Wetland hydrology, transport processes, and modeling ?· 1 Wetland hydrology, transport Institute of…

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    Wetland hydrology, transport

    InstituteofFoodandAgriculturalSciences(IFAS)

    processes, and modeling

    Wetland Biogeochemistry LaboratorySoil and Water Science Department

    June 23 26, 2008Gainesville, Florida

    6/22/2008 WBL 1

    Instructor:James Jawitz

    University of Florida

    Biogeochemistry of Wetlands: Wetland transport processes

    Science and ApplicationsScience and Applications

    OutlineLearning objectivesFlow in wetlandsWater-column/sediment exchange

    Advective fluxProcessesMeasurement

    Diffusive flux

    6/22/2008 WBL 2

    Diffusive fluxProcessesGradient-based measurementsOverlying water incubations

    Sediment movementSettlingResuspension

  • 2

    Biogeochemistry of Wetlands: Wetland hydrology

    Science and ApplicationsScience and Applications

    Learning ObjectivesHow is water velocity determined in wetlands?Different ways water flow through wetlands is describedProcesses for water-column/sediment exchange

    6/22/2008 WBL 3

    Differentiate advective and diffusive fluxMeasurement techniques for advective and diffusive flux

    Water flow in wetlands

    Velocity of water flowing through a wetland Manning's equation, velocimeter (current meter),

    nominal residence time, actual residence time (tracer)

    Mannings equation Flow driving force = bed slope Resistance to flow = friction from contact with solid

    surface (sediment) and vegetationsurface (sediment) and vegetation

    Qkn

    AR SH=2 3 1 2/ /

  • 3

    Mannings n with vegetation

    Same vegetation, same roughness, but not g , g ,same friction effect on flow

    n is a (power) function of flow depth (n = d-) depth increases, n decreases (less friction)

    Water flow in wetlands Hydraulic loading rate: q [L/T]

    q = Q/Aw Q = total flow into wetland [L3/T]Aw = surface area of the wetland [L2]

    Water velocity: v [L/T]v = Q/(Ac) = fraction of wetland volume that is

    water (usually high, ~ 0.9)Ac = cross sectional area for flow

    Nominal residence time: tntn = Vw/Q Vw = volume of watertn Vw/Q Vw volume of water

    Q = flow through the wetland

    Actual residence time: Mean residence time from a tracer test (residence time distribution)

  • 4

    Nominal vs actual residence time

    Ratio is hydraulic efficiencyRatio is hydraulic efficiency maximum 1 less than 1 indicates short-circuiting past dead

    zones where inflow water does not access before exiting

    Wang et al. 2006, Ecol. Eng. Rejuvenating the largest municipal treatment wetland in Florida

    Organic sediments were transported to a 40 acre pasture land and dumped. The area was leveled off with a bulldozer and planted with grass.

    Giant bulrush

  • 5

    Flux, flow, discharge

    Water flow solute flux mass dischargeWater flow, solute flux, mass discharge [MT-1L-2], [L3T-1], [MT-1] Discharge is mass flow (as opposed to

    volumetric flow), and flux is discharge per unit area

    Sediment/water column: Advective flux

    AdvectionAdvection solutes move with fluid (water) that is driven by

    hydraulic gradients contrast to convection, diffusion, dispersion

    Ja = CvaJa advective flux [MT-1L-2]C solute concentration [ML-3] v velocity [LT-1]

  • 6

    Advective flux processes

    Surface water/Groundwater exchangeSurface water/Groundwater exchange Bioturbation Phreatophytic mixing

    Figure 14.7

  • 7

    Figure 14.9 Data from Aller and Aller, 1992

    Cl-diffusion

    2 5 0 341.5

    2.0

    2.5

    Flux from bioturbation usually added to

    molecular diffusion

    Br-diffusion

    1.5

    2.0

    2.5

    y = 2.5x - 0.34

    R 2 = 0.64

    0.5

    1.0

    Meo

    faun

    a ad

    ded

    o u a d u o(e.g., Dtotal = Dm + Db)

    these data show to be ~ 2 times diffusion (slope of line ~ 2), which can be significant in the absence of other

    y = 2.2x - 0.04

    R 2 = 0.87

    0.5

    1.0

    0.5 0.7 0.9 1.1

    Control (no meofauna added)

    advective mechanisms not much data in

    wetlands

    Figure 14.8

    UndisturbedFloodwater Floodwater

    Bioturbated

    Dep

    th

    Anaerobic soil

    Mixed zone

    Anaerobic soil

    Aerobic soil Aerobic soil

    Concentration Concentration

    Even if not contributing significantly to solute flux (or internal load), bioturbation can affect the sediment biogeochemistry.

