INCREASING SOLUTE CONCENTRATIONS IN ALPINE LAKES OF THE CANADIAN ROCKIES OVER CA. 50 YEARS ATTRIBUTED TO INCREASED GROUNDWATER DISCHARGE RELATED TO CHANGING CLIMATE

We compare major ion concentrations and electrical conductivity of ~20 alpine lakes in Banff and Yoho National Parks in the Canadian Rockies collected in 2021-2023 to measurements completed approximately 50 years ago (~1965-1975). We found significant increases in the concentrations and ion proportions of sulfate and magnesium in most lakes sampled, although the percentage increase varied by lake. The increased concentrations are corroborated by increases in electrical conductivity indicating increased total dissolved solute (TDS) concentration of a similar magnitude.

Lake surface grab samples were collected during late July through early August, after snowmelt is complete and when the vertical temperature and concentration profiles are most stable in lakes that stratify. Seasonal variability was minimized by sampling each lake on the same ordinal day annually and minimizing differences between the ordinal sample day of the current and 50 years ago data sets.

Sulfate shows the largest concentration increases compared to 50 years ago. The average recent sulfate concentrations for all but one individual lake are 40% to 400% greater than 50 years ago. More than half of the lakes exhibit at least a doubling of the sulfate concentration. Typical concentration increases are 5 mg/L to 10 mg/L. Only one lake shows a decline in sulfate concentration. The magnesium concentration increases are more moderate than sulfate; all but two lakes show increased magnesium concentration.

We assert that increased solute concentrations are attributable to an increased proportion of groundwater compared to overland and/or interflow entering the lakes. This assertion is supported by comparison of groundwater and lake geochemical and isotopic composition.