Foreword

Groundwater velocity is a fundamental and important parameter in groundwater science but often our treatment of it is cavalier. This is particularly so when we sacrifice accuracy and precision for the convenience offered by Darcy’s Law. We do this in full knowledge of the large, well-known uncertainties that come from using estimates of hydraulic conductivity and hydraulic gradient to obtain the Darcy flux, then dividing by the bulk effective porosity. Even where no site-specific measurements of any of these parameters have been made, “guesstimates” of the parameter values are used to estimate a velocity value that can seem to satisfy the need. This approach has been useful for solving some groundwater problems, but is not the most desirable approach for cases in which velocity is a pivotal factor – a common occurrence in contaminant investigations.

In addition to the high uncertainty of the Darcy-based approach, a velocity value determined this way has inherent spatial and temporal scales that can be inappropriate for the problem at hand. These limitations might be overcome by conducting tracer tests across an entire study area but doing so requires multiple monitoring points and typically is fraught with difficulty. Of greater value are tracer tests conducted at smaller scales, particularly in individual monitoring wells or boreholes. A method for this, known as borehole dilution, appeared in the literature in the 1940’s and was expanded on in publications in the 1950’s, but was largely ignored in groundwater science. With the great expansion of attention to groundwater contamination in the late 20th and early 21st century, other approaches for measurement of velocity in single boreholes have been developed. The overall technology of velocity measurement has matured from a research activity to one that is ready for general use in solving diverse groundwater problems.

This book is the first of its kind in the groundwater education literature. Many of the single borehole methods, and consideration of Darcy velocity in the larger framework of velocity measurement, are examined in this book.

The author, Dr. J.F. Devlin, a professor at the University of Kansas, in the USA, has been at the forefront of development and testing of multiple methods for velocity measurement and has broad experience in applying this technology in a variety of groundwater conditions. With the publication of this book, our deficient respect for groundwater velocity as one of the key parameters in groundwater science can come to an end.

John Cherry, The Groundwater Project Leader
Guelph, Ontario, Canada, December 2020