This book is concerned with the dynamics of subterranean flows in the natural environment, with the transport and dispersal of contaminants that they may carry and the chemistry of the interactions between the matrix and the fluid that percolates through it in spatially random conduits or aquifer pores. It is intended for anyone with a quantitative interest in the world around them, and particularly for professionals and graduate students in hydrology, geology and environmental science and engineering. Many of the basic concepts originated in the late nineteenth or mid twentieth centuries, but they were usually developed in a very idealized and simplified form because few field measurements of actual sub-surface seepage or flow patterns had been attempted. Only recently have extensive and detailed hydrological field measurements been undertaken and their findings contain surprises that are re-defining the way we view this part of the natural world and begin to understand how it works.

In writing this book, I have relied heavily on the guidance and advice of many colleagues, friends and students who listened patiently, corrected gently and pointed in new directions. In particular, I must thank Lawrence Hardy, John Ferry, Jim Wood and Gordon Wolman, all colleagues, Robert Shedlock of the US Geological Survey and Emory Cleaves of the Maryland Geological Survey and my Cambridge colleagues, Andrew Woods and Herbert Huppert who always had something new to show me.