Natural Attenuation Tool

Natural Attenuation Models

Simple and powerful software tools for evaluating important processes

For decades, GSI has been at the forefront in developing modeling tools that provide effective approaches for evaluating natural attenuation processes at contaminated sites. These tools are designed specifically to make modeling easier for a wide audience, with the goal of improving site decision-making through better understanding of natural attenuation processes.

Each of these modeling tools have been developed in coordination with federal agencies, including the United Statement Environmental Protection Agency, the Air Force Civil Engineering Center, and the Department of Energy. They leverage GSI’s skills in modeling fate and transport processes to answer critical questions about attenuation of compounds in the subsurface. In several cases, GSI has partnered with academic and regulatory experts during the development processes. As free software, these models have been extensively downloaded and used by environmental professionals, and they have also served as valuable educational resources for understanding natural attenuation concepts.

Key Natural Attenuation Modeling tools developed by GSI include:

  • BIOCHLOR
  • BIOSCREEN
  • BIOBALANCE Toolkit
  • Source DK
  • RTM-CSIA-MNA

BIOCHLOR

Popular model for simulating natural attenuation of chlorinated solvents

BIOCHLOR is a screening model developed by GSI for the U.S. Environmental Protection Agency (USEPA) to simulate remediation of chlorinated solvents in groundwater by natural attenuation. The goal is to provide users with the information they need to evaluate site-specific data and decide whether monitored natural attenuation (MNA) is a suitable remedy for managing their site.

This software allows the user to enter site-specific geochemical and field parameter data in a step-by-step process to generate graphs of concentration versus distance. These predictions can then be compared to observed concentrations in wells along the centerline of the plume to estimate site-specific attenuation rates. The user can interactively observe the effect of parameter values on the predicted curves by changing the values and viewing the graphs. Users can also enter various simulation times to illustrate how the plume will change over time and confirm if MNA will achieve a threshold concentration at a downgradient point of compliance.

BIOCHLOR was developed for the USEPA and is hosted at their site. Click the link below to visit their page and download this free software.
https://www.epa.gov/water-research/biochlor-natural-attenuation-decision-support-system

BIOSCREEN

Well-recognized natural attenuation model for petroleum hydrocarbons

BIOSCREEN is a model developed by GSI for the U.S. Environmental Protection Agency (USEPA) to screen monitored natural attenuation (MNA) as an option for remediating petroleum fuel release sites. This tool allows remediation managers to identify sites where natural attenuation is most likely to be protective of human health and the environment. It is also designed to allow regulators to carry out an independent assessment of treatability studies and remedial investigations that propose the use of natural attenuation.

The model is designed to simulate biodegradation by both aerobic and anaerobic reactions.
This software, programmed in the Microsoft Excel spreadsheet environment and based on the Domenico analytical solute transport model, has the ability to simulate advection, dispersion, adsorption, and aerobic decay, as well as anaerobic reactions that have been shown to be the dominant biodegradation processes at many petroleum release sites. BIOSCREEN includes three different model types:

  • Solute transport without decay
  • Solute transport with biodegradation modeled as a first-order decay process (simple, lumped-parameter approach)
  • Solute transport with biodegradation modeled as an instantaneous biodegradation reaction with multiple soluble electron acceptors including dissolved oxygen, nitrate, and sulfate

BIOSCREEN was developed for the USEPA and is hosted at their site. Click the link below to visit their page and download this free software.
https://www.epa.gov/water-research/bioscreen-natural-attenuation-decision-support-system

BioBalance Toolkit

Practical tool for quantifying natural attenuation capacity

The BIOBALANCE Toolkit was developed by GSI with support from the Department of Energy as a way to perform mass balance calculations to support evaluations of Monitored Natural Attenuation (MNA) at chlorinated solvent sites. By using the mass balance approach, site managers can demonstrate that the assimilative capacity of a particular impacted aquifer system is either capable (or incapable) of managing the mass flux of chlorinated solvents emitted from a source zone. 

