Mitigation of Methylmercury in the Muskrat Falls Reservoir
The production of MeHg is a natural process, and it is very common for MeHg levels to increase after a reservoir is flooded. The amount of the increase depends on the characteristics of the reservoir- its size and depth, the speed at which the water flows, the amount of carbon present etc.
The only way to predict what the increase might be is to use computer models. The first complete model for the Muskrat Falls reservoir was completed by a Harvard researcher Dr. Ryan Calder (Calder et al, 2016). This model predicted that there will be a 10-fold increase in MeHg in the river downstream of Muskrat Falls and a 2.6-fold increase in the surface waters of Lake Melville. It further modelled what this would mean in terms of human exposure.
Models always have some amount of uncertainty. The IEC wanted to understand the factors that most contributed to uncertainty. As well, the human exposure depends on how MeHg moves through the food chain and how long any species (fish, seal etc.) spends time in the water that might be most affected by any changes in the MeHg. The IEC Indigenous Knowledge Experts provided valuable contributions due to their knowledge of where fish live and feed. For example, it was noted that salmon stop eating when they enter Lake Melville; this means that they are unlikely to be impacted by any changes in MeHg in the water or other fish. The former Harvard modeller, Ryan Calder used this new information and it was found that human exposures were somewhat reduced compared to earlier model results.
The IEAC was also interested in seeing the predictions of another model being developed by Reed Harris for Nalcor and asked that it be made available by February 15, 2018 (Recommendation #3). Unfortunately, the model was not completed by this date but the group was able to work with Reed Harris and see his preliminary findings. One notable observation was the fact that his prediction of the methylmercury levels in the reservoir waters at the highest point of production was within the same range of values predicted by the Harvard model.
The Harvard model only predicts the maximum (peak) change in MeHg concentration, not how it varies over time. Results from other reservoirs suggest that the MeHg peak in water takes one to five years and slightly longer for it to appear in fish. The highest concentrations in fish are maintained for a few years (depending on the type of fish) but take decades to return to pre-flood concentrations.
The IEAC looked at various ways to reduce (or mitigate) the amount of methylmercury that will be produced due to flooding of the Muskrat Falls reservoir. The experts committee (IEC) could not find a single example worldwide where action had been taken to control methylmercury production at a hydroelectric reservoir but there were some studies published in the scientific literature that offered suggestions.
All soils, in addition to vegetation, naturally contain inorganic mercury (a less toxic form) as well as varying amounts of organic carbon. After flooding, the naturally occurring bacteria that are present consume the carbon, deplete the oxygen at the bottom of the reservoir and this creates conditions that allows the conversion of inorganic mercury into methylmercury. This methylmercury can then flow downstream and enter the food chain.
The removal of topsoil and vegetation is one way to reduce the amount of carbon that is available for the bacteria to ‘eat’, thus resulting in conditions less favorable for methylmercury production. It is important to know how much of this carbon can be removed safely. In order to gather site-specific information on the option of removing organic carbon from the future reservoir, the IEAC made the following recommendations to the Minister of Municipal Affairs and Environment (NL) on Sept 22, 2017:
IEAC Recommendation #1: The IEAC recommends that a feasibility study be undertaken by December 20, 2017, for the removal of soil and vegetation from the future reservoir area.”
On December 22, 2017, the IEC received the draft report “Muskrat Falls – Soil and Vegetation Removal from the Future Reservoir Area”, prepared by SNC Lavalin for Nalcor. The following observations were made by the IEC on this draft report:
The report addressed the technical and economic factors associated with the removal of all the vegetation and topsoil from the entire reservoir area, up to 42m above sea level (asl), which is 3m above the full impoundment level, and did not exclude problematic areas such as steep slopes and unstable soils.
The constructability for full soil and vegetation removal was considered feasible within the project timeline, but was described as very challenging
Points of important note for the committee included: the greater than anticipated minimum depth of soil clearance (0.5 m in summer, 1.5 m in winter), which essentially removes the full soil organic profile; the need to re-profile cleared land, even on moderate slopes (>30%), in order to maintain ground stability; and the widespread erosion potentially associated with such extensive ground disturbance, which could unintentionally stimulate MeHg production.
The IEC decided that it would be better to identify certain areas for topsoil removal in order to make the work more feasible, improve safety and reduce the potential for unwanted side-effects that might actually stimulate methylmercury production.
The IEC struck a Reservoir Subcommittee tasked with examining the characteristics of the future reservoir including its physical geography, ecological land classifications, soil types and organic carbon pools with the goal of informing options for targeted mitigation. An examination of the environmental risks associated with carrying out large-scale soil disturbances was also undertaken, which is detailed in the memo “Effects of forestry practices and similar soil disturbance on environmental mercury concentrations.” (Jansen, W., September 27, 2017.)
