The future of applied tracers in hydrogeology

Abstract

Tracingtechniqueshave a broadapplicationinscience andhave demonstrated particular usefulness in hydrogeology.Applied tracers, which are defined as non-natural con-stituents that are intentionally introduced, are especiallypowerful investigative tools because the tracer application(or source term) is controlled and well characterized. Thispermits quantification of transport parameters and mea-surement of subsurface properties in a way often un-matched by standard physical methods. Furthermore,tracer tests directly measure properties in-situ and can beused to investigate very specific processes by selectingtracers with appropriate physicochemical properties. Inmany cases, tracer test methods offer the most accurate orpractical way to measure specific parameters, and in somecases, they are the only reliable investigative technique.Depending on the application, tracer tests can be used tocharacterize properties representative of large subsurfacevolumes or investigate small-scale transport phenomena.Applied tracers have been widely used for centuries tocharacterize flowpaths and estimate groundwater velocities.In fact, Kass (1998) notes that the Jewish historian FlaviusJosephus recorded in approximately 10 A.D. the use ofchaff as a tracer to link the spring source of the JordanRiver to a nearby pond. Quantitative tracer tests usingchloride, fluorescein, and bacteria were first conducted inthe large karst regions in Europe in the late 1800s and early1900s. After World War II, advances in chemical mea-surement technology permitted quantification of signifi-cantly lower tracer breakthrough concentrations and madehigh-frequency sampling economically feasible. Addition-ally, these technological advances lead to a significant in-crease in the diversity of constituents used as tracers.Historically, applied tracers in hydrogeology havebeen used mostly to characterize groundwater flow in kartregions. During the 1960s, benchmark studies exploitedapplied tracers to understand the controls on groundwaterrecharge (Horton and Hawkins 1965; Zimmerman et al.1966) and significantly advanced an understanding of theflowpaths of rainfall to the water table. During the past 30years, applied tracers have been used increasingly to un-derstand solute transport phenomena in porous media andfractured rock aquifers, motivated primarily by environ-mental concerns related to disposal of radioactive andother wastes. For example, the well-known large-scaletracer experiments conducted at the Borden, Cape Cod,and Macro Dispersion Experiment (MADE) sites in the1980s (Sudicky 1986; LeBlanc et al. 1991; Boggs et al.1992, respectively) were designed to compare observedfield-scale solute dispersion to macrodispersion predictedby stochastic analysis of independently-measured aquiferheterogeneity. These experiments have resulted in nu-merous important publications investigating the signifi-cance of local geologic heterogeneity, large-scale hy-draulic conductivity trends, sorption, and rate-limitedprocesses on solute transport. The results of these testshave also been used to evaluate the performance of nu-merical contaminant transport models.Tracing as a hydrogeologic investigative tool hasgrown significantly, and over the past decade, many newapplications of applied tracers have been developed toinvestigate advanced transport phenomena, includingmulti-species reactive transport, colloid-facilitated trans-port, pore-scale mixing, and fracture-matrix control. Thisincrease in tracer research is indicated by the increase inthe number of papers. The increase of papers published inHydrogeology Journal and other groundwater-relatedjournals over the past decade that incorporate either anapplied tracer technique as part of a broader hydrogeo-logical investigation or specifically develop a new use ofapplied tracers in hydrogeology. For example, Aggarwal