Contaminant Geochemistry: Interactions and Transport in the Subsurface Environment combines the earth science fields of subsurface hydrology and en- ronmental geochemistry and aims to provide a comprehensive background for s- dents and researchers interested in protection and sustainable management of the subsurface environment. This book focuses on the upper part of the earth crust, covering the region between the land surface and the groundwater zone; anthro- genic contamination occurs primarily in this well-defined geosystem. Water and land are limited natural resources, and it is incumbent on humankind to manage knowledge and technology in a way that avoids, or at least minimizes, deterioration of these resources. In this context, an understanding of the interactions between subsurface components (solid, liquid and gaseous phases) and chemical contaminants is required. Because the subsurface is an open system, contaminants may be transported, transformed, and redistributed in the subsurface under a variety of environmental influences. Contaminant interactions in the subsurface are subject to continuous changes, being affected by fluctuations in climatic conditions (p- ticularly precipitation) and microbiological activity. Additionally, these inter- tions are controlled by the structure and properties of the earth materials, the molecular properties of the contaminants, and the hydrogeology of each specific location. As a consequence, a multidisciplinary approach is fundamental to und- standing the governing processes.