Post on 05-Jul-2018
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Wetland hydrology, transport
Institute of Food and Agricultural Sciences (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
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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)
• Manning’s 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/ /
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Manning’s 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-β)– 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/(εAc) ε = 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: τMean residence time from a tracer test (residence time distribution)
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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
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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]
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Advective flux processes
• Surface water/Groundwater exchangeSurface water/Groundwater exchange• Bioturbation• Phreatophytic mixing
Figure 14.7
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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.
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Advective flux measurement
• Seepage metersSeepage meters– direct in-situ measurement– small area, short time (extrapolation)
• Piezometers– measure head difference and calculate with Darcy’s 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., ΔS = 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
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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
• Fick’s (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]
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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
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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)
η = porosity
1
2
>⎟⎟⎠
⎞⎜⎜⎝
⎛=
L
Lpθ
η
θDDs =
Diffusive flux measurement
• Gradient-basedGradient based– coring– pore-water equilibrators– multisamplers
• Overlying water incubations– benthic flux chambers– intact soil cores
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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
Δ x
Dep
th
Concentration
Δ C
Δ x
Δ C
Dep
th
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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
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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.
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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)
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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
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Figure 14.10
Before Sediment Resuspension
Floodwater
During Sediment Resuspension
After Sediment Resuspension
Cs
Cs
Sad
SadSad Cs Sad Cs
CsSad Cs
Soil/sediment
Schematic showing adsorption–desorption 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
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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
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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
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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
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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 Manning’s 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