Preface: Insights from environmental tracers in groundwater systems

Abstract

Hydrogeology developed into an independent field of scientific study over the course of the twentieth century. In the first part of the century, focus was mostly on resource development from aquifers, which depended primarily on an understanding of aquifer hydraulics through changes in water levels, with less focus on the actual movement of groundwater. By the middle of the century, water-quality interests had led to the development of groundwater geochemistry as a scientific field to understand the character and genesis of the natural constituents in groundwater (Foster 1950; Chebotarev 1955; Hem 1970). The discussion of chemical constituents in groundwater in textbooks continued to take a back seat to that of flow hydraulics, and the focus was either on water-quality concerns or upcoming issues pertaining to the transport of chemical pollutants (Todd 1959; Davis and DeWiest 1966; Freeze and Cherry 1979). The fields of physical and chemical hydrogeology were still separate from a practical standpoint illustrated by the fact that the historical contributions, well through the middle of the century, were most easily divided this way (Freeze and Back 1983; Back and Freeze 1983). With the development of mass spectrometers in the middle of the twentieth century and their improvements in accuracy and efficiency during the rest of the century, isotopic measurements of chemical tracers in groundwater became commonplace and the focus turned more toward movement of groundwater. The atmospheric nuclear testing in the late 1950s and early 1960s also created a convenient input signal to trace movement of tritium through the shallow hydrosphere during the rest of the century (Begemann and Libby 1957). At about that time (1961), the International Atomic Energy Agency (IAEA) began measurements for a Global Network of Isotopes in Precipitation (Aggarwal et al. 2007). Nuclear tests also released carbon-14 and chlorine-36 to the atmosphere, and introduction of other environmental tracers of anthropogenic origin such as chlorofluorocarbons (CFCs) and sulfur hexafluoride (SF6), soon followed. At the same time, physical hydrogeologists became aware that natural environmental tracers were good tools for understanding how groundwater moved through the subsurface over a broad range of time scales. Dating water by use of chemical and isotopic tracers yielded not only valuable conceptual information about residence times of water in aquifers, but also could be used to calibrate numerical groundwater-flow models. Thus, by the end of the century, environmental tracers had become an important field of study within groundwater geochemistry (Clark and Fritz 1997; Cook and Herczeg 2000). Today, in the twenty-first century, environmental tracers are critical tools that are used in many groundwater studies. Environmental tracers give information on current and past flow conditions that is independent of what can be determined by hydraulic analyses alone.