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Water and Gases

Explore the relationship between pure water and gases, equilibrium between air and gases, Henry’s Law, gas solubility, gas production, and groundwater chemistry processes. Learn the significance of diffusion, Fickian diffusion, and Darcy’s Law in groundwater movement. Understand the impact of minerals, gases, and respiration in groundwater chemistry. Discover the key processes involved in gas production, degassing, and biological gas production through oxygenic photosynthesis.

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Water and Gases

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  1. Water and Gases • Pure water (evaporation generally purifies water) interacts with gases in air: • Equilibrium between air and gases: H2O + CO2 H2CO3(aq) How do we determine conc. H2CO3(aq)??

  2. Henry’s Law • A description of gas solubility of gases in solution gas(g)↔ gas(aq) O2(g)↔ O2(aq)

  3. What is the difference in O2 at equilibrium with 5ºC water vs. 35ºC water?? • Compare that with SO2…

  4. Gas production? • What processes produce gases? • Degassing from melts • Formation and degassing from metamorphic reactions and hydrothermal systems • Diagenetic reactions, petroleum genesis • Biological production

  5. Oxygenic Photosynthesis Chlorphyll a (P680) is very oxidized (E0=+1.1V), enough to oxidize H2O. BUT e- excitation takes it to E0=-0.7V, not enough to reduce NADP+ to NADPH. Thus a need for 2 photosystems…. Water-oxidizing complex is key – Mn4Ca-complex that oxidizes H2O to O2 in 4 steps (S0 through S4)

  6. Diffusion, Fickian Diffusion from high to low levels.. Where D is the diffusion coefficient, dc/dx is the gradient, and J is the flux of material

  7. Groundwater • Precipitation that does not run-off into rivers percolates into soils, sediments, and into basement rock fractures to become groundwater • Water always flows down-hill • Darcy’s Law describes the rate of flow where,Q = volumetric flow rate (m3/s or ft3/s),A = flow area perpendicular to L (m2 or ft2),K = hydraulic conductivity (m/s or ft/s),l = flow path length (m or ft),H= hydraulic head (m or ft), andd= denotes the change in h over the path L.

  8. Groundwater Chemistry • Just like other waters, encounters minerals, gases, etc. • Some key differences from other waters: • PCO2 variable – respiration! • Segregation of flowpaths in different units

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