In addition, a detailed mass balance at a site can provide valuable insight on:
Which processes contribute to the overall assimilative capacity?
How the source term might change over the long-term lifetime of the source?
To what degree competing reactions interfere with solvent biodegradation processes?
How sustainable biodegradation is likely to be over the long term?

However, the mass balance approach can be difficult to apply at sites with current solute transport models—particularly numerical models—because they do not:
1. Have the ability to directly provide key mass flux data
2. Distinguish between mass loss due to different natural attenuation processes
3. Simulate long-term behavior of sources at sites

The BIOBALANCE Toolkit was developed to address these shortcomings. Other notable developments incorporated in BIOBALANCE include:
The ability to assess stability of plumes originating from both vadose and submerged source zones.
Evaluate plume stability (time and size) using an iterative approach that documents the relative contributions of various attenuation mechanisms.
Examine the sustainability of anaerobic degradation processes based on an approximate balance of electron acceptor and electron donor.
Provide an overarching accounting of mass balance results from the various modules in the form of a summary report.

Source DK

Versatile but simple model for estimating remediation timeframes

SourceDK is a computer decision support system for estimating remediation timeframes and assessing the uncertainty associated with those estimates. Developed by GSI Environmental Inc. for the Air Force Civil Engineering Center (AFCEC) and programmed in the Microsoft Excel, SourceDK uses three relatively simple approaches to estimate remediation timeframes and the uncertainty in the timeframe estimate.

The model can be used to predict concentrations of dissolved constituents in groundwater over time at a particular well or particular zone at a site. The modeling approach is designed to account for the presence of non-aqueous phase liquids (NAPLs) and other source materials in the saturated zone which control how long organic groundwater plumes will persist in the subsurface. All three approaches include methods to estimate the uncertainty in any remediation timeframe estimate

As a screening- and planning-level tool, SourceDK is a screening- and planning-level tool that can be applied to a variety of different types of source zones in groundwater. While SourceDK is primarily geared for natural attenuation processes, it can also be used to estimate source lifetimes for some flushing-based technologies, primarily groundwater pump-and-treat.

RTM-CSIA MNA Model

Innovative approach combining isotope analysis and reactive transport modeling

RTM-CSIA is an approach for combining compound-specific isotope analysis (CSIA) with reactive transport modeling (RTM) to support a more comprehensive evaluation of monitored natural attenuation (MNA) as a remedy for contaminated groundwater sites. This method was developed with researchers from the University of Oklahoma and VU University Amsterdam as part of a DoD-sponsored project (ER-201029) to create next-generation guidance for evaluating MNA. The goal was to design a new and novel way to strengthen interpretation of both CSIA and conventional analytical data. This so-called “Generation 3” MNA analyses provides stronger, clearer evidence of the strength of attenuation processes, with a focus on chlorinated solvent plumes.

The popularity of CSIA has risen rapidly among project managers as one line of evidence supporting MNA remedies. Compound-specific isotope analysis (CSIA) is a specialized laboratory method that can provide direct evidence of biological degradation of chlorinated solvents in groundwater plumes. CSIA can also support assessment of the strength of other natural attenuation processes. However, CSIA data alone can be difficult to interpret, especially at sites with complex hydrogeology or release histories. The CSIA-RTM Guidance Document (link below) describes methods for collecting and interpreting CSIA data. Simple data interpretation techniques described in the Guidance Document include carbon isotope mass balances, dual isotope plots and isotope ratio vs. fraction of contaminant remaining plots.

The guidance also provides detailed instructions on how to download the software and apply the RTM-CSIA methods, including examples from real sites. Reactive transport modeling simulates transport and contaminant degradation, using a simplified numerical representation of the features of the modeled site. RTMs enables to simulate complex reaction networks (e.g., sequential reductive dechlorination together with oxidative degradation) together with isotope fractionation (C, H, Cl), while accounting for physical processes that may influence isotope ratios.

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