Jake Gossen Presents at ESAA WaterTech
Monitored Natural Attenuation (MNA) has become a proven and viable approach to treat dissolved metals contamination at former industrial facilities. Following Hemmera’s recent development of a conceptual site model for metal release and attenuation, analytical results from sentinel wells indicate attenuation mechanisms are functioning and this model is consistent with field observations, offering significant time and cost savings during site closures. Thanks to MNA, multi-million-dollar remedial programs have been reduced to budgets in the order of $50,000.
At one confidential site where Hemmera’s geochemical assessment teams recently applied MNA, a dissolved-phase plume comprising several different metals was situated adjacent to a tidally influenced river, where water quality periodically fluctuates between fresh and saline conditions. A large industrial complex is located at this site, which was historically used for steel processing and manufacturing. Site investigations identified dissolved metal plumes (Al, B, Cd, Cu, Ni, Zn) related to a metal cleaning process utilizing sequential sulphuric acid, zinc phosphate, and borax dip baths. Analytical results indicated groundwater metal concentrations at the site were greater than federal and provincial guidelines and standards for freshwater and marine aquatic life.
Hemmera proposed a risk-based approach to achieve site closure. Following a review of historical analytical results, a fate and transport assessment was conducted involving a geochemical desktop study of the behaviour and mobility of the metals identified in groundwater at the site. Groundwater concentration contour figures indicated that the extent of cadmium, copper, nickel, and zinc coincided with the extent of the dissolved aluminum plume. Field parameters collected during groundwater sampling also indicated a zone of low pH that coincided with the dissolved aluminum plume, which appeared to originate at the metal cleaning baths. In addition, the floor below the metal cleaning baths was cracked, and sulphuric acid had infiltrated into groundwater, reducing the pH considerably (~7 to less than 4); gibbsite had dissolved and an aluminum plume had formed. Gibbsite is an adsorbent and its dissolution released the adsorbed divalent cations into the solution (cadmium, copper, and nickel).
Advection, dispersion, and rainwater infiltration subsequently diluted the groundwater and pH began to rebound back to neutral conditions. In addition, tidal fluctuations in river elevation periodically recharged the river bank at the site, creating a physical and geochemical barrier for further down-gradient migration of dissolved metals. Hemmera’s successful implementation of an MNA solution addressed site contamination issues at a fraction of the cost of conventional site remediation practices.
For more information about this project, contact Jake Gossen (firstname.lastname@example.org), an engineering hydrogeologist at Hemmera. This story was developed from a presentation for the 2015 ESAA Watertech Conference that took place on April 20 to 22 in Kananaskis, Alberta.