Drivers of hydrologic budgets in small terminal lakes in the Alberta prairies

Abstract

The prairies and boreal plain within the North Saskatchewan Watershed (NSW) of Central Alberta have numerous shallow ponds and lakes that sustain unique aquatic ecosystems and are critical habitat for migratory waterfowl in North America. However, over the past 20 years water levels have declined and the reasons are unresolved. Here we used a combination of inorganic geochemical analyses and stable water isotopes to constrain the hydrologic budgets of six lakes in the NSW. Our results show that the bedrock groundwater major element geochemistry is controlled by chemical weathering reactions along the flow paths and is dominated by lower δ18O and δ2H values (i.e. isotopically depleted), while the lake water generally shows unchanging evaporatively enriched stable isotope values and cation concentrations. An isotopic mass balance (IMB) technique combined with solution geochemical modelling using activity – activity plots reveals that deep groundwater input is negligible, while the lakes appear to lose a greater fraction of water inflows to evaporation (60%) than shallow groundwater and surface outflow (40%). The relative importance of shallow groundwater requires further study, as shallow groundwater sampling locations are scarce and surface outflow is negligible. The IMB technique also indicated that these prairie lakes have short water residence times, ranging from 1.8 to 10.4 yrs. Our results suggest that declining lake levels are likely the result of a changing relationship between precipitation and evaporation from the climatic norm.