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

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

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, C 0 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 C 0

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 - C 0 Constant Head Cell C=C 0 Recharge Cell Flow rate - Q Concentration - C 0

PHYSICAL AQUIFER CHARACTERISTICS 1. Transmissivity (ft 2 /s) – VPRM 2. Thickness (ft) – THCK 3. Dispersivity (ft) Longitudinal – BETA Ratio – DLTRAT = T xx /T yy 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: 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 –Dispersion –Mixing –Advection

INPUT PARAMETERS AFFECTING STABILITY OF MOC CELDIS - max distance per move –If CELDIS 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

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. Output control (cont.)