Groundwater geochemistry is an interdisciplinary science concerned with the chemistry in the subsurface environment. The chemical composition of groundwater is the combined result of the quality of water that enters the groundwater reservoir and reactions with minerals and organic matter of the aquifer matrix may modify the water quality. Apart from natural processes as controlling factors on the groundwater quality, in recent years the effect of pollution, such as nitrate from fertilizers and acid rain, also influences the groundwater chemistry. Due to the long residence time of groundwater in the invisible subsurface environment, the effect of pollution may first become apparent tens to hundreds of years afterwards. A proper understanding of the processes occurring in aquifers is required in order to predict what the effect of present day human activities will be on that scale. This book presents a comprehensive and quantitative approach to the study of groundwater quality. Practical examples of application are presented throughout the text.