Numerical Modeling of Natural Attenuation

• Amy B. Chan Hilton, Ph.D.
• Dept. of Civil & Environmental Engineering
• FAMU-FSU College of Engineering
• http://www.eng.fsu.edu/~abchan
• abchan@eng.fsu.edu

Natural Attenuation Processes

• Advection
• Dispersion
• Sorption
• Biodegradation
• Volatilization
• Hydrolysis

Available Simulation Models

• BIO1D/BIO2D
• BIOSCREEN
• BIOCHLOR
• BIOPLUME
• RT3D
• Florida RNA Toolkit
• MAROS

BIO1D

• Developed by Srinivasan and Mercer
• Advective and dispersive transport of hydrocarbon and electron acceptor (e.g.,oxygen)
• Aerobic biodegradation (Monod) and Anaerobic biodegradation (Michaelis-Menten)
• First-order degradation for both substances
• Linear, Freundlich, and Langmuir adsorption isotherms

BIOSCREEN

• Spreadsheet screening model for simulating NA of dissolved hydrocarbons (fuels)
• Developed by Newell et al. (1996) for AFCEE
• Analytical model of advection, dispersion, linear sorption
• Aerobic and anaerobic reactions (first-order or instantaneous)
• For simple GW flow conditions

BIOCHLOR

• Screening model that simulates NA of dissolved chlorinated solvents.
• Developed by Aziz et al. (1999)
• Similar to BIOSCREEN
• Solvent degradation using sequential first-order decay process, with one or two reaction zones
• Reductive dechlorination of chlorinated ethenes (PCE-ethene) & chlorinated ethanes (TCA-CA)

BIOPLUME II/III

• Natural attenuation model for transport of dissolved hydrocarbons under the influence of oxygen-limited biodegradation
• Advection, dispersion, sorption & ion exchange
• Based on the USGS MOC (Konikow and Bredehoeft, 1989)
• Aerobic and anaerobic biodegradation processes (first-order decay, instantaneous reaction and Monod kinetics)
• Electron acceptors: O2, NO3, Fe(III), SO4, CO2

RT3D

• 3-D, multi-species, reactive transport model
• Developed by Clement et al. (1998)
• Based MODFLOW and MT3D
• Instantaneous or kinetically limited biodegradation, using multiple electron acceptors
• Non-equilibrium sorption/desorption
• NAPL dissolution
• Double-porosity
• Reductive, anaerobic or aerobic biodegradation of PCE/TCE/DCE/VC

Florida RNA Toolkit

• Comprehensive Software used to assess applicability of RNA in accordance with the Florida Petroleum Cleanup Program.
• Developed by Groundwater Services, Inc. (1999)
• Analytical groundwater transport model based on BIOSCREEN
• Predicts long-term natural attenuation rates and derives source area target concentrations

MAROS

• Optimize long-term groundwater monitoring programs in accordance with AFCEE Long-Term Monitoring Optimization Guide http://www.afcee.brooks.af.mil/er/erproducts.htm
• Developed by Groundwater Services Inc. and University of Houston for AFCEE
• Decision support tool (database) based on statistical methods
• Tracks occurrence of contaminant migration

Software Availability

• BIO1D/BIO2D http://www.mines.edu/research/igwmc/
• BIOSCREEN, BIOCHLOR, BIOPLUME http://www.epa.gov/docs/ahaazvuc/csmos/models.html
• RT3D http://bioprocess.pnl.gov/rt3d.htm
• Florida RNA Toolkit http://www.gsi-net.com
• MAROS http://www.gsi-net.com/RBCATools/MAROS.htm

Applying Models

• Typically, calibrate model and then prediction
• "Ground-water models cannot be validated" (Konikow & Bredehoeft, Advances in Water Resources, v. 15, 75-83, 1992)
  • Sources of error: conceptual model, numerical errors, uncertainty/inadequacy in input data
  • Calibrating: nonunique solution
  • Prediction: length of prediction ~ duration of data
  • Postaudit: update model based on predictions and new data