The future of applied tracers in hydrogeology

Tracing techniques have a broad application in science and have demonstrated particular usefulness in hydrogeology. Applied tracers, which are defined as non-natural constituents that are intentionally introduced, are especially powerful investigative tools because the tracer application (or source term) is controlled and well characterized. This permits quantification of transport parameters and measurement of subsurface properties in a way often unmatched by standard physical methods. Furthermore, tracer tests directly measure properties in-situ and can be used to investigate very specific processes by selecting tracers with appropriate physicochemical properties. In many cases, tracer test methods offer the most accurate or practical way to measure specific parameters, and in some cases, they are the only reliable investigative technique. Depending on the application, tracer tests can be used to characterize properties representative of large subsurface volumes or investigate small-scale transport phenomena. Applied tracers have been widely used for centuries to characterize flowpaths and estimate groundwater velocities. In fact, Kass (1998) notes that the Jewish historian Flavius Josephus recorded in approximately 10 A.D. the use of chaff as a tracer to link the spring source of the Jordan River to a nearby pond. Quantitative tracer tests using chloride, fluorescein, and bacteria were first conducted in the large karst regions in Europe in the late 1800s and early 1900s. After World War II, advances in chemical measurement technology permitted quantification of significantly lower tracer breakthrough concentrations and made high-frequency sampling economically feasible. Additionally, these technological advances lead to a significant increase in the diversity of constituents used as tracers. Historically, applied tracers in hydrogeology have been used mostly to characterize groundwater flow in kart regions. During the 1960s, benchmark studies exploited applied tracers to understand the controls on groundwater recharge (Horton and Hawkins 1965; Zimmerman et al. 1966) and significantly advanced an understanding of the flowpaths of rainfall to the water table. During the past 30 years, applied tracers have been used increasingly to understand solute transport phenomena in porous media and fractured rock aquifers, motivated primarily by environmental concerns related to disposal of radioactive and other wastes. For example, the well-known large-scale tracer experiments conducted at the Borden, Cape Cod, and Macro Dispersion Experiment (MADE) sites in the 1980s (Sudicky 1986; LeBlanc et al. 1991; Boggs et al. 1992, respectively) were designed to compare observed field-scale solute dispersion to macrodispersion predicted by stochastic analysis of independently-measured aquifer heterogeneity. These experiments have resulted in numerous important publications investigating the significance of local geologic heterogeneity, large-scale hydraulic conductivity trends, sorption, and rate-limited processes on solute transport. The results of these tests have also been used to evaluate the performance of numerical contaminant transport models. Tracing as a hydrogeologic investigative tool has grown significantly, and over the past decade, many new applications of applied tracers have been developed to investigate advanced transport phenomena, including multi-species reactive transport, colloid-facilitated transport, pore-scale mixing, and fracture-matrix control. This increase in tracer research is indicated by the increase in the number of papers. The increase of papers published in Hydrogeology Journal and other groundwater-related journals over the past decade that incorporate either an applied tracer technique as part of a broader hydrogeological investigation or specifically develop a new use of applied tracers in hydrogeology. For example, Aggarwal Received: 29 May 2004 / Accepted: 11 November 2004 Published online: 25 February 2005