ASTM D6170-17

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Designation: D6170 − 97 (Reapproved 2010) D6170 − 17
Standard Guide for
Selecting a Groundwater Modeling Code
This standard is issued under the fixed designation D6170; the number immediately following the designation indicates the year of
original adoption or, in the case of revision, the year of last revision. A number in parentheses indicates the year of last reapproval. A
superscript epsilon (´) indicates an editorial change since the last revision or reapproval.
1. Scope Scope*
1.1 This guide covers a systematic approach to the determination of the requirements for and the selection of computer codes
used in a groundwater modeling project. Due to the complex nature of fluid flow and biotic and chemical transport in the
subsurface, many different groundwater modeling codes exist, each having specific capabilities and limitations. Furthermore, a
wide variety of situations may be encountered in projects where groundwater models are used. Determining the most appropriate
code for a particular application requires a thorough analysis of the problem at hand and the required and available resources, as
well as detailed description of the functionality of candidate codes.
1.2 The code selection process described in this guide consists of systematic analysis of project requirements and careful
evaluation of the match between project needs and the capabilities of candidate codes. Insufficiently documented capabilities of
candidate codes may require additional analysis of code functionality as part of the code selection process. Fig. 1 is provided to
assist with the determination of project needs in terms of code capabilities, and, if necessary, to determine code capabilities.
1.3 This guide is one of a series of guides on groundwater modeling codes and their applications, such as Guides D5447, D5490,
D5609, D5610, D5611, D5718, and D6025.
1.4 This guide offers an organized collection of information or a series of options and does not recommend a specific course
of action. This guide cannot replace education or experience and should be used in conjunction with professional judgement. Not
all aspects of this guide may be applicable in all circumstances. This guide is not intended to represent or replace the standard
of care by which the adequacy of a given professional service must be judged, nor should this guide be applied without
consideration of a project’s many unique aspects. The word “Standard” in the title of this document means only that the document
has been approved through the ASTM consensus process.
1.5 This standard does not purport to address all of the safety concerns, if any, associated with its use. It is the responsibility
of the user of this standard to establish appropriate safety and health practices and determine the applicability of regulatory
limitations prior to use.
2. Referenced Documents
2.1 ASTM Standards:
D653 Terminology Relating to Soil, Rock, and Contained Fluids
D5447 Guide for Application of a Groundwater Flow Model to a Site-Specific Problem
D5490 Guide for Comparing Groundwater Flow Model Simulations to Site-Specific Information
D5609 Guide for Defining Boundary Conditions in Groundwater Flow Modeling
D5610 Guide for Defining Initial Conditions in Groundwater Flow Modeling
D5611 Guide for Conducting a Sensitivity Analysis for a Groundwater Flow Model Application
D5718 Guide for Documenting a Groundwater Flow Model Application
D6025 Guide for Developing and Evaluating Groundwater Modeling Codes (Withdrawn 2017)
3. Terminology
3.1 Definitions of Terms Specific to This Standard:
This guide is under the jurisdiction of ASTM Committee D18 on Soil and Rock and is the direct responsibility of Subcommittee D18.21 on Groundwater and Vadose
Zone Investigations.
Current edition approved July 1, 2010Jan. 1, 2017. Published September 2010January 2017. Originally approved in 1997. Last previous edition approved in 20042010
as D6170 – 97 (2004).(2010). DOI: 10.1520/D6170-97R10.10.1520/D6170-17.
For referenced ASTM standards, visit the ASTM website, www.astm.org, or contact ASTM Customer Service at service@astm.org. For Annual Book of ASTM Standards
volume information, refer to the standard’s Document Summary page on the ASTM website.
The last approved version of this historical standard is referenced on www.astm.org.
*A Summary of Changes section appears at the end of this standard
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States
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FIG. 1 Checklist for Groundwater Modeling Needs and Code Functionality
3.1.1 analytical model—in groundwater modeling, a model that uses closed form solutions to the governing equations
applicable to groundwater flow and transport processes.
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FIG. 1 Checklist for Groundwater Modeling Needs and Code Functionality (continued)
3.1.2 code selection—the process of choosing the appropriate computer code, algorithm, or other analysis technique capable of
simulating those characteristics of the physical system required to fulfill the modeling project’s objective(s).