  • 8

    Advective flux measurement

    Seepage metersSeepage meters direct in-situ measurement small area, short time (extrapolation)

    Piezometers measure head difference and calculate with Darcys law hydraulic conductivity estimate neededhydraulic conductivity estimate needed

    Dyes tracer to track water movement perhaps best for qualitative rather than quantitative

    Advective flux in transient systems

    Water table rising brings solutesg g Water table drops, wetland drains out (slowly?) Measured/estimated from hydraulic heads, or

    from water balance (e.g., S = P-ET-G in cases where other terms are known to be zero)

    Broadly, advective fluxes are likely much higher than diffusive fluxes, but have received limited attention

  • 9

    Figure 14.4

    FIGURE 14.4 Schematic showing seepage cylinders placed together with one collection bag. From Rosenberg, D. O., Liminol. Oceanogr. Methods, 3, 131, 2005

    Diffusive flux processes

    Ficks (First) LawFick s (First) Law

    JD diffusive flux [MT-1L-2]D diffusion coefficient [L2T-1]

    dzdCDJ D =

    [ ]C solute concentration [ML-3] z depth [L]

  • 10

    Diffusion in soils Diffusion results from the thermally induced agitation of molecules

    (Brownian motion) In gases diffusion progresses at a rate of approximately 10 cm/min;In gases diffusion progresses at a rate of approximately 10 cm/min;

    in liquids about 0.05 cm/min and in solids about 0.00001 cm/min. Important diffusion processes in porous media include:

    diffusion of water vapor, organic vapors; diffusion of gases (O2, CO2, N2, etc.); diffusion of nutrients away from fertilizer granules and/or bands; diffusion of nutrients towards plant roots; and diffusion of solutes in the absence of advective flow diffusion is the dominant rate-limiting step for many physico-chemical

    processes of relevance in solute transport Diffusion occurs in the fluid phase (liquid and gaseous). Therefore,

    the porosity and pore-size distribution determine the geometry available for diffusion

    Table 1. Some typical diffusion coefficients

    1. Gas Phase Diffusion: (cm2 sec-1)

    O2 into air 0.209

    CO2 into air 0 163

    Diffusion in soils slower than in liquid phase

    CO2 into air 0.163

    2. Liquid Phase Diffusion:

    O2 in water 2.26 x 10-5

    CO2 in water 1.66 x 10-5

    NaCl in water 1.61 x 10-5

    Glucose in water 0.67 x 10-5

    3. Solid Phase Diffusion:

    Na in montmorillonite gel 4 x 10-6

    Na in vermiculite 6 x 10-9

    K in illite 10-23

    4. Diffusion in Soils:

    Cl in sandy clay loam ( =0.4) 9 x 10-6

    PO42- in sandy clay loam ( =0.4) 3.3 x 10-6

  • 11

    Figure 14.6

    A B

    Soil particles

    Pore space

    Tortuosity = solutes must follow an indirect path to move from A to B

    Lp = actual path lengthL = straight line from A to B

    Ds = effective diffusion coefficient in soil (less than D,diffusion coefficient in bulk fluid)

    = porosity

    1

    2

    >

    =

    L

    Lp

    DDs =

    Diffusive flux measurement

    Gradient-basedGradient based coring pore-water equilibrators multisamplers

    Overlying water incubations benthic flux chambers intact soil cores

  • 12

    Gradient methods

    In situ measurement of concentration gradient (dC/dz) and calculate diffusive flux based on known/estimated diffusion coefficient, porosity, tortuosity

    Pore water equilibrators (peepers) left in situ long enough for sample cells to reach equilibrium with

    porewater (>10 days) temporal average concentrationtemporal average concentration

    Multi-level samplers obtain instantaneous concentration, but depth resolution much less

