Groundwater: A Hidden Part of the Lake Ecosystem

The webinar on groundwater presented by Dr. Brian Smerdon of Alberta Geological Survey is posted on the website:

The groundwater section starts at about 53 mm 13 ss.

His presentation was based in part on this 2018 final Sylvan Lake Subbasin Groundwater modeling report by J.E. Liggett and A. Singh. The report graphics starting at Fig. 20 (p 31) are particularly relevant to the Sylvan Lake watershed.

One of the Smerdon topics was the ”seepage” or subsurface discharge of groundwater into lakes. The watershed community legend that Sylvan Lake is “spring fed” is the local folklore version of that process.

The difference in lake level data before/after ice formation extracted by the SLWSS from EC.GC hydrological records puts a limit on how much groundwater could possibly leak into (or out of) the lake. This photo gallery summarizes the lake level data together with the 2008 to 2019 precipitation record and long term average from the Alberta Agriculture Hespero weather station just west of Sylvan Lake.

The lake level has typically risen less than 100 mm from pre-freeze in the previous year to post-melt in the following one (when the EC.GC level sensor is again operational). Winter precipitation, some of which remains to melt and run off (if it does not sublime or evaporate away) is also about 100 mm. (That means the “seepage” is the difference between two numbers. The residual water is typically less than 50 mm of lake level, or about 2 million cubic metres of water.

The net result is that it is hard to accurately confirm significant groundwater inflow into Sylvan Lake, contrary to the local mythology of a “spring-fed lake”. The eastward outflow of groundwater towards Red Deer is also an unknown, except for what the AER team has modelled in recent years.

Relative to the massive fluxes of precipitation and evaporation through the open water season, and the continuous groundwater pumping of about 3 million tonnes (or cubic metres) for domestic use most of which is exported from the watershed in the wastewater system, the “seepage” record is in the lake water quality and isotope chemistry and not in the lake level data.

Remember that the typical summer lake evaporation rate is about 2.5 mm/day so the groundwater discharge rate is a relatively small contributor to the lake water balance.

Results of analysis of Sylvan Lake data on pre- vs post-freeze-up lake levels are summarized in this table and a graphic that allows quick inspection.

Year-NewOct-OldMay-NewDifferenceAnomaly, mYear-NewAnomaly, mm
Environment Canada lake level data show that in the period 2003 to 2016, the lake level increases by about 9 cm during the ice-covered periods. That includes lake snowmelt and land runoff before the level sensor starts recording data in the new year.
This graph shows the year-to-year change of lake level compared to the 13-year average change from pre-Fall freeze-up to post-Spring melt.

There has been so much variability from year to year in the winter precipitation, net residual snow cover at Spring melt time, air temperature profile through the ice cover period, that explanation of the record is complex and lost in the thermodynamic history of the lake that remains exempt from a federal government carbon tax.

It is almost impossible to relate old AENV lake depth profiles temperatures to the ambient weather conditions of any winter. The following photo gallery shows lake water temperature profile below the ice and the weather conditions above it for the same years.