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FIG. 1 Checklist for Groundwater Modeling Needs and Code Functionality (continued)
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FIG. 1 Checklist for Groundwater Modeling Needs and Code Functionality (continued)
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FIG. 1 Checklist for Groundwater Modeling Needs and Code Functionality (continued)
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FIG. 1 Checklist for Groundwater Modeling Needs and Code Functionality (continued)
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FIG. 1 Checklist for Groundwater Modeling Needs and Code Functionality (continued)
3.1.3 computer code (computer program)—assembly of numerical techniques, bookkeeping, and control language that
represents the model from acceptance of input data and instructions to delivery of output.
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FIG. 1 Checklist for Groundwater Modeling Needs and Code Functionality (continued)
3.1.4 conceptual model—an interpretation or working description of the characteristics and dynamics of the physical system.
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FIG. 1 Checklist for Groundwater Modeling Needs and Code Functionality (continued)
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FIG. 1 Checklist for Groundwater Modeling Needs and Code Functionality (continued)
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FIG. 1 Checklist for Groundwater Modeling Needs and Code Functionality (continued)
3.1.5 functionality—of a groundwater modeling code, the set of functions and features the code offers the user in terms of model
framework geometry, simulated processes, boundary conditions, and analytical and operational capabilities.
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FIG. 1 Checklist for Groundwater Modeling Needs and Code Functionality (continued)
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FIG. 1 Checklist for Groundwater Modeling Needs and Code Functionality (continued)
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FIG. 1 Checklist for Groundwater Modeling Needs and Code Functionality (continued)
3.1.6 groundwater modeling code—the non-parameterized computer code used in groundwater modeling to represent a
non-unique, simplified mathematical description of the physical framework, geometry, active processes, and boundary conditions
present in a reference subsurface hydrologic system.
3.1.7 mathematical model—(a) mathematical equations expressing the physical system and including simplifying assumptions;
(b) the representation of a physical system by mathematical expressions from which the behavior of the system can be deduced
with known accuracy.
3.1.8 model construction—the process of transforming the conceptual model into a parameterized mathematical form; as
parametrization requires assumptions regarding spatial and temporal discretization, model construction requires a priori selection
of a computer code.
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3.1.9 model schematization—simplification of a conceptualized groundwater system for quantitative, model-based analysis
commensurate with project objectives and constraints.
3.1.10 numerical model—in groundwater modeling, a model that uses numerical methods to solve the governing equations of
the applicable problem.
3.1.11 semi-analytical model—a mathematical model in which complex analytical solutions are evaluated using approximate
techniques, resulting in a solution discrete in either the space or time domain.
3.1 For definitions of other terms used in this guide, see Terminology D653.
3.2 Definitions of Terms Specific to This Standard:
3.2.1 analytical model—in groundwater modeling, a model that uses closed form solutions to the governing equations
applicable to groundwater flow and transport processes.
3.2.2 code selection—the process of choosing the appropriate computer code, algorithm, or other analysis technique capable of
simulating those characteristics of the physical system to fulfill the modeling project’s objective(s).
3.2.3 conceptual model—an interpretation or working description of the characteristics and dynamics of the physical system.
3.2.4 functionality—of a groundwater modeling code, the set of functions and features the code offers the user in terms of model
framework geometry, simulated processes, boundary conditions, and analytical and operational capabilities.
3.2.5 groundwater modeling code—the non-parameterized computer code used in groundwater modeling to represent a
non-unique, simplified mathematical description of the physical framework, geometry, active processes, and boundary conditions
present in a reference subsurface hydrologic system.
3.2.6 mathematical model—(a) mathematical equations expressing the physical system and including simplifying assumptions;
(b) the representation of a physical system by mathematical expressions from which the behavior of the system can be deduced
with known accuracy.
3.2.7 model construction—the process of transforming the conceptual model into a parameterized mathematical form; as
parametrization requires assumptions regarding spatial and temporal discretization, model construction requires a priori selection
of a computer code.
3.2.8 model schematization—simplification of a conceptualized groundwater system for quantitative, model-based analysis
commensurate with project objectives and constraints.
3.2.9 numerical model—in groundwater modeling, a model that uses numerical methods to solve the governing equations of the
applicable problem.
3.2.10 semi-analytical model—a mathematical model in which complex analytical solutions are evaluated using approximate
techniques, resulting in a solution discrete in either the space or time domain.
4. Significance and Use
4.1 Groundwater modeling has become an important methodology in support of the planning and decision-making processes
involved in groundwater management. Groundwater models provide an analytical framework for obtaining an understanding of the
mechanisms and controls of groundwater systems and the processes that influence their quality, especially those caused by human
intervention in such systems. Increasingly, models are an integral part of water resources assessment, protection, and restoration
studies, and provide essentialneeded and cost-effective support for planning and screening of alternative policies, regulations, and
engineering designs affecting groundwater.
