11.5 Trends in sulphate in the Slave River

Last Updated: 
December 17, 2015

This indicator tracks sulphate levels in a transboundary river1, the Slave River.

Sulphate is a naturally occurring substance that contains sulphur and oxygen. It is found in water as a result of the decomposition of leaves or the weathering of certain rocks and soils. Suphate forms salts with a variety of elements including barium, calcium, magnesium, potassium and sodium.

The Slave River downstream from the Rapids of the Drowned.
The Slave River downstream from the Rapids of the Drowned.

Levels of sulphate are routinely measured in the water samples collected from the Slave River near Fitzgerald. These water samples are collected monthly by Environment Canadastaff. Additional information from the GNWT-ENR community-based monitoring program is also provided.

NWT Focus

Although it is natural to find sulphate in surface waters, increased levels of sulphate can occur due to human activities. The burning of fossil fuels is a large source of sulphus to the atmosphere and may contribute sulphate to surface waters3. Discharges from coal mining, municipal treatment plants, tanneries and pulp mills as well as land use disturbances and agricultural runoff can also elevate levels above natural levels. These activities take place in the upper watershed in Alberta and British Columbia and have the potential to impact the quality of water flowing into the Northwest Territories (NWT). These are reasons for measuring sulphate in the water samples.

Current activities in the Peace, Athabasca and Lower Slave watersheds.
Current activities in the Peace, Athabasca and Lower Slave watersheds.

Current View: Status

Sulphate levels vary throughout the NWT. This is normal, as water is influenced by different landscapes, rocks and soils in different parts of the NWT.

Box plots from ENR's community-based monitoring program. Total sulphate data (2012-2014) highlighting the varying levels of total sulpate in rivers near select NWT communities. Slave River (Fort Resolution, Fort Smith), Mackenzie River (Fort Providence, Norman Wells, Fort Good Hope, Tsiigehtchic, Inuvik), Island River (Trout Lake), Peel River (Fort McPherson). Long-term trend data is not available for these sites.

Box plots can reveal a lot of information about water chemistry data. For instance, levels of total sulphate in the Slave River at Fort Smith are slightly higher than the levels in the Slave River near Fort Resolution. The graphic also highlights the very low variability in total sulphate levels in the Mackenzie River at Fort Good Hope compared to the levels at Norman Wells. The plots also show the lowest total sulphate value measured during the sampling periods was in the Slave River at Fort Resolution, while the highest were in the Mackenzie River at Fort Providence and Island River near Trout Lake.

Current View: Trend

Environment Canada has been monitoring dissolved sulphate in the Slave River near Fitzgerald for a long time. Long-term monitoring is important because it allows for the assessment of long-term trends. When assessed, a significant increasing trend was found1. This means that levels of sulpate have been steadily increasing since 1972 when regular sampling began on the Slave River. Even though the maximum dissolved sulphate level measured is 37 mg/L (1972-2010), which is well below the BC Ministry of the Environment freshwater aquatic life guideline for sulphate of 100 mg/L. Based on the trend assessment, it appears  levels of sulphate have increased through time. This increasing trend is occurring during all four seasons (see figure below).

Non flow-adjusted dissolved sulphate concentrations (log scale) at Fitzgerald (1972-2010). Statistically significant increasing trends were found in all four seasons at the 95% confidence interval.

Looking Forward

It is too early to tell where levels of sulphate might be in the future. Sulphate levels will continue to be measured and assessed. Environment Canada also reported a significant increasing trend in Slave River dissolved sulphate levels in the 1989-2006 data4. However, when data collected during the last decade only (1997-2006) was examined, the trend was not significant4. It was noted that although recent levels remained elevated above earlier levels, they were no longer increasing. As higher sulphate concentrations were noted in several other rivers, including mountain headwater streams4 , it was concluded the increase was a signal of global changes in water chemistry as opposed to inputs from point sources.

These findings highlight the need to continue to watch sulpate levels in the Slave River.


Found an error or have a question? Contact the team at NWTSOER@gov.nt.ca.


Ref. 1 - AANDC. 2012. Water and suspended sediment quality of the transboundary reach of the Slave River, Northwest Territories. 317pp.

Ref. 2 - Slave River near Fitzgerald surface water quality data.

Ref. 3 - BC MOE. 2000. Water quality - ambient water quality guideline for sulphate - overview report. Environmental Protection Division.

Ref. 4 - Glozier, N., D.B. Donald, R.W. Crosley, and D. Halliwell. 2009. Wood Buffalo National Park water quality: status and trends from 1989-2006 in three major rivers; Athabasca, Peace and Slave. Prairie and Northern Office, Water Quality Monitoring and Surveillance Division, Water Science and Technology Directorate, Environment Canada.