Response to the Letter from the Nunatsiavut Government (NG) to the Premier (dated August 15, 2017) regarding the increase in water levels at Muskrat Falls last winter and the effect on methylmercury production

Issued September 20, 2017

To address the question posed by the Nunatsiavut Government, the IEC examined the conclusions of a technical memorandum (dated 31 July 2017) provided to Nalcor from Azimuth Consulting Group Partnership (Azimuth). Azimuth had commented on the trends in water sample results for the period 14 Oct 2016 to 2 May 2017. An IEC working group also subjected all available water sample results (from points above the reservoir to the centre of Goose Basin) for the period 14 Oct to 19 July 2017 to a rigorous analysis.

Response to NG letter: In the opinion of the Independent Expert Committee, there is inconclusive evidence linking increases in water levels (to approximately 21-22 m asl) in the Muskrat Falls headpond and methylmercury production downstream in the Churchill River and Lake Melville (based on data between November 2016 and the most recent available data for July 19, 2017).
Key reasons for an inconclusive analysis at this time include:

  • Lack of adequate baseline data, especially during previous spring and summer seasons, against which to compare methylmercury concentrations recorded during and after water level increases;
  • Hesitation to causally link observed transient increases in concentrations of total methylmercury immediately following increases in water levels in February 2017, due to the absence of similar responses to changing water levels in November 2016;
  • Inability to separate the effects of higher water levels and higher temperatures on increased variability in methylmercury concentrations, which was observed at most sampling sites between the headpond and Goose Bay since February 2017; and,
  • The temperature dependence of methylmercury production means that there likely is a time lag before water level increases in the winter of 2016/17 are potentially reflected in higher mean methylmercury concentrations in the reservoir and downstream. Thus, although persistent increases in methylmercury may not have been detected in the analyzed data (up to 19 July 2017), the remaining summer and fall data (once available) are necessary for a more conclusive analysis.

The IEC is continuing its analysis of water monitoring data as it becomes available.