4.2 Many different groundwater modeling codes are available, each with their own capabilities, operational characteristics and
limitations. Furthermore, each groundwater project has its own requirements with respect to modeling. Therefore, it is important
that the most appropriate code is selected for a particular project. This is even more important for projects that require extensive
modeling, or where costly decisions are based, in part, on the outcome of modeling-based analysis.
4.3 Systematic and comprehensive description of project requirements and code features provides the necessary basis for
efficient selection of a groundwater modeling code. This standard guide is intended to encourage comprehensive and consistent
description of code capabilities and code requirements in the code selection process, as well as thorough documentation of the code
selection process.
5. Code Selection Process in Groundwater Modeling
5.1 Code selection in groundwater modeling is a crucial step in the application of groundwater models (see Guide D5447). Each
groundwater project in which computer-based modeling is performed should include a code selection phase.
National Research Council (NRC), Committee on Ground Water Modeling Assessment, Water Science and Technology Board, Ground Water Models: Scientific and
Regulatory Applications, National Academy Press, Washington, DC, 1990.
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5.2 Code selection is in essence the process of matching a project’s modeling needs with the documented capabilities of existing
computer codes.
5.3 Selecting an appropriate code requires analysis and systematic description of both the modeling needs and the characteristics
of existing groundwater modeling codes.
5.4 A perfect match rarely exists between desired code characteristics or selection criteria and the capabilities or functionality
of available codes. Therefore, the selection criteria are divided into the following two groups: essential code capabilities and
non-essential code capabilities. If a candidate code does not include the essential capabilities, it should be removed from
consideration.
5.5 The relative importance of the non-essential code capabilities needs to be assessed. This may be done by assigning
weighting factors to the considered capabilities (for example, using weights from one to five according to their relative
importance). Although such weighing factors are often not explicitly mentioned in the code selection process, candidate codes are
often ranked implicitly using some kind of weighting of the non-essential capabilities. Assigning weighting factors is a rather
subjective procedure; if a match is difficult to obtain, reassessment of these factors may be necessary. Hence, code selection may
turn out to be a rather iterative process requiring a significant level of professional judgment and experience.
5.6 Selecting the right code is critical in ensuring necessary for an optimal trade-off between effort and result in a modeling
project. The result can be expressed as the expected effectiveness of the modeling tasks in terms of prediction accuracy. The effort
is basically represented by the modeling costs, such as incurred in becoming familiar with the code, model schematization and
model construction, and model-based scenario analysis. Such costs should not be considered independently from those of field data
acquisition, especially those requiredneeded for the modeling effort. For a proper assessment of modeling cost, consideration
should be given to the choice of developing a new code (or modifying an existing one) versus acquisition of an existing code, the
implementation and maintenance of the code, computer platform requirements, and the development and maintenance of databases.
NOTE 1—The availability of or familiarity with a particular code, or both, may lead to modeling overkill by using a pre-chosen code requiring
significantly more preparation in data gathering and model construction than necessary for the project. Such modeling overkill may also result from the
user’s inability to limit the number of “essential” code features, or to discriminate between non-essential code features.
NOTE 2—The belief that use of the “best” or most mathematically advanced codes will automatically provide predictive reliability and scientific
credibility is false. The technical capability of the modeler or the modeling team involved in the modeling project has the greatest impact on the overall
results.
5.7 If different project questions need to be addressed, more than one code might be needed or different combinations of
functions of a single code may be utilized. This is often the case when models are used in different stages of the project. For
example, in an early stage of a remediation project, a model is used to assist in problem scoping and system conceptualization,
while during the design phase of the project, a model is used to screen between alternative remediation techniques and to detail
the selected remediation approach.
5.8 If, as a result of the code selection process, a code is selected that requires modification, proper quality assurance procedures
for code development and testing need to be followed (see Guide D6025).
6. Defining Modeling Needs
6.1 Following are major steps in evaluating modeling needs: formulating the project-related modeling objectives; determining
the required level of analysis (that is, modeling complexity) and reliability in terms of prediction accuracy and sensitivity of the
project for incorrect or imprecise answers (that is, acceptable level of uncertainty); conceptualizing and characterizing the
groundwater system involved; and analyzing the constraints in human and material resources ava
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