Hydrogeology of Minnesota
Calcareous Fens:
How do they work?
James E. Almendinger
St. Croix Watershed Research Station, Science Museum of Minnesota
Jeanette H. Leete
Division of Waters, Minnesota Department of Natural Resources
Calcareous fens in the Minnesota River Basin:
1992-94 project funded by MDNR and USGS

I. Introduction
– Importance and problem
– Purpose and scope
– Methods

II. Physical hydrogeology
– Regional setting
– Local setting

III. Geochemistry
– Basics of carbonate chemistry
– Regional setting
– Local setting

IV. Summary and Conclusions
I. Introduction

Importance and problem
– Calcareous fens are rare wetlands that receive large discharges
of calcareous groundwater and that harbor a disproportionately
large number of rare and threatened species.
– Calcareous fens were protected by legislation in 1991 -- but
they can’t be protected unless we understand how they work
(function)

Purpose and scope
– To characterize the physical hydrology and geochemistry of
selected fens
– Six fens chosen in the Minnesota River Basin for study in
1992-94
I. Introduction:
Scope
I. Introduction:
Methods



Peat cored to determine depth and composition
Two “nests” of wells installed at each fen
Water levels measured to determine gradients and flow
– Slug tests used to determined hydraulic conductivity

Water samples collected to determine chemistry
I. Introduction:
Methods



Slug tests used to determine hydraulic conductivity K
K determined every 50 cm depth
K biased by anisotropy
– Horizontal K underestimated a little
– Vertical K overestimated a lot (probably)
II. Physical hydrogeology: Regional setting

Landform 1: Linear peat apron along valley terrace
– Convex along flow lines, relatively diffuse discharge
II. Physical hydrogeology: Regional setting

Landform 2: Subcircular peat mound over aquifer window
– Convex along flow lines, relatively local, focused discharge
– Can form central “chimney” that spills over the top

Peat aprons and mounds can be mixed together
– End members of a continuum
II. Physical hydrogeology: Regional setting

Sioux Nation Fen -- a prime example of a peat mound calcareous fen
overlying an aquifer window
II. Physical hydrogeology: Local setting: Groundwater levels


Sub-peat water
level nearly
always above
peat surface
Water table
typically at peat
surface -- but
can drop 10-40
cm seasonally
(Ignore data from Nicols Meadow -- a sick fen damaged by pumping)
II. Physical hydrogeology: Local setting: Groundwater fluxes

Vertical flux
likely overestimated -- by
a factor of
about 6 to 50...
II. Physical hydrogeology: Inference from regional + local settings

How big is the recharge area for a calcareous fen?
– I.e., how much of a recharge area is necessary to produce observed
discharge at a fen?

Rule of thumb: 1 foot of recharge over 1 square mile
each year produces (about) 1 cfs discharge

Examples
– Recharge can be about 6”/yr in the eastern part of the state, and less
than 1”/yr in the western part of the state.
– For example, assuming 6” recharge in the East, and 1” in the West:


East: each cfs of discharge needs about 2 sq. mi. of recharge area
West: each cfs of discharge needs about 12 sq. mi. of recharge area
III. Geochemistry: The Big Picture
III. Geochemistry: CaCO3 basics

Calcite (CaCO3) is really soluble in water, right??
– NO -- at least not in pure water: only 2.23 x 10-4 mg/L
– YES -- if water has a little acidity from dissolved CO2: 75 mg/L at ambient
atmospheric CO2 pressures

Conclusion:
– Dissolution of CaCO3 depends on gaining dissolved CO2
 CO2 from atmosphere (PCO2 = 10-3.5 atm)
 CO2 from decaying/respiring organic matter
– Precipitation of CaCO3 depends on losing dissolved CO2
 CO2 degasses to atmosphere (when PCO2(aq) > 10-3.5 atm)
 CO2 extracted from solution by photosynthesizing plants (esp. algae)

THE Equation:
– CaCO3(s) + CO2 + H2O <==> Ca2+ + 2HCO3– K = 10-5.87 ( for concentrations in moles/L and CO2 in atm)
III. Geochemistry: What is the source of CO2?


THE equation:
– CaCO3(s) + CO2 + H2O <==> Ca2+ + 2HCO3SO -- is CO2 from the ambient atmosphere enough to
dissolve the CaCO3 that is delivered to fens??
– NO -- the ambient atmosphere can never supply enough CO2 to both
dissolve CaCO3 and physically degas at the fen surface, which would be
necessary to precipitate CaCO3 in the fen!!
– Dissolution of CaCO3 uses up the dissolved CO2, reducing its concentration
below equilibrium -- so the water would dissolve, rather than degas, CO2
when re-exposed at the fen surface.

Conclusion:
– The ambient atmosphere is not the dominant source of CO2 to the
groundwater -- there must be another source
– That source is the SOIL ATMOSPHERE, from decaying organics and root
respiration in the upper soil horizons, where PCO2 can be 20-50 times that in
the ambient atmosphere
III. Geochemistry: The Big Picture revised
(1) Large amounts of
CO2 are dissolved
from the soil
atmosphere during
infiltration
III. Geochemistry: What is the source of CaCO3?