    Figure 14.5

    x

    Dep

    th

    Concentration

    C

    x

    C

    Dep

    th

  • 13

    Figure 14.11

    High spatial (vertical) resolutionLow temporal resolution

    Incubation methods

    Directly measure mass dischargeDirectly measure mass discharge change in concentration in overlying water

    column, multiplied by volume of water = M column cross-sectional area = A duration of experiment = T

  • 14

    Figure 14.12

    12 VSample portTygon tubing

    DissolvedOxygen

    Flux box

    Recirc. pump

    ygmeter

    70 cm

    70 cm

    Benthic chambers = in situ, but (i) small area, (ii) inconvenient, and (iii) difficult

    Air Samplingport

    Figure 14.13a

    20 cm

    40 cm

    Sediment

    Water Column

    Floc

    Intact cores = ex situ, but relatively easy (therefore much more common); possible scale issues.

  • 15

    Figure 14.13b

    Figure 14.15

    2

    4

    6

    8

    10

    solv

    ed O

    xyge

    n (m

    g/L)

    00 24 48 72 96 120D

    iss

    Time (hours)Time (hours)

    0.4

    0.5

    0.6

    ve P

    (mg/

    L)

    AnaerobicAnaerobic

    Oxygen flux from water to soil, and P flux from soil to water.

    0.0

    0.1

    0.2

    0.3

    0 200 400 600 800 1000Dis

    solv

    ed R

    eact

    iv

    Time (hours)Time (hours)

  • 16

    Sediment movement

    Settling settling velocity = f(particle radius^2, particle density

    vs fluid density, fluid viscosity)

    Resuspensionimportant in shallow systems important in shallow systems

    likely orders of magnitude greater flux than diffusion ~ 10x for P in Lake Okeechobee (Fisher and Reddy, 1991) even greater for ammonium flux in Potomac estuary (Simon,

    1988)

    Figure 14.16

    tal nded

    cles

    tal nded

    cles

    Distance from inflow

    Tot

    Sus

    peP

    artic

    Distance from inflow

    Tot

    Sus

    peP

    artic

  • 17

    Figure 14.10

    Before Sediment Resuspension

    Floodwater

    During Sediment Resuspension

    After Sediment Resuspension

    Cs

    Cs

    Sad

    SadSad Cs Sad Cs

    Cs Sad Cs

    Soil/sediment

    Schematic showing adsorptiondesorption regulating solute concentration in the water column, as a result of resuspension and diffusive flux from sediment. Sad is solute adsorbed on sediment particles and Cs is solute in solution.

    Coupled hydrologic and biogeochemical modeling in wetlands

  • 18

    Modeling to address treatment wetland management questions

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    Example model application: Comprehensive description of P cycling

    Wang and Mitsch, Ecol. Modeling, 2000

    What can the model be used for? An example application...

    Wang and Mitsch, Ecol. Modeling, 2000

  • 20

    Solute Transport Model Hydraulicsy

    Inlet/outlet locations and flow rates Hydrodynamics

    Internal mixing Chemistry/Biology

    Sorption Sorption Uptake Release Degradation/Sequestration

    Velocity Vectors m/d

    200160 140

    Measured Flows1.5 cfs ~ 1 MGD

    0 450 50 100 150 200 250 300 350 400

    20140140

    150

    30 20

    430 m/d0 m/d

  • 21

    Total P concentration in WCA-2A soil (0-10 cm)

    1990 1998

    Total P concentration in WCA-2A soil (0-10 cm)

    T = 20 yrs T = 30 yrs

    0 12000 2000 4000 6000 8000 10000

  • 22

    T = 3, 15, 39, 66, 100, 133 years

    Upon completion of this course, participants should be able to:

    Biogeochemistry of Wetlands: Wetland hydrology

    Science and ApplicationsScience and Applications

    Describe how water velocity is determined in wetlandsExplain how/why Mannings roughness varies with depthUnderstand the biogeochemical implications of residence timeDifferent ways water flow through wetlands is describedUnderstand advective and diffusive processes for water-

    column/sediment exchangeDescribe measurement techniques for advective fluxDescribe the advantages and disadvantages of gradient-based vs

    6/22/2008 WBL 44

    Describe the advantages and disadvantages of gradient-based vs incubation-based measurement techniques for diffusive flux

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