LIF in Action
The Use of Laser Induced Fluorescence (LIF) to Enhance Remedial Efforts at 5 Wing Goose Bay, Labrador
In 2008 Defence Construction Canada (DCC), as part of the Goose Bay Remediation Project, contracted AMEC Earth and Environmental to conduct a feasibility study of remedial technologies for subsurface petroleum plumes at the Main Gate property, located at 5 Wing Goose Bay.
Based on the feasibility study and pilot testing results, multi-phase vacuum extraction (MPVE or MPE) was selected as the remedial technology for LNAPL recovery and implemented in November 2008 at an area known as Main Gate 1 (MG1).
MPE was highly effective at recovering free phase hydrocarbon from the subsurface, operating at depths of 15 – 16m (depths conventionally assumed to be limiting to successful MPE application).
In summer 2009, the project was expanded to include another portion of the plume in an area known as Main Gate 2 (MG2). Both MG1 and MG2 operated highly efficiently for another year with product recovery levels approaching 200,000 litres and price per litre (LNAPL) as low as $0.44. In 2010 the projects were re-tendered and an additional system was added in what is the presumed source area of the Main Gate plume, Main Gate 3 (MG3).
Over the following two years the three systems continued to perform with aggressive monitoring and optimization protocols in place. During this highly effective recovery period analysis was conducted to monitor overall recovery trends. As predicted initial recovery rates were declining and recovery curves were trending toward asymptotic. All stakeholders were beginning to feel that the recoverable portion of the product had been reduced to a point where successful completion of the LNAPL recovery phase was imminent.
In the spring of 2012 however, groundwater levels reached a deep low that had not yet been encountered in the 6 years of project monitoring. With this dramatic decline in groundwater elevation, the reappearance of product was extensive. Recovery rates increased to levels observed in the early days of the project.
Based on this incident, various stakeholders felt it necessary to further understand the reason(s) for such a dramatic increase in the recoverability of LNAPL. Questions concerning the extent of LNAPL impacts below the average water table, the periodicity of these “deep low“events and their cumulative impact on LNAPL management were posed. The dynamics of these environmental processes could not be underestimated from a responsible management perspective.
Multiple lines of evidence were sought to gain a site-specific understanding of the impacts of water table elevation on LNAPL recovery. Hydrographs were consulted as a first step to help quantify the relationship between groundwater elevation and LNAPL recovery. Extensive groundwater monitoring had been ongoing as a part of the contract; this data was analyzed in the context of LNAPL recovery.
Historical precipitation data was also reviewed in an attempt to determine the predictability and potential magnitude of this type of event in the future. Sixty years of precipitation data was gathered but only 6 years of groundwater data existed.
The hydrograph data indicated that the drop in spring 2012 was well below the average water table and previous lows encountered over the six years of monitoring. Analysis of weather data and correlation with available groundwater data suggested that the main driver for major water table flux at the Main Gate sites is the volume and timing of snowmelt in spring.
The appearance of LNAPL associated with this unprecedented ground water low suggested that there was LNAPL trapped below the average low water table mark. The variance displayed in precipitation and snowmelt data dating back prior to the spill suggested the possibility of greater then expected water table variability trending with mesoscale climactic fluctuations. These findings suggested that the smear zone might extend deeper than previously anticipated. Delineating the LNAPLs full vertical impact was an important next step for site management.
With little information available to quantify the extent of submerged LNAPL, DCC initiated a High Resolution Site Characterization (HRSC) investigation. Specifically Laser Induced Fluorescence (LIF) was utilized on site.
The LIF investigation took place in 2012 and repeated in 2013 based on positive, cost-effective results. The investigation included the Main Gate areas as well as numerous sites throughout 5 Wing Goose Bay. During the investigation it quickly became apparent that the smear zone extended beneath the water table roughly 2 m at MG 1 and MG 2 and even deeper in the source zone at MG3, these results were verified by soil samples.
This data was helpful in explaining the circumstances that lead to the mobilization of product associated with the “deep low”. LIF results also provided important information through correlation of LNAPL concentrations with strata permeability to help assess the mobility and recoverability of the submerged product. With this new data, changes to the operation conditions of the current systems, and the potential incorporation of new technologies could be proposed to help reach management goals.
Most importantly this tool helped eliminate uncertainty and assured all stakeholders that if this submerged product were recovered, a further decline in the water levels would not likely mobilize more product.
The data collected with this HRSC tool, has become a key line of evidence in the understanding of LNAPL recovery, risk, and potential mobility at the site.
Please contact SCG to learn more about LIF and how it can add value to your free phase projects.