Water is a basic necessity for life, and exerts a primary control on virtually all geological, chemical and biological processes occurring at or near the Earth’s surface. Because these water-rock-life interactions take place at interfaces, both fluid composition and the physical and chemical structure of porous media must be treated as coevolving phenomena. Such complex and interrelated processes can hinder both interpretation and prediction of key environmental processes. One avenue of addressing this complexity is the use of multicomponent numerical methods that combine the governing equations of flow, transport and reactivity. In this presentation I will demonstrate the construction and application of multi-component reactive transport models to address key hydrogeochemical problems, with an emphasis on the balance between simulations of complex reactivity versus highly heterogeneous hydrologic conditions. Examples include stable isotope fractionations during microbially-mediated redox cycling and reactivity in highly resolved permeability structures. The goal is to demonstrate how simulations can be used to interrogate complex field data and thus provide new insights into the processes governing hydrogeochemical systems.