1. Introduction

There is a continuing need for quantitative information about the subsurface environment. Applications include developing and managing new or existing water supplies, storm water management, energy storage and recovery, environmental remediation, artificial recharge, gas and oil production, carbon sequestration, and many others. The spectrum of processes that might be of interest include essentially all the physical, chemical, and biological processes that are operational in any natural or engineered environment. Physical processes might include fluid flow by advection and mixing/spreading processes such as dispersion, conduction, and convection that apply to transport of matter and energy. Chemical processes might include the wide range of chemical reactions that are possible in water/mineral/atmosphere systems such as acid/base, precipitation/dissolution, sorption, surface complexation, and oxidation/reduction reactions. Biological processes might include any of the diverse metabolic activities displayed by indigenous microorganisms as they obtain energy and nutrients for growth and include processes related to nutrient cycling, detoxification of contaminants, and many others. Of course all of these processes are coupled so that physical properties will influence fluid flow, which controls the flow of substrates that in turn influence microbial growth, or solutes that participate in chemical reactions. There is no satisfactory term to describe the coupling among all these processes; “biogeochemical processes” will be used here with the explicit understanding that physical processes are also included.