The Salinity of the St. Lawrence River Under Scrutiny: For a Better Understanding of the Anticipated Impacts of its Variability on the Drinking Water Supply of the CMQ

The project has provided a better understanding of current and future spatial and temporal variability of salinity in the transition zone to help guide stakeholders in making sound decisions.

Project details

Scientific program

2014-2019 programming

Theme(s) and priority(s)

Water Management

Start and duration

May 2017 • 5 years

Project Status

Completed

Principal(s) investigator(s)

Charles-Éric Bernier

CMQ

Context

Studies have hypothesized that the salinity transition zone in the metropolitan Québec City region may rise as a result of climate change. Impacts on the drinking water supply could affect more than 200,000 people due to increased water salinity at the drinking water treatment plants.

In fact, high salinity peaks have been observed in recent years at some drinking water treatment plants. Although these peaks are below the potability threshold, the municipal governments of Québec and Lévis are concerned about the situation. In this context, a study of the vulnerability to salinity of St. Lawrence River drinking water sources for Québec City and Lévis was conducted.

Objective(s)

The aim of this research project was to gain a better understanding of current and future sources of salinity observed at drinking water treatment plants through:

An analysis of the current temporal and spatial variability of the water salinity observed near and at drinking water intakes in the St. Lawrence River and identification of the causes
Modelling of the salt front displacement based on scenarios of sea level rise and changes in river flow levels over the next 25 and 50 years to assess the likelihood of such a phenomenon occurring

Methodology

Analyze the current temporal and spatial variability of the water salinity observed near and at drinking water intakes in the river, and identify the causes.

Model the salt front displacement based on scenarios of sea level rise and changes in river flow levels over the next 25 and 50 years to assess the likelihood of such a phenomenon occurring. This step is the subject of this report.

Results

Analysis of data from the Sainte-Foy, Lévis and Saint-Romuald drinking water treatment plants between 2007 and 2019 reveals that the salinity peaks observed occur in the winter when precipitation hovers around 0°C. Although the salinity peaks remain below the threshold considered unfit for consumption, salt spread on the roads in the winter and the melting of snow dumps pose a risk to drinking water treatment plants drawing from the St. Lawrence River.

The analysis of water samples taken at 433 stations at three times of the year between 2018 and 2020 revealed that the metropolitan Québec City section of the river, including the Québec City and Lévis drinking water treatment plants, is currently beyond the reach of the salinity transition zone front.

On the other hand, in a future climate, displacement of this zone could add to the causes of increased salinity at the Québec City and Lévis drinking water treatment plants. Although there is uncertainty as to the extent of the salinity transition zone in a future climate due to the complexity of the phenomenon and the lack of multi-annual monitoring in the field, the fact remains that changes in salinity conditions will occur in the medium and long term.