Transcript BioPlume Slides

BIOPLUME II
Introduction to Solution Methods and
Model Mechanics
What does it do?
• Two dimensional finite difference model for simulating
natural attenuation due to:
– advection
– dispersion
– sorption
– biodegradation
How Does BPIII Solve Equations?
• Contaminant transport solved using the Method of
Characteristics
• Particles travel along Characteristic lines determined
by flow solution.
• Particles carry mass
• Advection solved via particle movement
• Dispersion solved explicitly
• Reaction solved explicitly
– First order decay
– Instantaneous Biodegradation
Particle Movement
Ini tial locati on of particle
New l ocati on of particle
Flow l ine and direction of fl ow
Computed path of parti cl e
Limitations/Assumptions
• Darcy’s Law is valid
• Porosity and hydraulic conductivity constant in time,
porosity constant in space
• Fluid density, viscosity and temperature have no effect on
flow velocity
• Reactions do not affect fluid or aquifer properties
• Ionic and molecular diffusion negligible
• Vertical variations in head/concentration negligible
• Homogeneous, isotropic longitudinal and transverse
dispersivity
Limitations of Biodegradation
• No selective or competitive biodegradation of
hydrocarbons (lumped hydrocarbons)
• Conceptual model of biodegradation is a simplification
of the complex biologically mediated redox reactions
that occur in the subsurface
BIOPLUME II Flowchart
Start
Read Geologic, Hydrogeologic
& Chemical Input Data
Generate Uniformly Distributed Particles
Compute Hydraulic Gradients
Compute Ground Water Velocities
Compute Dispersion Coefficients
Determine ²t for Explicit Calculations
Move Particles
N
N
Y
End of
Time Step?
Make New/Remove Old Particles @ Edges
End of
Pumping
Period?
Y
Calc. Avg. Conc. in Each Cell
Summarize and Print Results
Compute Explicitly Conc. at Nodes
Adjust Concentration of Each Particle
Stop
Compute Mass Balance
Y
End of
Simulation?
N
HOW TO SET UP A MODEL
1. Data Collection & Analysis
2. Modeling Scale
3. Discretization
4. Boundary Conditions
5. Parameter Estimation
6. Calibration
7. Sensitivity Analysis
8. Error Estimation
9. Prediction
SOURCE DATA
• Mass of contaminant
• Q, C0
• Discrete vs. Continuous
Nature of contaminant
• Chemical stability
• Biological stability
• Adsorption
PARAMETER ESTIMATION
1. Porosity
2. Dispersivity
3. Storage coefficients
4. Hydraulic conductivity
5. Thickness of unit
6. Recharge rates
REGIONAL SCALE QUANTITATIVE
•
•
•
•
Aquifer characteristics
Background gradients
Geology
Recharge sources
LOCAL SCALE - WATER QUALITY
•
•
•
•
•
Site history
Site characterization
Source definition
Nature of contamination
Plume delineation
MOC TIMING PARAMETERS
Total Simulation Time
1st pumping period
2nd
NPMP = 2
For Each Pumping Period
PINT = pumping period in yrs
NTIM = # of time steps in pumping period
MOC BOUNDARY CONDITIONS
Two types
• Constant Head
– Water Table = constant
• Constant Flux
– Flow rate Q
– Concentration C0
MOC BOUNDARY CONDITIONS
Specifications of NCODES
For Each Code in NOEID map
• LEAKANCE (s-1)
– vertical hyd. conduct. / thickness
• CONCENTRATION OF CONTAMINANT
• RECHARGE RATE (ft/s)
NOTE
For constant head cells set LEAKANCE to 1.0
MOC SOURCE DEFINITION
Injection well
• Flow rate - Q
• Concentration - C0
Constant Head Cell
• C=C0
Recharge Cell
• Flow rate - Q
• Concentration - C0
PHYSICAL AQUIFER
CHARACTERISTICS
1. Transmissivity (ft2/s) – VPRM
2. Thickness (ft) – THCK
3. Dispersivity (ft)
Longitudinal – BETA
Ratio – DLTRAT = Txx/Tyy
4. Porosity – POROS
5. Storativity – S
NOTE
For transient problems
TIMX – increment multiplier
TINIT – size of initial time step
MOC REACTION PARAMETERS
NREACT
Flag to instruct MOC to expect reaction data
0 - no reactions
1 - reactions taking place
expect card # 4 free format
Two types of reaction:
RETARDATION
KD - Distribution coefficient
RHOB - Bulk density
RADIOACTIVE DECAY
THALF - Half life of solute
INPUT PARAMETERS AFFECTING
ACCURACY FOR HYDRAULIC
CALCULATIONS
ITMAX
Maximum allowable number of iterations: 100-200
Increase ITMAX if hydraulic mass balance error is >
1%
NITP
Number of iteration parameters
USE 7
TOL
Convergence criteria: <0.01
Decrease TOL to get less hydraulic mass balance error
PARAMETERS AFFECTING
ACCURACY OF TRANSPORT
NPTPND - Number of particles in a cell
NPMAX - Maximum number of particles
= NX • NY • NPTPND
STABILITY CRITERIA FOR MOC
MOC may require dividing NTIM or PINT into smaller
move time steps
•t minimum of
0.5
– Dispersion
– Mixing
D yy
Dxx

dx2 dy2
nbi , j ,k 
Wi , j ,k
 x
– Advection
Vx max
 y
V 
y max
INPUT PARAMETERS AFFECTING
STABILITY OF MOC
CELDIS - max distance per move
– If CELDIS < space between particles MOC will oscillate for
N yrs BUT gives smallest Mass Balance errors for T>N
– If CELDIS = Stability Criteria DO a sensitivity analysis on
CELDIS
NPTPND - initial # of particles
– Accuracy of MOC directly proportional to NPTPND
– Runtime inversely proportional to NPTPND
RULE OF THUMB
– Initially set NPTPND=4 or 5 and CELDIS=0.75 or 1
– For final runs use NPTPND=9 and CELDIS=0.5
Output control
NPNTMV
Number of particle moves after which output is requested. Use 0 to
print at end of time steps
NPNTVL
Printing velocities
0 - do not print
1 - print for first time step
2 - print for all time steps
Output control (cont.)
NPNTD
Print dispersion equation coefficients
NPDELC
Print changes in concentration
NPNCHV
Do not use this option. Always set to 0. It is used to request cards to
be punched.