1.1 This guide covers an overview of subsurface fluid-flow (ground-water) modeling. The term subsurface fluid flow is used to reduce misunderstanding regarding ground water, soil water, vapors including air in subsurface pores, and non-aqueous phase liquids. Increased understanding of fluid-flow phenomena is the combined result of field investigations and theoretical development of mathematical methods to describe the observations. The results are methods for modeling viscous fluids and air flow, in addition to water, that are practical and appropriate.

1.2 This guide includes many terms to assist the user in understanding the information presented here. A ground-water system (soils and water) may be represented by a physical, electrical, or mathematical model, as described in 6.4.3. This guide focuses on mathematical models. The term mathematical model is defined in 3.1.11; however, it will be most often used to refer to the subset of models requiring a computer.

1.3 This guide introduces topics for which other standards have been developed. The process of applying a ground-water flow model is described in Guide D5447. The process includes defining boundary conditions (Guide D5609), initial conditions (Guide D5610), performing a sensitivity analysis (Guide D5611), and documenting a flow model application (Guide D 5718). Other steps include developing a conceptual model and calibrating the model. As part of calibration, simulations are compared to site-specific information (Guide D5490), such as water levels.

1.4 Model use and misuse, limitations, and sources of error in modeling are discussed in this standard. This guide does not endorse particular computer software or algorithms used in the modeling investigation. However, this guide does provide references to some particular codes that are representative of different types of models.

1.5 Typically, a computer model consists of two parts; computer code that is sometimes called the computer program or software, and a data set that constitutes the input parameters that make up the boundary and initial conditions, and medium and fluid properties. A standard has been developed to address evaluation of model codes (see Practice E978).

1.6 Standards have been prepared to describe specific aspects of modeling, such as simulating subsurface air flow using ground-water flow modeling codes (see Guide D5719) and modeling as part of the risk-based corrective action process applied at petroleum release sites (see Guide ES 38).

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.