THE equation:
– CaCO3(s) + CO2 + H2O <==> Ca2+ + 2HCO3OK, fine -- now that the water is supercharged with CO2 from
the soil atmosphere, it needs to percolate through limestone
bedrock to dissolve enough CaCO3, right?
– NO!! Calcareous bedrock is not necessary -- there is plenty of CaCO3 in
calcareous drift
– In most settings, most of the dissolution probably occurs in the unsaturated
zone during infiltration or shallow saturated zone (according to the literature)
 (Unless soils are very thin and well-leached... as in SE MN)
– Once groundwater reaches saturation with CaCO3 (e.g., calcite), it will not
dissolve more, no matter how much limestone it percolates through

Conclusion:
– Most CaCO3 dissolution occurs early in the flow path, relatively near the soilatmosphere source of CO2
III. Geochemistry: The Big Picture revised again
(1) Large amounts of
CO2 are dissolved
from the soil
atmosphere during
infiltration
(2) CaCO3 dissolved
from drift (or
shallow bedrock)
early in the flow path
III. Geochemistry: What happens at the fen?


THE equation:
– CaCO3(s) + CO2 + H2O <==> Ca2+ + 2HCO3So, groundwater, supercharged with CO2 and saturated with
CaCO3, reaches the fen surface, physically degasses CO2, and
causes CaCO3 to precipitate, right?
– YES!! Finally...

Umm, how come CaCO3 (marl) precipitation in lakes is so
common, while CaCO3 precipitation in fens is rare?
– Because lakes don’t have to rely on physical degassing of CO2 -photosynthetic algae can deplete dissolved CO2 faster than it can dissolve from
the ambient atmosphere, thereby raising pH and causing carbonate precipitation
III. Geochemistry: The Big Picture revised yet again
(1) Large amounts of
CO2 are dissolved
from the soil
atmosphere during
infiltration
(2) CaCO3 dissolved
from drift (or
shallow bedrock)
early in the flow path
(3) CO2 physically degasses
from groundwater reaching
the fen surface, thereby
precipitating CaCO3
III. Geochemistry: Local setting -- Geochemical reactions

How did the shallow fen water attain its chemical
composition (which the fen plants depend on)?
– Or, what reactions or processes transformed the sub-peat
source water that feeds the fen into the shallow fen water?
III. Geochemistry: Local setting -- Geochemical reactions

Reactions and processes considered:
–
–
–
–
–
CO2 dissolution and degassing
CaCO3 (calcite) dissolution and precipitation
SO4 reduction and S-2 oxidation
Cation (Ca, Mg, and Na) adsorption, desorption, and exchange
Rain water mixing (dilution)
III. Geochemistry: Local setting -- Geochemical reactions

Western fens
– CO2 degassing and CaCO3 precipitation, as expected
– Shallow water = 6% rain water + 94% groundwater
III. Geochemistry: Local setting -- Geochemical reactions

Eastern fens
– CO2 dissolution and CaCO3 dissolution, NOT as expected (?!)
– Shallow water = 13% rain water + 87% groundwater
III. Geochemistry: Local setting -What is the peat composition?




Surface zone:
>10% CaCO3
(ave. 27%)
Carbonatedepleted zone:
<10% CaCO3
(ave. 4%)
Lower zone:
>10% CaCO3
(ave. 42%)
Why a CaCO3
depleted zone?
III. Geochemistry: Local setting -Why would CaCO3 dissolve, rather than precipitate?

CaCO3 dissolves when water table drops below critical depth
III. Geochemistry: Using peat composition to infer water-table fluctuations

Fort Snelling and
Sioux Nation:
– Some WT levels
below surface

Ottawa Bluffs:
– WT nearly always
at surface

Nicols Meadow:
– OK at top, but
major WT drop
down to 1 m
III. Geochemistry: Revisiting groundwater levels




Fort Snelling
and Sioux
Nation do drop
below surface,
at least a little
Ottawa Bluffs
appears to be in
great shape
Nicols Meadow
has been
hammered by
pumping
Savage Fen
looks to be in
trouble...
IV. Summary and Conclusions

Summary -- Hydrology
– Fens are in river valleys or flanks of moraines, and underlain
by coarse deposits discharging large quantities of calcareous
groundwater
– Fens form peat aprons where discharge is diffuse and peat
mounds where discharge is localized
– Sub-peat groundwater levels were commonly 30 to 70 cm
above the peat surface
– Water-table levels were commonly at the peat surface, but
levels 10 to 40 cm lower were not uncommon
– Groundwater discharge at three fens averaged 40 L m-2 day-1
(but this is an overestimate)
IV. Summary and Conclusions

Summary -- Geochemistry
– Movement of CaCO3 to the fens begins with high CO2 in the
soils of the recharge area and CaCO3 dissolution (probably)
early in the flow path
– Fen peats commonly have >10% carbonate content at the
surface, which may overlie a carbonate-depleted zone. Basal
peats commonly have a very high carbonate content.
– Shallow fen water was a mix of about 80 to 95% groundwater
and about 5 to 20% rain water
– CaCO3 precipitation in fens ultimately depends on many
factors and occurs when the water table is above a critical
level, which may be near the base of the surface zone
IV. Summary and Conclusions

Conclusions
– Rare vegetation of calcareous fens appears to be associated
with CaCO3 precipitation at the fen surface
– CaCO3 precipitation depends on many factors along the entire
hydrologic flow path, from soils in the recharge zone to water
levels in the fen
– Therefore, sustenance of rare vegetation may need protection
of the entire hydrologic flow path, especially requiring the
maintenance of water tables above a critical level in fens for
much (most?) of the year