Calcareous fens in Minnesota are spring-seepage peatlands with adistinctive flora of rare calciphilic species. Peat characteristics andgroundwater geochemistry were determined for six calcareous fens in theMinnesota River Basin to better understand the physical structure andchemical processes associated with stands of rare vegetation. Onset of peataccumulation in three of the fens ranged from about 4,700 to 11,000 ¹⁴C yrs BP and probably resulted from acombination of climate change and local hydrogeologic conditions. Most peatcores had a carbonate-bearing surface zone with greater than 10%carbonates (average 27%, dry wt basis), an underlyingcarbonate-depleted zone with 10% or less carbonates (average4%), and a carbonate-bearing lower zone again with greater than10% carbonates (average 42%). This carbonate zonation washypothesized to result from the effect of water-table level on carbonateequilibria: carbonate precipitation occurs when the water table is above acritical level, and carbonate dissolution occurs when the water table islower. Other processes that changed the major ion concentrations inupwelling groundwater include dilution by rain water, sulfate reduction orsulfide oxidation, and ion adsorption or exchange. Geochemical modelingindicated that average shallow water in the calcareous fens during the studyperiod was groundwater mixed with about 6 to 13% rain water.Carbonate precipitation in the surface zone of calcareous fens could bedecreased by a number of human activities, especially those that lower thewater table. Such changes in shallow water geochemistry could alter thegrowing conditions that apparently sustain rare fen vegetation.