The committee agreed that an emphasis would be placed on practical considerations such as existing roads/tracks, slopes less than 30% etc., to reduce slope hazards, erosion and runoff (Jansen, Sept 27, 2018). In January 2018, the Subcommittee completed draft specifications for two Targeted Mitigation Scenarios, which were finalized in cooperation with Nalcor and its contractor SNC Lavalin, forming the basis of a new Statement of Work for Nalcor and its contractor SNC Lavalin. These two scenarios are summarized here:
Scenario A (Capping):
- Cap all fen and low shrub bog (but not marsh) wetlands ELC areas between 23.5 and 39 m asl with sediments that are low in total organic carbon, locally available and that will be stable (resistant to erosion from water flow) on the reservoir bed.
- Stability of sediment cap is more important than thickness, but assume 50 cm thick for this scenario. Cap should isolate the organic wetland soils, particularly peat accumulations, from the water column.
- Conduct work during frozen ground conditions to minimize ground disturbance.
Scenario B (Targeted Soil Removal):
- Remove soil from areas that have been previously cleared of trees and vegetation and are accessible by existing roads, between the 23.5 masl contour and the 39 masl contour.
- Exclude areas of slopes greater than 30% and other areas that would require re-profiling.
- Exclude areas that potentially contain sensitive clays (glaciofluvial and glaciomarine)
- Exclude riparian areas.
- Prioritize work on steeper slopes during frozen ground conditions, moving towards flatter areas during thawed ground conditions (to limit runoff from clearance activities).
On February 26, SNC Lavalin provided a brief summary of the preliminary information regarding the feasibility of the 2 Targeted Mitigation Scenarios, and provided costing estimates (SNC Lavalin, Feb 26, 2018 – 3 documents). The conclusions are summarized as follows:
- Both Scenarios A and B were considered feasible within the current July 2019 impoundment schedule
- Scenario B was described as a challenging undertaking to complete within the current July 2019 impoundment schedule.
(Note: on March 22, 2018 Nalcor distributed a report that described this information in greater detail – Muskrat Falls- Soil and Vegetation Removal from the Future Reservoir Area – Targeted Scenarios, 2018)
Some details would be worked out during the actual engineering design but some features are known.
Scenario A (capping) affects areas that are relatively small (1031 to 1756 ha; less than 1%) compared to the full mitigation originally examined. The material to be used for covering, or capping, these areas would be obtained locally, have low organic carbon content, and would cover the areas to a depth of 0.5 to 0.7m. The feasibility study noted that it would be possible to cap the areas before and, possibly after, flooding. This cap would prevent the organic carbon within these areas from being available for the production of methylmercury.
One expert had observed organic material (peat) floating to the surface in other reservoirs; preventing this at Muskrat Falls would reduce overall methylmercury production.
Scenario B (targeted topsoil removal) is a considerably greater engineering challenge as it entails volumes (5 – 9 million cubic metres) that are roughly one-third of the full mitigation option. Much of the work would have to be undertaken in cold weather to reduce the risk of unwanted production of methylmercury.
The soil being removed would have to be moved above the high-water mark of the future reservoir and properly contained so that this soil does not contaminate other watershed areas. A detailed design of the disposal areas was not provided but, according to the report, a typical soil disposal area would be approx. 200 m long, 50 m wide and 6 m deep – equivalent to two football fields end to end. It was thought that approximately 100 to 200 such areas would be needed.
We only have the models to estimate the potential reduction in methylmercury that could be achieved by capping and/or targeted soil removal. One such estimate was obtained from Dr. Ryan Calder (who developed his model while at Harvard University). This model suggested that soil removal might reduce the amount of methylmercury by approximately 20-25% (relative to the predicted increase with no mitigation). Capping seemed to be less effective (2% reduction) but this is because the model only looks at the surface area of the areas. If the organic material is allowed to float into the water, as has been seen in other reservoirs, then capping could produce a much greater reduction in overall methylmercury production over time.
There is evidence that natural events that result in large-scale soil disturbance, such as windfall events during storms and soil disturbance due to logging, can influence the production of methylmercury. Such effects may vary from site to site and depend on where the disturbance is relative to the waterbody of concern, the area topography and the soil type. It is known that projects carried out in winter months result in less impact than those carried out in times when soils are not frozen.
One expert felt that targeted soil removal, if not done correctly, may partially or fully offset the expected decrease in methylmercury concentrations in fish that mitigation was expected to accomplish; others felt that cold temperature excavation and appropriate precautions could reduce this risk of what has been called the unwanted side-effects of mitigation.