Ground-water modeling has become an important methodology in support of the planning and decision-making processes involved in ground-water management. Ground-water models provide an analytical framework for obtaining an understanding of the mechanisms and controls of ground-water 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 essential and cost-effective support for planning and screening of alternative policies, regulations, and engineering designs affecting ground water.³

There are many different ground-water modeling codes available, each with their own capabilities, operational characteristics, and limitations. If modeling is considered for a project, it is important to determine if a particular code is appropriate for that project, or if a code exists that can perform the simulations required in the project.

In practice, it is often difficult to determine the capabilities, operational characteristics, and limitations of a particular ground-water modeling code from the documentation, or even impossible without actual running the code for situations relevant to the project for which a code is to be selected due to incompleteness, poor organization, or incorrectness of a code’documentation.⁴

Systematic and comprehensive description of a code’features based on an informative classification provides the necessary basis for efficient selection of a ground-water modeling code for a particular project or for the determination that no such code exists. This guide is intended to encourage correctness, consistency, and completeness in the description of the functions, capabilities, and limitations of an existing ground-water modeling code through the formulation of a code classification system and the presentation of code description guidelines.

1.1 This guide presents a systematic approach to the classification and description of computer codes used in ground-water modeling. Due to the complex nature of fluid flow and biotic and chemical transport in the subsurface, many different types of ground-water modeling codes exist, each having specific capabilities and limitations. 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 a detailed description of the functionality of potentially applicable codes.

1.2 Typically, ground-water modeling codes are nonparameterized mathematical descriptions of the causal relationships among selected components of the aqueous subsurface and the chemical and biological processes taking place in these systems. Many of these codes focus on the presence and movement of water, dissolved chemical species and biota, either under fully or partially saturated conditions, or a combination of these conditions. Other codes handle the joint movement of water and other fluids, either as a gas or a nonaqueous phase liquid, or both, and the complex phase transfers that might take place between them. Some codes handle interactions between the aqueous subsurface (for example, a ground-water system) and other components of the hydrologic system or with nonaqueous components of the environment.

1.3 The classification protocol is based on an analysis of the major function groups present in ground-water modeling codes. Additional code functions and features may be identified in determining the functionality of a code. A complete description of a code's functionality contains the details necessary to understand the capabilities and potential use of a ground-water modeling code. Tables are provided with explanations and examples of functions and function groups for selected types of codes. Consistent use of the descriptions provided in the classification protocol and elaborate functionality analysis form the basis for efficient code selection.

1.4 Although ground-water modeling codes exist for simulation of many different ground-water systems, one may encounter situations in which no existing code is applicable. In those cases, the systematic description of modeling needs may be based on the methodology presented in this guide.

1.5 This guide is one of a series of guides on ground-water modeling codes and their applications, such as Guides D 5447, D 5490, D 5609, D 5610, D 5611, and D 5718.

1.6 Complete adherence to this guide may not be feasible. For example, research developments may result in new types of codes not yet described in this guide. In any case, code documentation should contain a section containing a complete description of a code's functions, features, and capabilities.

1.7 This guide offers an organized collection of information or a series of options and does not recommend a specific course of action. This document cannot replace education or experience and should be used in conjunction with professional judgment. Not all aspects of this guide may be applicable in all circumstances. This ASTM standard 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 document 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.