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Ground Water

Ground Water. Geochemistry at Sulfate Reduction- Methangenesis Transition Zone in an Anoxic Groundwater. Kristina Loen Wei Zheng. Introduction. Groundwater important of drinking Pollution industry/agriculture: near surface abandoned, obtained from deeper anoxic aquifers.

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Ground Water

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  1. Ground Water

  2. Geochemistry at Sulfate Reduction-Methangenesis Transition Zone in an Anoxic Groundwater Kristina Loen Wei Zheng

  3. Introduction • Groundwater important of drinking • Pollution industry/agriculture: near surface abandoned, obtained from deeper anoxic aquifers. • Anoxic redox processes important for water quality in deep aquifers • Anoxic RØMØ aquifer: inorganic geochemical processes+microbiologically mediated redox processes+thermodynamics

  4. Introduction • This Study • Focus: geochemistry of Fe-oxide reduction/sulfate reduction/methanogenesis mediated by microorganisms. • Shallow marine sand rather dune sand of RØmØ • High flow rate, different infiltration composition, lithologically less homogeneous • Useful interpreting other anaerobic aquifer

  5. Site Description • Northern Zealand, Denmark • 10m deep phreatic postglacial sandy aquifer, lower 7-8m occasional gravely, with pebbles; upper 2-3m homogeneous eolian sand with occasional paleosols. • Porosities 25-30%, Hydraulic conductivity 1.3×10-4 m/s • Groundwater table 1.2mbs (meter below surface)

  6. Methods • Groundwater: stainless steel drive point piezometers • H2 sampling: a bundle of 10mm PVC with 20mm disc-shape 20μm nylon screen, field measure: bubble stripping (Chapelle & McMahon 1991) • Methane: syringe, injected pre-weighted 13ml evacuated blood vial, frozen below -18°C • Others(anions, acetate,formate): filtered anaerobically through 0.2μm filter, 5ml polypropylene vials, frozen below -18°C • pH, O2, conductivity: field measured. • Alkalinity: Gran titration • Fe2+ , H2S: spectrophotometric • In Lab: Cation-AAS; Anion-ion chromatography; methane-gas chromatography; acetate/formate: ion exclusion chromatography

  7. Methods • CO2 reduction-H14 CO3- 22h • Acetate -14 CH3COONa 14h • Sulfate reduction-H235 SO4 18h • incubation ended by freezing cores to -50°C • Radiotracer Rate: 50mmID, 1.5mm thick, stainless steel tubing ; After retrieval core, 1mm holes and 12.5~25uL radiotracer injected, interval of 10~12cm. Incubation

  8. Methods α=1.06 (SRR-Sulfate Reduction Rate) (Jakobsen&Postma 1994) α=1.08 (Hansen, 1998) • (CO2 Reduction Rate) • α=1.08 (Acetate Turnover Rate)

  9. Methods • Sediment Parameters: Fe, Organic and Inorganic carbon, Sulfide as AVS (Acid Volatile) and CRS (Chromium Reducible) • Sediment bound organic carbon: non acid desorbable sedimentary organic carbon (NADSOC) Inorganic carbon=TC-NADSOC-ADSOC

  10. Results Reduction of Fe-oxides With increase Ca, Mg Inorganic compounds /shell

  11. Results Dry Deposition /Earlier Inundation

  12. Results Fe-oxidereduction/sulfate reduction/ increase in methane Dry Deposition in Pyrite Oxidation

  13. Results

  14. Results Degrade/Oxidation Organic Matter release

  15. Transport organic matter from surface to aquifer

  16. Equilibrium Control

  17. Exchange • Na+ slightly delayed in terms of vertical transportation • Ion exchange affect cations, also affect Ca2+, Mg2+ , K+ • Mg2+ displace Ca2+ • Ca2+ affected by dissolution of calcite, ion exchange release Ca2+ , precipitate Ca2+ • Al3+ not affected by ion exchange

  18. Rates and Sediment Composition • Sulfate reduction rate highly correlated with where sulfide found in sediment • AVS (Acid Volatile Sulfur) only in 5~6 mbs, transform of AVS to CRS (Chromium Reduced Sulfur) • Sulfate reduction rate extremely small, sulfate input higher, so sulfate reduction took place in large volume of sediment. • Organic matter low

  19. Bioenergetics

  20. Bioenergetics Average -4.5kJ/mol, adequate for ATP synthesis High Low

  21. Conclusion Similar to RØmØ aquifer, but • No pool AVS below sulfate reducing zone, indicating enough sulfide for conversion, related to higher measurable sulfide concentration • H2 level high enough to sustain methanogenesis, removing need for stagnant microniches. • Data indicating influx organic matter from soil, sustaining redox processes in system

  22. Questions?

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