Sc. Komor, GEOCHEMISTRY AND HYDROLOGY OF A CALCAREOUS FEN WITHIN THE SAVAGE FEN WETLANDS COMPLEX, MINNESOTA, USA, Geochimica et cosmochimica acta, 58(16), 1994, pp. 3353-3367
Savage Fen is a wetlands complex at the base of north-facing bluffs in
the Minnesota River Valley. The complex includes 27.8 hectares of cal
careous fen that host rare calciphile plants whose populations are dec
lining in Minnesota. Water and sediment compositions in the calcareous
fen were studied to gain a better understanding of the hydrologic sys
tem that sustains the rare vegetation. Groundwater in the fen is a cal
cium-magnesium-bicarbonate type with circumneutral pH values. The grou
ndwater composition is the result of interactions among water, dissolv
ed and gaseous carbon species, carbonates, and ion exchangers. Shallow
groundwater is distinguished from deep groundwater by smaller concent
rations of chloride, sulfate, magnesium, and sodium, and larger concen
trations of calcium, bicarbonate, hydrogen sulfide, and ammonium. Magn
esian calcite is the prevalent carbonate in unconsolidated sedimentary
fill beneath the fen and is an important source and sink for dissolve
d calcium, magnesium, and inorganic carbon. Calcite concentrations jus
t below the water table are small because aerobic and anaerobic oxidat
ion of organic matter increase the partial pressure of carbon dioxide
(P(CO2)), decrease pH, and cause calcite to dissolve. Thick calcite ac
cumulations just above the water table, in the root zone of calciphile
plants, result from water table fluctuations and attendant changes in
P(CO2). Groundwater beneath Savage Fen recharges in lakes and ponds s
outh of the fen and upwells to the surface within the fen. Water at th
e water table is a mixture of upwelling groundwater and water near the
surface that flows downslope from higher elevations in the fen. Chang
es in oxygen and hydrogen isotope compositions of shallow groundwater
indicate that the proportion of upwelling groundwater in shallow groun
dwater decreases downgradient in the calcareous fen. Encroachment of r
eed grasses into the calcareous fen may reflect human-caused disturban
ces in the recharge area.