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Hydrogeology of Minnesota Calcareous Fens: How do they work?

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.

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Hydrogeology of Minnesota Calcareous Fens: How do they work?

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  1. 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

  2. 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

  3. 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

  4. I. Introduction:Scope

  5. 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

  6. 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)

  7. II. Physical hydrogeology: Regional setting • Landform 1: Linear peat apron along valley terrace • Convex along flow lines, relatively diffuse discharge

  8. 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

  9. II. Physical hydrogeology: Regional setting • Sioux Nation Fen -- a prime example of a peat mound calcareous fen overlying an aquifer window

  10. 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)

  11. II. Physical hydrogeology: Local setting: Groundwater fluxes • Vertical flux likely over-estimated -- by a factor of about 6 to 50...

  12. 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

  13. III. Geochemistry: The Big Picture

  14. 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)

  15. III. Geochemistry: What is the source of CO2? • THE equation: • CaCO3(s) + CO2 + H2O <==> Ca2+ + 2HCO3- • SO -- 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 CaCO3uses 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

  16. III. Geochemistry: The Big Picture revised (1) Large amounts of CO2 are dissolved from the soil atmosphere during infiltration

  17. III. Geochemistry: What is the source of CaCO3? • THE equation: • CaCO3(s) + CO2 + H2O <==> Ca2+ + 2HCO3- • OK, 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 soil-atmosphere source of CO2

  18. 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

  19. III. Geochemistry: What happens at the fen? • THE equation: • CaCO3(s) + CO2 + H2O <==> Ca2+ + 2HCO3- • So, 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

  20. III. Geochemistry: The Big Picture revised yet again (1) Large amounts of CO2 are dissolved from the soil atmosphere during infiltration (3) CO2 physically degasses from groundwater reaching the fen surface, thereby precipitating CaCO3 (2) CaCO3 dissolved from drift (or shallow bedrock) early in the flow path

  21. 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?

  22. 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)

  23. III. Geochemistry: Local setting -- Geochemical reactions • Western fens • CO2 degassing and CaCO3 precipitation, as expected • Shallow water = 6% rain water + 94% groundwater

  24. III. Geochemistry: Local setting -- Geochemical reactions • Eastern fens • CO2 dissolution and CaCO3 dissolution, NOT as expected (?!) • Shallow water = 13% rain water + 87% groundwater

  25. III. Geochemistry: Local setting --What is the peat composition? • Surface zone: >10% CaCO3 (ave. 27%) • Carbonate-depleted zone: <10% CaCO3 (ave. 4%) • Lower zone: >10% CaCO3 (ave. 42%) • Why a CaCO3 depleted zone?

  26. III. Geochemistry: Local setting --Why would CaCO3 dissolve, rather than precipitate? • CaCO3 dissolves when water table drops below critical depth

  27. 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

  28. 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...

  29. 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)

  30. 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

  31. 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

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