# Foreword

In textbooks published about hydrogeology over the decades, the emphasis is on the flow of groundwater based on the assumption that the groundwater has a spatially constant density. This common conceptualization of groundwater systems gives the impression that in nature groundwater behaves as if this assumption is generally valid. However, this is not the case. The assumption is a convenience for teaching and textbook purposes, and to justify simpler calculations. The inclusion of variable density in the consideration of groundwater behavior adds complexity for which understanding requires time and effort beyond what can be included in most groundwater courses and consulting projects. In nature, there are many circumstances where density effects are sufficiently strong that the character of the groundwater flow system is entirely different from the case where density effects are absent. Including the influence of variable density in evaluating groundwater systems has profound consequences for groundwater flow and transport behavior, because it results in a completely different flow regime, with much faster and more effective mixing, over larger spatial scales, and shorter time scales.

Density effects occur in groundwater when there is a significant contrast in the density of water masses from different sources. One example is where fresh groundwater below land flows toward the coast where salt water occurs in the aquifer below the sea. Another example is where groundwater flows through a chemically reactive zone in which minerals dissolve adding mass to the water. This creates a density contrast between the geochemically altered water and the ambient groundwater. An additional example is where groundwater is contaminated by a source such as a landfill or septic system and the dissolved constituents increase the density of the water forming a plume that sinks into the ambient groundwater flow system. Yet another example is where a zone of groundwater is heated, perhaps by localized geothermal activity deep in the earth causing convective flow cells to form in the groundwater system.

This book presents the theory of variable-density flow and provides many examples based on published field and laboratory studies as well as numerical model simulations showing that density variations can result in complex patterns of salinity and/or temperature distribution in groundwater.

The authors, Dr. Vincent Post in the Netherlands and Dr. Craig Simmons in Australia have published extensively on topics related to variable-density groundwater flow including numerical simulations, as well as laboratory and field studies, giving them a balanced viewpoint of the topics. This book describes the approaches, tools, techniques, and methods for conceptualizing, understanding, predicting, modeling and measuring variable-density groundwater flow behavior. The graphs and simulation results in this book provide the reader with an informed basis for evaluating circumstances for which variable density can be important to groundwater flow and envisioning how the flow system will deviate from constant density systems.

John Cherry, The Groundwater Project Leader

Guelph, Ontario, Canada, May 2022