Hydrologic Controls on the Extent of the Kasota 7 Calcareous Fen, Le Sueur County, Minnesota

Calcareous fens are defined as ground- and surface-water supported wetlands that contain a plant assemblage dominated by calciphilic (i.e., “calcium-loving”) plant indicator species. These fens are also defined by a set of criteria that include continuous groundwater discharge that is sufficient to maintain soil saturation, histosol soils containing a significant fraction of calcium carbonate minerals, and various water quality parameters.

The Kasota 7 calcareous fen is a legally protected wetland area located in Le Sueur County, Minnesota, in the floodplain of the Minnesota River. Geochemical and head data indicate that fen waters are derived from about 35% fresh water (river and local meteoric water) and 65% groundwater discharge from the Cambrian Jordan Sandstone aquifer, with no contribution from other potential surface and groundwater sources.

Soil samples from the calcareous fen indicate average Ca concentrations of about 21% (dry weight) for the calcareous fen soils vs. <8% for non-calcareous peat soils adjacent to the fen. Geochemistry and stable isotope data indicate that the carbonate material in the fen soils are precipitates from groundwater discharge rather than detrital carbonate minerals derived from overlying Ordovician Prairie du Chien Group sediments. Carbon-14 data indicate a maximum age of 850 (±100) years bp for fen peat material.

Based on the local groundwater hydrology, we should expect to see calcareous fens throughout the local floodplain of the Minnesota River. Fen occurrence, however, is limited to a few isolated locations, suggesting that some non-groundwater-related factor is controlling and limiting floodplain fen occurrence. Using field observations, survey data, and historic river stage data, we have concluded that calcareous fen occurrence is controlled by the maximum river stage during major flooding events. We hypothesize that fresh water flood events flush out carbonate particulate matter, rendering the most frequently flooded peat soils unsuitable for the calciphilic plant species.