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Solute transport in sediments

Solute transport in sediments. Physical properties Transport: Diffusion “Irrigation” Advection. Sediments magnified. -- Water + solids -- Solids a complex mixture of components. Burdige, 2006. General Features of the solids - Grain size. Burdige 2006. Porosity.

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Solute transport in sediments

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  1. Solute transport in sediments Physical properties Transport: Diffusion “Irrigation” Advection

  2. Sediments magnified -- Water + solids -- Solids a complex mixture of components Burdige, 2006

  3. General Features of the solids - Grain size Burdige 2006

  4. Porosity Definition: the volume of connected pore space, per volume of bulk sediment Measurement: measure sediment (1) wet, and (2) after drying in (typically) a 65° oven Then: Typical densities of dry sediments: 2.6-2.7 g/cm3

  5. Porosity in near surface sediments -An example From a typical fine-grained (“clayey silt”) coastal sediment

  6. Transport of solutes:Diffusion Definition : the process by which matter is transported as a result of random molecular motions (Crank, 1975) Small scale Randomly directed ==> diffusion transports matter from regions of high concentration to regions of low concentration Boudreau, 1996

  7. The diffusion coefficient: Mathematically: C X J ~ mol / cm2/s D ~ cm2/s Increasing concentration Direction of diffusive transport

  8. Diffusion coefficients of ions in seawater Depend on: water properties ion properties Ion properties: it’s transport we’re interested in: The “limiting equivalent conductivity” Water properties: the viscosity Bockris and Reddy, 1970

  9. Data for diffusion coefficients(see Boudreau, 1996, for tables and procedures) Limiting equivalent conductivities have been measured (sometimes as ƒ(T)) for many ions. With this, R = gas constant F = Faraday constant Z = ion charge = lim. Equiv. conductivity Accounting for solvent viscosity: For temperature dependence: Either tabulated, or For seawater: 0.92 (25°) to 0.95 (0°)

  10. What about for uncharged species? An empirical relationship… T = absolute temperature µ = dynamic viscosity of water Vb= molar volume of nonelectrolyte at normal b.p. of solvent And empirical equations based on experimental data for important solutes - eg O2 and CO2

  11. Back to diffusion of solutes in sediments A fraction of the path is blocked by particles porosity A “tortuous” path tortuosity

  12. Determining tortuosity:The “formation factor” A 4-pronged probe: Voltage difference is proportional to Specific resistance of the medium Measure (1) bulk sediment (2) overlying water Measure voltage difference Apply current

  13. Example F is closely related to porosity for a given Sediment: C~1; n~2-4

  14. Finally… The diffusive flux in a porous medium:

  15. One last remark A general, qualitative relationship… Can be derived from consideration of diffusion as a “random walk” The “average” (root mean square) distance of transport of a substance by diffusion in the time interval, t:

  16. Solute transport by “irrigation”… mostly driven by macrofauna Example: pore water dissolved oxygen profiles (a shallow water site) Data from Fred Sayles

  17. Comparing fluxes:Diffusive fluxes from pore water profiesvsFluxes from in situ benthic flux chambersLocation: Massachusetts Bay, water depth 35m

  18. Bill Marti Comparisons in Offshore, Continental Slope Sediments 2x “Biological enhancement factor” vs. water depth ~50 µmol O2/cm2/y

  19. Organic C oxidation ratesContinental Margin, NW Atlantic

  20. Are sediments really “layered”?

  21. Example: Rn-222 deficits in Buzzards Bay:winter: alpha ~ 0spring / summer: alpha > 0

  22. Rn results:Continental slope Filled : Rn-222 production rate Open: Rn-222 measured in pore water Dashed line: Rn-222 model: diffusion only

  23. Problem…“reaction mosaic” around burrows

  24. Another Problem…Solute concentrations in burrows are not constant

  25. And: 1. Flushing can affect reaction rates e.g. : removal of products 2. Burrows are active environments on their own

  26. Viewing sediments as a 2-dimensional system:The “average microenvironment”

  27. …Applying the 2-D model to data… Parameters: r1 = 0.05 cm: est. based on numerically dominant poychaete L = 15 cm: est. from x-radiographs R = based on incubations for SO4 and NH4+; for SiO2, assume Ceq = 577µM; vary rate constant to fit r2 = 2.1 cm: varied to fit NH4+ profile; used for other 2 solutes

  28. Advective flow through sediments Permeability: Relates the velocity of fluid flow through a porous medium in response to a pressure gradient Unit = darcy Permeability is related to Grain size:

  29. Where are there sandy sediments?

  30. Relict sands -- e.g. on continental shelves --do contain fine-grained particlesand are sites of diagenetic cycling Excess Pb-210 Inventories -- Contintental shelf

  31. Advective flow through permeable sediments in response to bottom currents flowing over “rough” features : e.g., mounds or dunes High pressure Low pressure

  32. Flow+filtering of particles from flow … Measurements of dissolved Oxygen around a mound -- Results from experiment in a flume

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