancouver (G3TV) grain export terminus (the Site) required management of historical contamination. Concentrations of copper, zinc and cadmium in groundwater exceeded the applicable Groundwater Quality Guidelines and applicable marine discharge criteria. The groundwater contamination coincided with a proposed excavation required to construct facilities for receiving grain by rail (referred to as the Rail Receiving Pit) requiring dewatering, and discharge to the Lynn Creek estuary.
This situation highlights a common problem faced by developers. The metal contamination in groundwater at the Site was shown to naturally attenuate to below the WQGs along the groundwater flow path before reaching the marine environment. Construction dewatering eliminates these natural attenuation processes creating a water disposal challenge. Large flow volumes were also predicted given the Site geology. G3TV proposed to mitigate the high flow volumes by installing a control density slurry mix (CDSM) soil cement cofferdam to isolate the excavation.
Since the location and geometry of the source of the contamination was unknown and metal contamination on the Site was dispersed and irregular, the concentration of metals in groundwater to seep into the RRP during construction was uncertain. To address this data gap, PGL designed a pumping test to create a capture zone equivalent to the size of the anticipated RRP excavation.
The pumping test demonstrated the desired capture zone and flowrate. It was concluded that metals concentrations in groundwater extracted from the Rail Receiving Pit would remain above the WQGs over the course of construction and treatment would be required.
McCue was contracted to supply and operate the temporary water treatment system following the installation of the CDSM soil cement cofferdam in spring 2017. Within two months of startup, the inlet contaminant concentrations started to climb and the system could not meet the discharge standards for copper and zinc. The strict copper and zinc standards, the presence of salts from seawater intrusion, and construction activities in the excavation made water treatment technically challenging.
In late October, the system was shut down. McCue undertook a system redesign, which included new technology assessments and a series of chemical bench scale tests. In an accelerated, two-week timeframe McCue deconstructed and relocated equipment for construction of a new plant that incorporated an unconventional multi step metals precipitation process. The new plant was operated by McCue for seven months and consistently met the treatment objectives for copper and zinc at a fraction of the cost of other methods. McCue will review the challenges involved.
McCue will also review the bench scale tests, which were based on work McCue previously completed in the mining sector. The test results and data from the G3 water treatment system contributed to the body of knowledge for heavy metals precipitation treatment performance. For instance, previously, literature indicated that no or negligible removal could be achieved for copper. McCue’s work has provided valuable data for copper that could benefit EA work at mine sites globally.
Cecilia Garcia Marra – Chemical Engineer, Project Engineer
Cecilia has a Chemical Engineering degree from the University of Buenos Aires (Argentina) and she is currently registered as an E.I.T in B.C., Canada. She has four years of relevant experience in designing and cost estimation of water/wastewater treatment systems. Cecilia joined McCue in January 2018 as a Project Engineer and contributes to the Engineering Team with process design, budget estimation, and project management. She also leads, plans, and oversees, most of the laboratory work conducted in McCue laboratory.
Cory Nelson, B.Sc., P.Ag., PGL Environmental Consultants, Environmental consultant with 12 years of experience in environmental site assessment, remediation and hydrogeology throughout western Canada. Projects have included marine water lots, waterfront development sites, industrial facilities and urban development properties.