Preface

In 1998 I was involved in a project that sequenced in situ remediation technologies to treat a plume of contaminated groundwater consisting of hydrocarbons and chlorinated solvents. An anaerobic treatment zone relying on zero-valent-iron was placed in the ground and followed by an aerobic biosparge zone. Laboratory work was undertaken to carefully measure the chemical transformation rates and sorption coefficients, as well as evaluating biodegradation rates and volatilization. Flow was directed through a funnel and gate system in a highly controlled fashion, with monitoring points and piezometers placed to maximize the characterization efforts. The treatment system was designed with a factor of safety that should have ensured that no anaerobically degradable substances passed into the downstream aerobic treatment zone. Nonetheless, the lesser chlorinated compounds, cis 1,2 DCE and vinyl chloride passed through the zero-valent-iron zone and into the aerobic zone. Why did this happen? Was the problem with the estimated transformation rates or did the water find preferred pathways through the iron that reduced the residence time of contaminants in the iron? Follow up laboratory work found plausible chemical reasons for the breakthrough of contaminants, but tracer tests and Darcy calculations were unable to satisfactorily rule out physical flow contributions to the breakthrough. The failure of the hydrogeological portion of this assessment was both surprising and disturbing. It ignited an interest in alternative methods and technologies for measuring groundwater velocity. Apparently, this experience — or the thinking it inspired — was not unique. Over the past couple of decades, a substantial and growing literature has appeared that offers alternatives for velocity estimation. Some of the methods represent brand new approaches while others are modifications of earlier ideas; some methods have gained increasing attention since their introduction and others have gone dormant. The lesson here is that researchers and practitioners have increasingly recognized the importance of groundwater velocity and the limitations of the conventional methods for its estimation. The goal of this book is to compile the reasons for hydrogeologists to consider alternative methods of groundwater velocity measurement, and to present a subset of the technologies that have gained attention through the years. Through the presentation of this material, it is hoped that both experienced and upcoming hydrogeologists may begin to explore the insights that the novel velocity measurements can offer, and perhaps themselves be inspired to imagine new and better ways to make these measurements.