Long-Term Hydrologic Sustainability of Calcareous Fens Along Glacial Lake Agassiz Beach Ridges, Northwestern Mn

Abstract

Calcareous fens are peat-accumulating wetlands fed by calcium-rich groundwater that support several threatened species of plants that evolved to thrive in these geochemical conditions. Fifty-three of Minnesota’s nearly 300 identified calcareous fens are located in the Glacial Lake Agassiz beach ridge complex in northwestern Minnesota. Each of these fens is located immediately downslope of large sand/gravel beach ridges, where peat aprons have accumulated on the seepage face. This investigation characterizes the hydrology and landscape setting of two calcareous fens that are typical of the larger groups. Three potential sources of water to the fens are considered: groundwater from the surficial beach ridge aquifers, underlying confined aquifers, or a combination of the two influenced by seasonal hydrology. Water levels in wells in the confined aquifers, surficial beach ridge aquifers, and in and below the fens were compared with rainfall hydrographs to identify hydrologic connections. Hydrologic responses to rainfall events and associated hydraulic gradients suggest the calcareous fens are well-connected to the beach-ridge aquifer. Wells in the beach-ridge aquifers and wells in and below the fens respond synchronously to rainfall events. Water chemistry and stable isotopes are similar within the beach ridge aquifer and calcareous fens and differ significantly from water in confined aquifers. Beach ridge aquifer complexes are relatively thin (<8-10 m) and overly thick clay/clay loam till. These shallow aquifers exhibit high seasonal recharge and have permanent saturated zones, providing a continual source of water for the fens. Electrical resistivity profiles and 3D aquifer models characterized the glacial stratigraphy and highlight the well-developed physical connection between beach ridge aquifers and calcareous fens.